Chemical Reactions

QUESTION 1 2 marks Criterion A

Easy

A student mixes two colourless solutions in a test tube. She notices bubbles of gas forming rapidly, and the test tube becomes warm to touch.

State two signs from this description that suggest a chemical reaction has occurred.

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Sign 1 — gas production: bubbles forming rapidly show a new gas is being released, which is evidence a new substance has been made.

Sign 2 — temperature change: the test tube becoming warm shows energy is being released, which happens when a chemical reaction takes place (an exothermic reaction).

QUESTION 2 2 marks Criterion A

Easy

Ice melting into water is not evidence of a chemical reaction, even though its appearance changes from solid to liquid.

Explain why melting is a physical change, not a chemical change.

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Melting is a physical change because no new substance is formed — the ice and the water it melts into are both still the same substance, $\text{H}_2\text{O}$, just in a different state. The change is also easily reversible (the water can be frozen back into ice), unlike most chemical reactions.

QUESTION 3 2 marks Criterion A

Easy

Read the questions below.

a. Define what is meant by a precipitate forming in a reaction.

[1]

b. Give an example of two solutions that react together to form a precipitate, and describe what you would see.

[1]

Show complete worked solution

(a)

A precipitate is an insoluble solid that forms and appears suddenly (often as a cloudiness or a settling solid) when two solutions are mixed together and react.

(b)

Mixing lead nitrate solution with potassium iodide solution produces a bright yellow precipitate of lead iodide — the previously clear, colourless solutions instantly become cloudy with a bright yellow solid.

QUESTION 4 4 marks Criterion A

Medium

A student burns a strip of magnesium ribbon by holding it in a Bunsen burner flame.

a. Describe two observable signs that a chemical reaction is taking place.

[2]

b. State whether this reaction is exothermic or endothermic, and how you know.

[1]

c. Explain why the white ash produced cannot easily be turned back into magnesium ribbon.

[1]

Show complete worked solution

(a)

1. A very bright white light (and heat) is given off as the ribbon burns.

2. The shiny grey ribbon turns into a dull white powder/ash — a clear change in colour and appearance.

(b)

It is exothermic — light and heat are released to the surroundings during burning, which is the definition of an exothermic reaction.

(c)

Burning is a chemical reaction: the magnesium has reacted with oxygen to form a completely new substance, magnesium oxide, with different properties. Unlike a physical change, this cannot be reversed by a simple physical process such as cooling.

QUESTION 5 4 marks Criterion A

Medium

For each change below, state whether it is a physical change or a chemical change, and give one reason for your answer.

a. Dissolving sugar in water.

[1]

b. Toasting a slice of bread.

[2]

c. Breaking a glass window.

[1]

Show complete worked solution

(a)

Physical change — no new substance is formed (the sugar particles simply spread out among the water particles), and it can be reversed by evaporating the water.

(b)

Chemical change — new substances are formed (the bread turns brown, develops a new smell and taste, and becomes crisp); this cannot be reversed back to soft, pale bread by any simple physical process.

(c)

Physical change — the glass is still the same substance, just broken into smaller pieces; no new substance has been formed.

QUESTION 6 4 marks Criterion A

Medium

Explain why a colour change is not always reliable evidence that a chemical reaction has happened. Give an example of a colour change that is actually a physical change.

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A colour change can also happen during a purely physical process, so on its own it does not prove a new substance has formed. For example, dissolving blue copper sulfate crystals in water changes the appearance from a solid blue crystal to a pale blue solution — the colour clearly changes, but it is still a physical change, because the copper sulfate particles are still the same substance, just spread out (dissolved) among the water particles, and the solid could be recovered again by evaporating the water.

QUESTION 7 5 marks Criterion A

Hard

A student adds vinegar to a spoonful of baking soda in a beaker. Bubbles form rapidly, the beaker feels colder to touch, and a faint hissing sound is heard.

a. Identify three separate signs of a chemical reaction from this description.

[2]

b. This reaction is endothermic. Explain what "endothermic" means, using evidence from the description.

[1]

c. Suggest a test the student could carry out on the gas produced to help identify it.

[2]

Show complete worked solution

(a)

1. Gas bubbles forming (effervescence).

2. A temperature decrease (the beaker feels colder).

3. A hissing sound being produced.

(b)

Endothermic means the reaction absorbs energy (heat) from its surroundings rather than releasing it. This is shown by the beaker feeling colder — thermal energy has been taken from the beaker and its contents to drive the reaction.

(c)

She could bubble the gas through limewater (calcium hydroxide solution). If the gas is carbon dioxide (as expected from a vinegar and baking soda reaction), the limewater will turn cloudy/milky, confirming its identity.

QUESTION 8 6 marks Criterion B

Medium

A student wants to investigate whether measuring temperature change can reliably distinguish a physical change from a chemical reaction. She plans to compare (1) dissolving salt in water, and (2) mixing vinegar with baking soda.

a. State the independent and dependent variables in this investigation.

[2]

b. State one variable that must be controlled, and explain why.

[1]

c. Describe a method, including equipment, to collect temperature data for both processes fairly.

[3]

Show complete worked solution

(a)

Independent variable: which mixture is tested (salt + water, or vinegar + baking soda). Dependent variable: temperature change of the mixture (°C), measured before and after mixing.

(b)

The starting temperature and volume of liquid used must be controlled (kept the same for both tests) — if one test started warmer, or used more liquid, any difference in temperature change would not be a fair comparison between the two processes.

(c)

Measure the same volume (e.g. $50\,\text{cm}^3$) of water into an insulated cup for each test, and record the starting temperature with a thermometer.
Add the same mass of salt (for test 1) or the same volumes of vinegar and baking soda (for test 2), and stir gently.
Record the temperature every 30 seconds for 3 minutes, noting the maximum or minimum reached.
Repeat each test 3 times and calculate a mean temperature change, to improve reliability.

QUESTION 9 6 marks Criterion B

Medium

A student mixes small marble chips (calcium carbonate) with dilute hydrochloric acid and notices fizzing. She wants to measure how much gas is produced over time, as evidence that a chemical reaction is occurring.

a. Identify the independent and dependent variables.

[1]

b. Describe the equipment and method she could use to measure the volume of gas produced over time.

[3]

c. State one variable she should control to make the investigation a fair test, explaining why.

[2]

Show complete worked solution

(a)

Independent variable: time (s). Dependent variable: volume of gas produced (cm³).

(b)

She should react the marble chips and acid in a conical flask connected by a delivery tube to a gas syringe (or an upturned measuring cylinder full of water, collected by downward displacement of water in a trough). As soon as the acid is added, she should start a stopwatch and record the volume of gas collected every 30 seconds for 5 minutes, plotting a graph of volume against time.

(c)

The mass and size of the marble chips (and the concentration/volume of acid) must be controlled — using bigger chips, more chips, or a more concentrated acid would speed up gas production for reasons unrelated to the time being tested, making the comparison unfair.

QUESTION 10 8 marks Criterion B

Hard

Trial	1	2	3	4	5
Maximum temperature reached (°C)	34.5	36.0	34.0	38.5	34.2

A student is investigating the temperature change when two solutions react. She repeats the same experiment five times, each time mixing $20\,\text{cm}^3$ of solution A with $20\,\text{cm}^3$ of solution B in an open beaker, and recording the maximum temperature reached with a thermometer read by eye.

a. Identify the trial that gives an anomalous result, and calculate the mean of the other four trials.

[2]

b. Suggest a likely source of error causing the variation between trials, referring to the equipment used.

[3]

c. Suggest a specific improvement to the equipment that would give more reliable, consistent results, and explain why.

[3]

Show complete worked solution

(a)

Trial 4 ($38.5\,^\circ\text{C}$) is anomalous — it is clearly higher than the other four fairly consistent readings (around $34$–$36\,^\circ\text{C}$). Mean of the remaining four: $$ \frac{34.5+36.0+34.0+34.2}{4} = \frac{138.7}{4} = 34.7\,^\circ\text{C} \ (\text{1 d.p.}) $$

(b)

Using an open beaker means heat is continuously lost to the surrounding air, and how quickly the maximum temperature is read (by eye) can vary — a short delay in reading the thermometer, a draught in the room, or incomplete stirring could all cause the recorded maximum to be lower or higher than the true value from trial to trial.

(c)

Carry out the reaction in an insulated cup with a lid (with a small hole for the thermometer) instead of an open beaker. This reduces heat loss to the surroundings during the reaction, so the maximum temperature reached should be more consistent and closer to the true value across repeats.

QUESTION 11 3 marks Criterion C

Easy

Test tube	Substances mixed	Colour before	Colour after	Bubbles observed?
1	Iron filings + sulfur (heated)	grey / yellow	dull grey-black solid	No
2	Copper sulfate solution + water	blue	pale blue	No
3	Hydrochloric acid + zinc	colourless / grey	colourless (solid dissolves)	Yes
4	Sand + water	tan	tan (mixed, unchanged)	No

The table shows the results of a class experiment mixing four different substances.

a. Which tube(s) show evidence of a chemical reaction? State the sign used in each case.

[2]

b. Explain why tube 4 (sand and water) is only a physical change.

[1]

Show complete worked solution

(a)

Tube 1 — the grey iron and yellow sulfur change into a completely new dull grey-black solid (a colour/form change to a new substance).

Tube 3 — bubbles of gas are produced as the solid zinc dissolves and reacts (gas production).

(b)

No new substance is formed — the sand does not dissolve or react, it is simply mixed with (suspended in) the water, and could be separated again by filtering.

QUESTION 12 5 marks Criterion C

Medium

Concentration of sodium thiosulfate (g/dm³)	10	20	30	40	50
Time for cross mark to disappear (s)	240	120	80	60	20

Sodium thiosulfate solution reacts with acid to produce a cloudy, pale-yellow precipitate of sulfur, which is used as a sign that a chemical reaction has occurred (the solution slowly turns cloudy until a cross mark underneath the beaker can no longer be seen). The table shows results from an investigation into how concentration affects the time taken for this to happen.

a. What is the appearance of a precipitate evidence of?

[1]

b. Identify the anomalous result.

[1]

c. Explain how you identified it, using the pattern in the rest of the data.

[2]

d. State what the student should do with this anomalous result before drawing a graph.

[1]

Show complete worked solution

(a)

It is evidence that a new (insoluble) substance has formed — a sign that a chemical reaction has taken place.

(b)

$50\,\text{g/dm}^3$, $20\,\text{s}$.

(c)

For the first four results, concentration $\times$ time is roughly constant: $10\times240=2400$, $20\times120=2400$, $30\times80=2400$, $40\times60=2400$. Following this pattern, $50\,\text{g/dm}^3$ should give a time of about $2400\div50=48\,\text{s}$ — the recorded $20\,\text{s}$ breaks this otherwise consistent pattern.

(d)

Repeat that trial to check whether $20\,\text{s}$ was a genuine result or a timing error, and exclude the original anomalous reading from the graph if it cannot be reproduced.

QUESTION 13 4 marks Criterion C

Medium

Time (s)	0	20	40	60	80	100	120
Volume of gas collected (cm³)	0	15	26	33	37	38	38

Marble chips were reacted with excess dilute hydrochloric acid, and the volume of gas produced was recorded over time.

a. At approximately what time did the reaction stop producing gas? Explain how you know.

[2]

b. What is the total volume of gas produced by the reaction?

[1]

c. State the sign of a chemical reaction shown by this data.

[1]

Show complete worked solution

(a)

From $t=100\,\text{s}$ onwards, the volume stays constant at $38\,\text{cm}^3$ — a flat (zero gradient) section of the data means no further gas is being released, so the reaction has essentially finished by around $t=100\,\text{s}$ (one of the reactants, the marble chips, has been fully used up).

(b)

$38\,\text{cm}^3$.

(c)

Gas production (effervescence).

QUESTION 14 5 marks Criterion C

Medium

Two beakers, P and Q, were each set up by mixing two different pairs of solutions. The temperature of each beaker was recorded over time and plotted below.

a. Which beaker, P or Q, shows evidence that a chemical reaction took place? Explain using the graph.

[2]

b. State the sign of a chemical reaction shown in beaker P's graph.

[1]

c. Suggest why beaker Q's temperature might have changed very slightly, even though no chemical reaction occurred.

[2]

Show complete worked solution

(a)

Beaker P — its temperature rises steadily and clearly (from $19\,^\circ\text{C}$ to $28\,^\circ\text{C}$), showing energy is being released into the solution, which is evidence of an exothermic chemical reaction. Beaker Q's temperature stays almost completely flat, suggesting no reaction is releasing (or absorbing) energy.

(b)

A rise in temperature (heat given off).

(c)

The two solutions mixed in Q may simply have started at slightly different temperatures; mixing them causes a small, purely physical change in temperature as the liquids reach thermal equilibrium with each other, not because any new substance was formed.

QUESTION 15 4 marks Criterion C

Medium

Time (s)	0	10	20	30	40
Colour of solution	pink	pink	pink	colourless	colourless

A student slowly adds acid, drop by drop, to an alkali containing a few drops of phenolphthalein indicator (which is pink in alkali and colourless in acid), recording the colour of the mixture over time.

a. Between which two times did the colour change occur?

[1]

b. What does this colour change indicate about the reaction taking place?

[2]

c. Suggest one way the student could pinpoint the exact time of the colour change more precisely.

[1]

Show complete worked solution

(a)

Between $t=20\,\text{s}$ and $t=30\,\text{s}$.

(b)

The colour change shows the chemical composition of the solution has changed — the alkali has been completely used up (neutralised) and the mixture has become acidic, evidence that a chemical reaction (neutralisation) has occurred, not just a dilution.

(c)

Take readings more frequently (e.g. every 2 seconds instead of every 10 seconds) around the expected change, or add the acid more slowly (drop by drop) as the colour begins to change.

QUESTION 16 7 marks Criterion C

Hard

Mass of magnesium ribbon (g)	0.1	0.2	0.3	0.4	0.5
Temperature rise (°C)	4	8	12	16	20

A student reacted increasing masses of magnesium with excess dilute hydrochloric acid in an insulated cup, recording the temperature rise each time.

a. Describe the relationship between the mass of magnesium and the temperature rise shown by the data.

[2]

b. Predict the temperature rise for $0.6\,\text{g}$ of magnesium, showing your reasoning.

[2]

c. Explain how this consistent pattern supports the conclusion that a genuine chemical reaction (not experimental error) is producing the heat.

[3]

Show complete worked solution

(a)

The temperature rise is directly proportional to the mass of magnesium used — each extra $0.1\,\text{g}$ of magnesium produces a further $4\,^\circ\text{C}$ rise, and the pattern would pass through the origin (approximately $0\,\text{g}$ giving $0\,^\circ\text{C}$ rise).

(b)

Continuing the pattern of $+4\,^\circ\text{C}$ per extra $0.1\,\text{g}$: $$ 20 + 4 = 24\,^\circ\text{C} $$

(c)

Because the temperature rise scales predictably and repeatably with the amount of magnesium — more magnesium means more particles reacting, releasing proportionally more energy. Random experimental error would be expected to scatter the results unevenly above and below a trend, not produce such a smooth, consistent, repeatable pattern across five different masses.

QUESTION 17 7 marks Criterion C

Hard

Concentration of acid (mol/dm³)	0.5	1.0	1.5	2.0	2.5
Mass lost from open flask in 60 s (g)	0.12	0.24	0.35	0.49	0.33

A student reacted marble chips with hydrochloric acid of different concentrations in an open flask on a balance, recording the mass lost (as gas escaped) after 60 seconds each time.

a. Calculate the rate of mass loss (g/s) for the $2.0\,\text{mol/dm}^3$ trial.

[2]

b. Identify the anomalous result, explaining your reasoning.

[2]

c. Explain how mass loss provides evidence that a chemical reaction (producing a gas) is taking place, referring to conservation of mass.

[3]

Show complete worked solution

(a)

$$ \text{rate} = \frac{0.49}{60} = 0.0082\,\text{g/s} \ (\text{2 s.f.}) $$

(b)

$2.5\,\text{mol/dm}^3$ ($0.33\,\text{g}$) is anomalous. The mass lost increases steadily with concentration for the first four results ($0.12\to0.24\to0.35\to0.49$), so at $2.5\,\text{mol/dm}^3$ the mass lost should be even greater than $0.49\,\text{g}$ — but $0.33\,\text{g}$ is actually lower than the previous result, breaking the increasing trend.

(c)

The law of conservation of mass states the total mass of reactants equals the total mass of products. In this open flask, the carbon dioxide gas produced escapes into the air, so it is no longer being weighed — the mass "lost" from the flask is really the mass of the new gaseous substance that has left the system. This mass loss is therefore strong evidence that a new substance (a gas) has been formed, i.e. that a chemical reaction has occurred.

QUESTION 18 5 marks Criterion D

Medium

Some breathalyzer devices detect alcohol using a chemical reaction: a driver's breath is passed through orange crystals of potassium dichromate, which turn green as they react with alcohol vapour — a colour change that is a sign of a chemical reaction.

Discuss one benefit and one drawback of relying on this chemical-reaction-based test for road safety.

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Benefit: The colour-change reaction gives police a quick, cheap, and portable way to screen drivers for alcohol at the roadside, without needing a laboratory. This allows officers to make rapid decisions and remove suspected drink-drivers from the road immediately, helping to prevent accidents and keep other road users safe.

Drawback: Chemical colour-change tests can sometimes be triggered or affected by other substances (such as strong mouthwash or certain foods), risking a false result. The chemical used, potassium dichromate, is also toxic and must be handled and disposed of carefully. For these reasons, a positive dichromate-based test is usually only used as an initial screening step, with a more accurate method (such as a blood test) required before it can be used as legal evidence.

QUESTION 19 5 marks Criterion D

Medium

Glow sticks work using a chemical reaction: mixing the chemicals inside produces light (a sign of a chemical reaction) without needing batteries or electricity.

Discuss one benefit and one drawback of using this chemical-reaction-based technology.

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Benefit: Glow sticks provide safe emergency lighting that does not need electricity or an open flame, making them extremely useful in situations such as natural disasters, power cuts, underwater diving, or near flammable gases where a battery-powered light or naked flame could be dangerous.

Drawback: Glow sticks are single-use — once the reaction has finished producing light, they cannot be reused or recharged like a battery-powered torch. This creates ongoing plastic and chemical waste, especially when used in large quantities (for example, at concerts or festivals), and the chemicals inside can be an irritant if the stick breaks and leaks.

QUESTION 20 6 marks Criterion D

Hard

The thermite reaction (aluminium reacting with iron oxide) is an extremely exothermic reaction, producing molten iron, bright sparks, and temperatures of over $2500\,^\circ\text{C}$. It is sometimes used to weld railway tracks together on site.

Evaluate the impact of using this reaction for welding, discussing both a benefit and a concern it raises.

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Benefit: Because the reaction generates its own extreme heat directly at the joint, workers can weld railway tracks together on location without needing to transport bulky electrical welding equipment or a power supply to remote sections of track. This produces very strong, permanent joints and allows repairs to be completed relatively quickly, reducing how long a railway line needs to be closed.

Concern: The reaction is extremely hazardous — the molten iron, bright sparks, and temperatures far exceeding $2500\,^\circ\text{C}$ pose a serious burn and fire risk to workers and surrounding materials if not carefully controlled. It requires specially trained operators and strict safety equipment and procedures, showing that a highly useful chemical reaction can also carry a significant safety cost if the risks are not properly managed.

QUESTION 1 2 marks Criterion A

Easy

Complete the word equation for the reaction between magnesium and oxygen:

magnesium + oxygen → ___

Show complete worked solution

$$ \text{magnesium} + \text{oxygen} \rightarrow \textbf{magnesium oxide} $$

QUESTION 2 2 marks Criterion A

Easy

Identify the reactants and the products in the following word equation:

methane + oxygen → carbon dioxide + water

Show complete worked solution

Reactants: methane, oxygen (the substances present at the start, on the left of the arrow).

Products: carbon dioxide, water (the new substances formed, on the right of the arrow).

QUESTION 3 2 marks Criterion A

Easy

Word equations follow a set format.

a. What symbol is used in a word equation to mean "reacts to produce"?

[1]

b. Write a word equation for the reaction between iron and sulfur to form iron sulfide.

[1]

Show complete worked solution

(a)

An arrow, $\rightarrow$.

(b)

$$ \text{iron} + \text{sulfur} \rightarrow \text{iron sulfide} $$

QUESTION 4 4 marks Criterion A

Medium

Methane burns completely in oxygen, producing carbon dioxide and water.

a. Write the word equation for this reaction.

[2]

b. Name the general type of reaction this is.

[1]

c. State one sign that would show this reaction is taking place.

[1]

Show complete worked solution

(a)

$$ \text{methane} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} $$

(b)

A combustion (oxidation) reaction.

(c)

A flame is produced, with heat and light given off.

QUESTION 5 4 marks Criterion A

Medium

Calcium carbonate breaks down when strongly heated.

a. Complete the word equation: calcium carbonate $\xrightarrow{\text{heat}}$ ___ + ___

[2]

b. What type of reaction is this called, and what does writing "heat" above the arrow mean?

[2]

Show complete worked solution

(a)

$$ \text{calcium carbonate} \xrightarrow{\text{heat}} \text{calcium oxide} + \text{carbon dioxide} $$

(b)

This is called a thermal decomposition reaction — a single compound breaking down into two or more simpler substances. Writing "heat" above the arrow shows that heat is a required condition for the reaction to happen, not a reactant or product itself.

QUESTION 6 4 marks Criterion A

Medium

Hydrochloric acid reacts with sodium hydroxide solution.

a. Write the word equation for this reaction.

[2]

b. Name the salt formed.

[1]

c. State the general word equation pattern for acid + base reactions.

[1]

Show complete worked solution

(a)

$$ \text{hydrochloric acid} + \text{sodium hydroxide} \rightarrow \text{sodium chloride} + \text{water} $$

(b)

Sodium chloride.

(c)

$$ \text{acid} + \text{base} \rightarrow \text{salt} + \text{water} $$

QUESTION 7 6 marks Criterion A

Hard

Magnesium is more reactive than copper. When magnesium metal is added to copper sulfate solution, a displacement reaction occurs.

a. Write the word equation for this reaction.

[2]

b. Explain, in terms of reactivity, why this reaction happens.

[2]

c. Predict whether copper metal added to magnesium sulfate solution would react. Explain your reasoning.

[2]

Show complete worked solution

(a)

$$ \text{magnesium} + \text{copper sulfate} \rightarrow \text{magnesium sulfate} + \text{copper} $$

(b)

Magnesium is more reactive than copper, so it can displace (push out) copper from its compound — the magnesium atoms take copper's place in the sulfate compound, releasing copper metal, because more reactive metals form compounds more readily than less reactive ones.

(c)

No reaction would occur. Copper is less reactive than magnesium, so it cannot displace magnesium from its compound — a less reactive metal can never displace a more reactive one from a solution of its compound.

QUESTION 8 6 marks Criterion B

Medium

A student reacts a piece of unknown metal with dilute hydrochloric acid and wants to work out the word equation for the reaction, including confirming the identity of the gas produced.

a. If she investigates how the mass of metal affects the volume of gas collected, state the independent and dependent variables.

[1]

b. Describe a test she could carry out to identify the gas released, including the expected positive result.

[3]

c. Using the result of this test, complete the general word equation: metal + hydrochloric acid $\rightarrow$ metal chloride + ___.

[2]

Show complete worked solution

(a)

Independent variable: mass of metal (g). Dependent variable: volume of gas produced (cm³).

(b)

She should collect a sample of the gas in a test tube and hold a lit splint at its mouth. If the gas is hydrogen, it will ignite with a distinctive "squeaky pop" sound — the standard positive test for hydrogen gas.

(c)

$$ \text{metal} + \text{hydrochloric acid} \rightarrow \text{metal chloride} + \textbf{hydrogen} $$

QUESTION 9 6 marks Criterion B

Medium

A student wants to rank four metals (magnesium, zinc, iron, copper) in order of reactivity by testing each metal in solutions of the other three metals' sulfates, observing which combinations react (shown by a colour change or a new coating forming on the metal).

a. State the independent and dependent variables.

[1]

b. Describe how she should control the test to make it a fair comparison across metals.

[2]

c. Explain how she would use her results to both rank the reactivity of the metals and write word equations only for the combinations that actually react.

[3]

Show complete worked solution

(a)

Independent variable: which metal / salt solution combination is tested. Dependent variable: whether a reaction occurs (a colour change in the solution, or a new metal coating forming).

(b)

She should use the same sized/cleaned strip of each metal, the same volume and concentration of each salt solution, and leave each combination for the same length of time before checking for a reaction.

(c)

A reaction occurring (a colour change or new coating) shows the metal added is more reactive than the metal already in the solution, since it has displaced it. She can rank the metals by how many of the three other solutions each metal reacts with — the metal that reacts with the most solutions is the most reactive. A word equation (metal + salt solution $\rightarrow$ new salt + displaced metal) should only be written for the pairs that actually showed a reaction, since only these are genuine displacement reactions.

QUESTION 10 8 marks Criterion B

Hard

Trial	1	2	3	4	5
Result of lit-splint test	loud pop	faint pop	loud pop	no sound	loud pop

A student repeats a metal-plus-acid reaction five times, collecting gas each time and testing it with a lit splint.

a. Explain why the test gave an inconsistent result across the five trials.

[2]

b. Suggest an improved method for collecting the gas sample that would give a more reliable, purer sample for testing, and explain why it works.

[3]

c. Given that a "pop" (of any loudness) was heard in four of the five trials, explain what conclusion the student can still draw about the identity of the gas, and what word equation this supports.

[3]

Show complete worked solution

(a)

The gas collected was likely mixed with air in some trials — if the collection tube wasn't given time to fill completely, or air wasn't fully purged from it before the gas was collected, the diluted hydrogen sample would produce a weaker pop, or none at all.

(b)

Collect the gas by downward displacement of water — an upturned, water-filled test tube in a trough of water, with the delivery tube feeding gas into it. This excludes air, because the tube starts completely full of water, and the gas physically pushes the water out as it collects, so only pure hydrogen ends up in the tube.

(c)

Since a lit splint producing a "pop" sound is the characteristic positive test for hydrogen, and this was observed (even if inconsistently) in the majority of trials, the student can reasonably conclude the gas is hydrogen. This supports the word equation: $$ \text{metal} + \text{hydrochloric acid} \rightarrow \text{metal chloride} + \text{hydrogen} $$

QUESTION 11 3 marks Criterion C

Easy

Test performed	Result
Limewater test on gas produced	turns milky / cloudy

A student burns a candle (a hydrocarbon wax) in a sealed bell jar and passes the gas produced through limewater.

a. What does the result of the limewater test tell us about one of the products?

[1]

b. Complete the word equation: wax (hydrocarbon) + oxygen $\rightarrow$ carbon dioxide + ___.

[2]

Show complete worked solution

(a)

It confirms that carbon dioxide gas is one of the products, since limewater turning milky/cloudy is the positive test for carbon dioxide.

(b)

$$ \text{wax} + \text{oxygen} \rightarrow \text{carbon dioxide} + \textbf{water} $$

QUESTION 12 4 marks Criterion C

Medium

Reaction	Gas test used	Result
X: zinc + hydrochloric acid	lit splint	squeaky pop
Y: hydrogen peroxide decomposition	glowing splint	relights
Z: calcium carbonate + hydrochloric acid	limewater	turns milky

Three separate reactions, X, Y and Z, were each tested to identify the gas produced.

a. Identify the gas in reaction X, and complete: zinc + hydrochloric acid $\rightarrow$ zinc chloride + ___.

[1]

b. Identify the gas in reaction Y, and name the general type of reaction: hydrogen peroxide $\rightarrow$ water + ___.

[1]

c. Identify the gas in reaction Z, and complete: calcium carbonate + hydrochloric acid $\rightarrow$ calcium chloride + water + ___.

[2]

Show complete worked solution

(a)

Hydrogen (squeaky pop test): $$ \text{zinc} + \text{hydrochloric acid} \rightarrow \text{zinc chloride} + \text{hydrogen} $$

(b)

Oxygen (relights a glowing splint); this is a decomposition reaction: $$ \text{hydrogen peroxide} \rightarrow \text{water} + \text{oxygen} $$

(c)

Carbon dioxide (turns limewater milky): $$ \text{calcium carbonate} + \text{hydrochloric acid} \rightarrow \text{calcium chloride} + \text{water} + \text{carbon dioxide} $$

QUESTION 13 4 marks Criterion C

Medium

The word equation below represents the combustion of ethanol:

ethanol + oxygen → carbon dioxide + water + energy

A student claims this word equation is wrong because "energy should not be included as a product." Evaluate this claim, referring to the type of reaction taking place.

Show complete worked solution

The student's claim is broadly correct. Word equations are used to show only the substances (matter) involved in a reaction, as reactants and products — energy is not a physical substance, so it should not be written as a "product" alongside carbon dioxide and water. However, combustion genuinely is an exothermic reaction that releases a large amount of energy; this fact is usually shown separately, for example by describing the reaction as "exothermic," rather than by writing "energy" into the word equation itself.

QUESTION 14 5 marks Criterion C

Medium

Time (min)	0	2	4	6	8	10
Mass of white solid remaining (g)	10.0	9.2	8.5	8.9	7.9	7.9

A white solid, calcium carbonate, was heated continuously and its mass recorded every 2 minutes.

a. Identify the anomalous reading, explaining why it must be an error.

[2]

b. At what time does the reaction appear to be complete?

[1]

c. Complete the word equation for this reaction, and name the gas that escaped, based on the mass loss observed.

[2]

Show complete worked solution

(a)

The reading at $t=6\,\text{min}$ ($8.9\,\text{g}$) is anomalous, because the mass increased from the previous reading ($8.5\,\text{g}\to8.9\,\text{g}$). As the reaction releases gas to the surroundings, the total mass being weighed can only decrease or stay the same over time — it can never increase — so this must be a measurement or recording error.

(b)

From $t=8\,\text{min}$ onwards, since the mass stays constant at $7.9\,\text{g}$.

(c)

$$ \text{calcium carbonate} \xrightarrow{\text{heat}} \text{calcium oxide} + \text{carbon dioxide} $$ Carbon dioxide is the gas that escaped, matching the total mass lost ($10.0-7.9=2.1\,\text{g}$).

QUESTION 15 4 marks Criterion C

Medium

Mass of hydrogen used (g)	Mass of oxygen used (g)	Mass of water formed (g)
2	16	18

Two students disagree about the word equation for making water. Student A writes: hydrogen + oxygen → water. Student B writes: hydrogen + water → oxygen.

a. Using the table, evaluate which student's word equation is consistent with the data given.

[2]

b. State which check confirms Student A's equation is consistent with the law of conservation of mass, showing the numbers.

[2]

Show complete worked solution

(a)

Student A's equation is consistent with the data: hydrogen and oxygen are both used up (they behave as reactants), while water is newly formed (a product) — matching the masses given, where hydrogen and oxygen are consumed and water appears. Student B's equation is inconsistent, since it treats water as something being used up, but no water was present or consumed at the start.

(b)

Mass of reactants = mass of products: $$ 2 + 16 = 18\,\text{g} $$ which exactly matches the mass of water formed, confirming the equation obeys conservation of mass.

QUESTION 16 7 marks Criterion C

Hard

A student carries out two separate unknown reactions and records these observations:

Reaction 1: A grey metal is added to a blue solution. The blue colour fades to colourless, and a pink-brown solid coats the metal.

Reaction 2: A white powder is heated strongly. A colourless gas is given off, which turns limewater milky. A white solid remains.

a. Identify the gas in reaction 2, and the general type of both reactions.

[3]

b. Reaction 1 is between iron and copper sulfate solution. Write the word equation, using the colour evidence to justify the products.

[2]

c. Reaction 2 is the decomposition of calcium carbonate. Write the word equation and state which piece of evidence identifies each product.

[2]

Show complete worked solution

(a)

Gas: carbon dioxide (turns limewater milky). Reaction 1 is a displacement reaction (a more reactive metal displaces a less reactive one from solution); Reaction 2 is a thermal decomposition reaction (heat breaks a compound into simpler substances).

(b)

$$ \text{iron} + \text{copper sulfate} \rightarrow \text{iron sulfate} + \text{copper} $$ The blue colour of copper sulfate solution fading confirms it is being used up (a reactant), and the pink-brown solid coating the iron is deposited copper metal (a product).

(c)

$$ \text{calcium carbonate} \rightarrow \text{calcium oxide} + \text{carbon dioxide} $$ The gas turning limewater milky identifies the carbon dioxide; the white solid remaining is the calcium oxide.

QUESTION 17 7 marks Criterion C

Hard

Test	Result
Glowing splint held in the gas	relights and burns brightly
Limewater	no change (stays clear)

A student is trying to identify the gas produced when hydrogen peroxide solution decomposes with a manganese dioxide catalyst, and carries out the two tests above. Another student claims the gas must be carbon dioxide, "because all gases produced in chemical reactions are carbon dioxide."

a. Evaluate the other student's claim, using the two test results given.

[3]

b. Identify the gas, giving the evidence.

[2]

c. Complete the word equation: hydrogen peroxide $\xrightarrow{\text{manganese dioxide}}$ ___ + ___.

[2]

Show complete worked solution

(a)

The claim is incorrect. If the gas were carbon dioxide, the limewater would have turned milky/cloudy — but it stayed clear. A glowing splint relighting is also the positive test for oxygen, not carbon dioxide (carbon dioxide would extinguish a glowing splint, not relight it). Both results point to oxygen, not carbon dioxide — and in general, different reactions produce different gases depending on the substances reacting, not always carbon dioxide.

(b)

Oxygen — a glowing splint relighting is the standard positive test for oxygen gas.

(c)

$$ \text{hydrogen peroxide} \xrightarrow{\text{manganese dioxide}} \text{water} + \text{oxygen} $$

QUESTION 18 5 marks Criterion D

Medium

The Haber process uses the word equation nitrogen + hydrogen → ammonia to manufacture ammonia on a huge industrial scale, most of which is used to make fertilisers.

Discuss one benefit and one drawback of this reaction being used at such a large scale.

Show complete worked solution

Benefit: Large-scale ammonia production enables the manufacture of vast quantities of fertiliser, which has massively increased how much food can be grown worldwide, helping to feed billions of people who would otherwise face food shortages.

Drawback: The Haber process is very energy-intensive, and this energy is largely generated by burning fossil fuels, releasing significant amounts of carbon dioxide and contributing to climate change. In addition, excess fertiliser can run off farmland into rivers and lakes, causing water pollution and harming aquatic ecosystems (eutrophication).

QUESTION 19 5 marks Criterion D

Medium

Rusting can be represented by the word equation iron + oxygen + water → hydrated iron oxide. Knowing that both oxygen and water are needed for rusting to occur has practical uses.

Discuss one drawback of rusting and one benefit of understanding this word equation.

Show complete worked solution

Drawback: Rusting costs economies enormous amounts of money every year through damage to vehicles, bridges, pipelines, and other steel structures, and can also create serious safety hazards if structural steel is weakened without being noticed.

Benefit: Knowing that rusting requires both oxygen and water means engineers can prevent it by keeping either one away from the iron — for example, by painting or galvanising steel to form a protective barrier. This understanding, based directly on the word equation, saves significant money on repairs and improves the safety of structures.

QUESTION 20 6 marks Criterion D

Hard

Combustion of fuels can be represented by word equations such as hydrocarbon fuel + oxygen → carbon dioxide + water (complete combustion), or, with a limited oxygen supply, incomplete combustion producing toxic carbon monoxide and soot instead.

Evaluate the impact of combustion reactions on society, discussing both a benefit and a concern.

Show complete worked solution

Benefit: Understanding the word equations for combustion allows engineers and scientists to control and harness the release of energy from fuels reliably, powering transport, electricity generation, heating, and countless other technologies that modern life depends on.

Concern: Even complete combustion of fossil fuels releases large amounts of carbon dioxide, a greenhouse gas that is a major driver of climate change; incomplete combustion is worse still, producing toxic carbon monoxide gas and soot (carbon particles) that harm air quality and human health. This shows that while combustion reactions are hugely useful, their widespread use also causes serious environmental and health concerns that societies must manage.

QUESTION 1 2 marks Criterion A

Easy

State the law of conservation of mass, in your own words.

Show complete worked solution

The law of conservation of mass states that mass cannot be created or destroyed in a chemical reaction — the total mass of the reactants equals the total mass of the products formed (in a closed system).

QUESTION 2 2 marks Criterion A

Easy

$20\,\text{g}$ of substance A reacts completely with $15\,\text{g}$ of substance B in a sealed container.

Calculate the total mass of products formed.

Show complete worked solution

By the law of conservation of mass, mass of products = mass of reactants: $$ 20 + 15 = \textbf{35}\,\text{g} $$

QUESTION 3 3 marks Criterion A

Easy

$12\,\text{g}$ of magnesium reacts completely with oxygen gas in a sealed flask to form $20\,\text{g}$ of magnesium oxide.

a. What feature of the apparatus is essential for accurately testing conservation of mass in this way?

[1]

b. Calculate the mass of oxygen that reacted.

[2]

Show complete worked solution

(a)

A sealed (closed) container, so that no gas can escape from or enter the system while it is weighed.

(b)

$$ \text{mass of oxygen} = 20 - 12 = \textbf{8}\,\text{g} $$

QUESTION 4 4 marks Criterion A

Medium

In a sealed flask, $5.3\,\text{g}$ of sodium carbonate reacts with $7.3\,\text{g}$ of hydrochloric acid solution. The total mass of products (including the carbon dioxide gas produced) is measured as $12.6\,\text{g}$.

a. Show, with a calculation, that this data obeys the law of conservation of mass.

[2]

b. If the flask were left open instead, and the gas allowed to escape, explain what would happen to the measured mass on a balance, and why this does not break the law of conservation of mass.

[2]

Show complete worked solution

(a)

$$ \text{mass of reactants} = 5.3 + 7.3 = 12.6\,\text{g} $$ This exactly matches the mass of products given ($12.6\,\text{g}$), so the law of conservation of mass is obeyed.

(b)

The measured mass on the balance would appear to decrease, as carbon dioxide gas escapes into the surrounding air. This does not break the law, because that escaped gas still has mass — it has simply left the container and can no longer be weighed; if the escaped gas were captured and weighed too, the total mass would still be found to be conserved.

QUESTION 5 4 marks Criterion A

Medium

A piece of iron wool with mass $4.0\,\text{g}$ is left in open air and slowly rusts, combining with oxygen from the air. After several days, the rusted mass is measured as $4.6\,\text{g}$.

a. Calculate the mass of oxygen that combined with the iron.

[2]

b. Explain why the total mass increased, even though this is an open (not sealed) system, without breaking the law of conservation of mass.

[2]

Show complete worked solution

(a)

$$ 4.6 - 4.0 = \textbf{0.6}\,\text{g} $$

(b)

The increase happens because the reaction took in oxygen gas from the surrounding air and incorporated it into a new solid, iron oxide (rust). No mass was created — it simply moved from the air (part of the wider, open system) into the solid being weighed. If the mass lost by the surrounding air were also measured, the total mass of the whole system (iron wool + air) would still be conserved.

QUESTION 6 4 marks Criterion A

Medium

$15\,\text{g}$ of hydrogen gas reacts completely with excess oxygen to form $135\,\text{g}$ of water, in a sealed container.

a. Calculate the mass of oxygen that reacted.

[3]

b. State the law that allows you to make this calculation without directly measuring the oxygen.

[1]

Show complete worked solution

(a)

$$ \text{mass of oxygen} = 135 - 15 = \textbf{120}\,\text{g} $$

(b)

The law of conservation of mass: total mass of reactants = total mass of products.

QUESTION 7 5 marks Criterion A

Hard

$50\,\text{g}$ of calcium carbonate is heated completely and decomposes into calcium oxide and carbon dioxide gas in a sealed container. The mass of calcium oxide produced is $28\,\text{g}$.

a. Calculate the mass of carbon dioxide gas produced.

[2]

b. If this reaction were carried out in an open container instead, predict and explain what the balance would show immediately after the reaction, compared to before.

[2]

c. Calculate the percentage of the original $50\,\text{g}$ that was lost as gas.

[1]

Show complete worked solution

(a)

$$ 50 - 28 = \textbf{22}\,\text{g} $$

(b)

The balance reading would drop as the carbon dioxide gas escapes into the surrounding air, no longer contributing to the mass measured in the container. Only the solid calcium oxide would remain to be weighed, so the balance would read approximately $28\,\text{g}$ — a drop of $22\,\text{g}$ from the original $50\,\text{g}$.

(c)

$$ \frac{22}{50}\times100 = \textbf{44\%} $$

QUESTION 8 6 marks Criterion B

Medium

A student wants to test whether mass is conserved when marble chips react with hydrochloric acid, a reaction that produces carbon dioxide gas. She compares mass change in a sealed flask (with a balloon fitted over the neck to trap any gas) versus an open flask.

a. State the independent and dependent variables.

[1]

b. Describe the method and equipment she should use to fairly compare mass change in the two setups.

[3]

c. Predict and explain what she should observe for each setup, in terms of conservation of mass.

[2]

Show complete worked solution

(a)

Independent variable: type of setup (sealed with balloon, or open). Dependent variable: mass change measured on the balance (g), before and after the reaction.

(b)

Use the same mass of marble chips and the same volume/concentration of acid in both trials.
Place the marble chips and acid in a conical flask on a digital balance and record the starting mass.
For setup 1, immediately stretch a balloon tightly over the neck of the flask to trap any gas produced; for setup 2, leave the flask open.
Wait until bubbling stops in each case, then record the final mass.
Repeat each setup 3 times and calculate a mean mass change.

(c)

The sealed (balloon) flask should show almost no mass change, since the gas produced is still trapped and counted in the total mass. The open flask should show a clear mass decrease, because the carbon dioxide gas escapes into the room and is no longer weighed — even though mass is genuinely conserved overall in both cases.

QUESTION 9 6 marks Criterion B

Medium

A student wants to investigate the mass gained by steel wool as it rusts in a sealed conical flask of air over one week, and use this to demonstrate the law of conservation of mass.

a. State the independent and dependent variables.

[1]

b. Describe how she should set up the experiment, including how to ensure it is a sealed system so no air can enter or leave.

[3]

c. Explain how measuring both the mass gained by the steel wool AND the volume of trapped air used up would let her demonstrate the law of conservation of mass most convincingly.

[2]

Show complete worked solution

(a)

Independent variable: time (days). Dependent variable: mass of the steel wool (g) (and/or the volume of trapped air remaining).

(b)

Place slightly damp steel wool inside a conical flask, and seal the mouth of the flask with a rubber bung fitted with a delivery tube leading to an inverted, water-filled measuring cylinder standing in a trough of water. As oxygen from the trapped air is used up, water will be drawn up into the measuring cylinder, showing the volume change without ever opening the sealed flask. She should check the bung fits snugly with no visible gaps.

(c)

If she can show that the mass gained by the steel wool is equal to the mass of oxygen that disappeared from the trapped air, this directly proves that nothing was created or destroyed — the oxygen simply transferred from being a gas in the flask's air to becoming part of the solid rust. This is far more convincing evidence for the law than only observing that the steel wool got heavier.

QUESTION 10 8 marks Criterion B

Hard

Trial	1	2	3	4	5
Mass change in "sealed" balloon setup (g)	-0.02	-0.05	-0.03	-0.15	-0.04

A student repeats the sealed balloon setup from a mass-conservation experiment five times and records the mass change each time.

a. Explain why the sealed setup should theoretically show a mass change of $0\,\text{g}$ in each trial.

[2]

b. The measured values are all slightly negative rather than exactly $0$, and trial 4 is much more negative than the others. Explain a likely source of error, and identify the anomalous trial.

[3]

c. Suggest a specific improvement to the setup that would give results closer to the expected $0\,\text{g}$ change, and explain why.

[3]

Show complete worked solution

(a)

No matter is entering or leaving the closed system — the reactants turn into products, including the gas, which stays trapped by the balloon, so total mass should be identical before and after.

(b)

The small negative values in most trials are likely due to the expanding balloon pushing against (displacing) the surrounding air, creating a small buoyancy effect that makes the flask appear slightly lighter on the balance, or a very slow, slight leak around the neck. Trial 4 ($-0.15\,\text{g}$) is anomalous — it is roughly 3–5 times larger in magnitude than the other four fairly consistent readings, suggesting the balloon wasn't fitted tightly enough that time and some gas actually escaped.

(c)

Use a rigid sealed flask with a bung instead of an expanding balloon (weighing the whole sealed flask-plus-bung system), or seal the balloon on tightly with tape or an elastic band. A rigid sealed container removes the buoyancy effect of an expanding balloon and prevents slow gas leaks, so the measured mass change would stay much closer to the true, theoretically conserved value of $0\,\text{g}$.

QUESTION 11 3 marks Criterion C

Easy

Trial	Mass A (g)	Mass B (g)	Mass of product (g)
1	10	5	15
2	8	6	?
3	?	4	16

Substance A reacts completely with substance B, in a sealed container, to form a single product.

a. Calculate the missing mass in trial 2.

[1]

b. Calculate the missing mass in trial 3.

[2]

Show complete worked solution

(a)

$$ 8 + 6 = \textbf{14}\,\text{g} $$

(b)

$$ 16 - 4 = \textbf{12}\,\text{g} $$

QUESTION 12 4 marks Criterion C

Medium

Trial	Total mass of reactants (g)	Total mass of products (g)
1	24	24
2	31	31
3	18	22
4	40	40

Four sealed-container reactions were weighed before and after the reaction.

a. Which trial's data appears to break the law of conservation of mass?

[1]

b. Suggest two possible explanations for why this trial's results don't balance, without assuming the law is wrong.

[2]

c. Is the law of conservation of mass ever really broken in real chemical reactions? Explain.

[1]

Show complete worked solution

(a)

Trial 3 ($18\,\text{g}$ reactants vs $22\,\text{g}$ products — not equal).

(b)

A measurement/weighing error — for example, the balance not being correctly zeroed/tared, or an extra item (such as a container or residue) being accidentally weighed along with the products; or matter from outside the system (such as moisture) entering the products' container between the two measurements.

(c)

No — the law always holds true for real chemical reactions. Discrepancies like trial 3 arise only from experimental error, not because mass is actually being created or destroyed.

QUESTION 13 4 marks Criterion C

Medium

In a school experiment, $2500\,\text{mg}$ of reactant X is fully reacted with $1.8\,\text{g}$ of reactant Y in a sealed tube. The total mass of products formed is measured as $4.3\,\text{g}$.

a. Convert $2500\,\text{mg}$ to grams.

[1]

b. Show, with a calculation, whether this data is consistent with the law of conservation of mass.

[3]

Show complete worked solution

(a)

$$ 2500\,\text{mg} = \textbf{2.5}\,\text{g} $$

(b)

$$ \text{mass of reactants} = 2.5 + 1.8 = 4.3\,\text{g} $$ This matches the measured mass of products ($4.3\,\text{g}$) exactly, so the data is consistent with the law of conservation of mass.

QUESTION 14 5 marks Criterion C

Medium

The graph shows the mass of an open flask (containing a reacting mixture) measured over time, as a gas is produced and escapes.

a. Calculate the total mass lost as gas by the time the reaction is complete.

[2]

b. At approximately what time does the graph show the reaction is complete?

[1]

c. Explain, using conservation of mass, why the $8\,\text{g}$ "lost" from the flask has not actually disappeared from existence.

[2]

Show complete worked solution

(a)

$$ 50 - 42 = \textbf{8}\,\text{g} $$

(b)

Around $t=80\,\text{s}$, since the mass becomes constant (the graph plateaus) from that point onward.

(c)

The $8\,\text{g}$ corresponds exactly to the mass of gas produced by the reaction that escaped from the open flask into the surrounding air. Conservation of mass means this gas still exists and still has mass — it has simply left the container being weighed, so the total mass of the flask and the escaped gas together is unchanged.

QUESTION 15 4 marks Criterion C

Medium

A student claims: "If I weigh a sealed flask before and after a reaction and the mass is exactly the same, this proves no chemical reaction happened." She tests this by reacting $10\,\text{g}$ of vinegar with $6\,\text{g}$ of baking soda in a sealed flask (fitted with a balloon to trap gas), and finds the mass is unchanged at $16.0\,\text{g}$ both before and after — even though she can see bubbles have filled the balloon.

a. Evaluate the student's claim, using her own observation and the law of conservation of mass.

[2]

b. Explain what the unchanged mass of $16.0\,\text{g}$ actually shows about this reaction.

[2]

Show complete worked solution

(a)

The student's claim is incorrect. She directly observed bubbles filling the balloon — clear visual evidence that a chemical reaction did occur (a gas, carbon dioxide, was produced). An unchanged total mass does not mean no reaction happened; it is exactly what conservation of mass predicts for a genuine reaction in a sealed system, since none of the matter (including the new gas) could escape to be lost from the measurement.

(b)

It confirms the reaction obeyed the law of conservation of mass: the total mass of the reactants ($10+6=16\,\text{g}$) equals the total mass of the products (the trapped gas plus the remaining liquid, still $16\,\text{g}$ combined), since the system was sealed and nothing entered or left.

QUESTION 16 7 marks Criterion C

Hard

A student burns $4.6\,\text{g}$ of a hydrocarbon fuel completely in a sealed container already containing $20.0\,\text{g}$ of oxygen gas. After combustion, the products are $12.4\,\text{g}$ of carbon dioxide, $6.9\,\text{g}$ of water vapour, and some unreacted oxygen gas.

a. Calculate the total mass inside the sealed container after the reaction, using conservation of mass.

[2]

b. Calculate the mass of oxygen gas that must have reacted.

[3]

c. Calculate the mass of oxygen gas remaining unreacted in the container.

[2]

Show complete worked solution

(a)

Mass before $= 4.6 + 20.0 = 24.6\,\text{g}$. Since the container is sealed, the total mass after the reaction must also be $\textbf{24.6}\,\text{g}$ (nothing enters or leaves).

(b)

Mass of products formed from the reacting fuel and oxygen $= 12.4 + 6.9 = 19.3\,\text{g}$. All $4.6\,\text{g}$ of the fuel reacted (complete combustion), so: $$ \text{mass of oxygen reacted} = 19.3 - 4.6 = \textbf{14.7}\,\text{g} $$

(c)

$$ 20.0 - 14.7 = \textbf{5.3}\,\text{g} $$ Check: $5.3\,(\text{O}_2) + 12.4\,(\text{CO}_2) + 6.9\,(\text{H}_2\text{O}) = 24.6\,\text{g}$, matching part (a).

QUESTION 17 8 marks Criterion C

Hard

Trial	Reactants mass (g)	Products mass (g), measured
1	20.0	20.3
2	25.0	25.3
3	30.0	30.4
4	18.0	18.2
5	22.0	22.3

A student weighs the reactants and products of the same type of reaction across five different trials.

a. Calculate the difference (products ? reactants) for each trial.

[2]

b. Explain what this consistent small positive difference across all five trials suggests about the source of error, as opposed to a random error.

[3]

c. Explain how the student could adjust her method to test whether her explanation for the error is correct.

[3]

Show complete worked solution

(a)

$0.3,\ 0.3,\ 0.4,\ 0.2,\ 0.3$ (all in g) — a consistent difference of roughly $+0.2$ to $+0.4\,\text{g}$ in every trial.

(b)

Because the difference is small and consistently positive in every single trial (rather than randomly scattered above and below zero), this points to a systematic error rather than a random one — most likely an uncalibrated balance that reads a fixed amount too high when weighing the products (for example, not being correctly zeroed/tared beforehand), rather than the law of conservation of mass genuinely failing.

(c)

She should carefully re-zero (tare) the balance with nothing on it immediately before every single measurement, and repeat all five trials again. If the systematic $+0.2$ to $+0.4\,\text{g}$ offset disappears and the products' masses now closely match the reactants' masses, this confirms an uncalibrated balance was the cause, rather than the law of conservation of mass being broken.

QUESTION 18 5 marks Criterion D

Medium

Chemical manufacturing plants use conservation of mass to carry out "mass-balance" calculations, precisely tracking how much of each raw material goes in and how much product (and waste) comes out, in order to minimise wasted material.

Discuss one benefit and one drawback of using conservation-of-mass calculations in this way.

Show complete worked solution

Benefit: By precisely tracking where every gram of raw material ends up, a factory can identify and reduce waste, improving efficiency, cutting costs, and reducing the amount of raw material that needs to be mined or extracted from the environment in the first place.

Drawback: Even a highly efficient, mass-balanced industrial process can still use enormous quantities of raw materials and energy at large scale, and the pursuit of efficiency and profit does not automatically guarantee that companies invest in safely disposing of unavoidable waste products or protecting workers — the underlying scale of industrial chemical production remains a significant resource and environmental concern.

QUESTION 19 5 marks Criterion D

Medium

Pharmaceutical (medicine) manufacturers rely on accurate mass-balance calculations, based on the law of conservation of mass, to make sure large batches of a drug are produced with the correct purity and dosage.

Discuss one benefit and one drawback related to this use of conservation of mass.

Show complete worked solution

Benefit: Careful mass-balance tracking helps ensure medicines are manufactured efficiently and, crucially, safely — confirming that the correct proportions of ingredients have reacted and that the final product contains the intended concentration of the active drug, protecting patients from receiving an incorrect or dangerously high or low dose.

Drawback: If any mass — such as unreacted starting material or an unwanted by-product — is not carefully accounted for using conservation of mass, this could go undetected and contaminate the medicine, or simply be released as chemical waste that must be safely and expensively disposed of, raising an environmental concern from large-scale drug manufacturing.

QUESTION 20 6 marks Criterion D

Hard

Because mass is always conserved, scientists can use the mass of fossil fuel burned to precisely calculate the mass of carbon dioxide released into the atmosphere, informing government climate policy and international emissions targets.

Evaluate the impact of this use of conservation of mass, discussing both a benefit and a concern.

Show complete worked solution

Benefit: Precise, science-based mass calculations allow governments and international bodies to accurately track how much carbon dioxide different countries and industries are actually emitting. This data underpins climate agreements and carbon-pricing policies, and gives clear, trustworthy targets that motivate investment in cleaner technology.

Concern: These same calculations reveal the true, sobering scale of the problem — billions of tonnes of $\text{CO}_2$ released globally every year — and raise difficult questions of fairness, since countries and companies differ hugely in their historical and current emissions. Accurately monitoring and enforcing mass-based emissions targets across every country and industry worldwide is also a major practical and political challenge.

QUESTION 1 2 marks Criterion A

Easy

Define the terms "exothermic" and "endothermic" in terms of energy transfer to or from the surroundings.

Show complete worked solution

Exothermic: a reaction that releases/transfers energy (heat) to the surroundings, so the temperature of the surroundings increases.

Endothermic: a reaction that absorbs/takes in energy (heat) from the surroundings, so the temperature of the surroundings decreases.

QUESTION 2 2 marks Criterion A

Easy

A student mixes two chemicals in a test tube, and the temperature of the mixture rises from $20\,^\circ\text{C}$ to $35\,^\circ\text{C}$.

Is this reaction exothermic or endothermic? Explain your answer.

Show complete worked solution

Exothermic. The temperature rose because the reaction released energy (heat) into the surrounding mixture, rather than absorbing it.

QUESTION 3 3 marks Criterion A

Easy

Classify each of the following reactions or processes as exothermic or endothermic.

a. Combustion (burning) of a fuel.

[1]

b. Photosynthesis in a green plant.

[2]

Show complete worked solution

(a)

Exothermic — burning releases heat and light energy.

(b)

Endothermic — photosynthesis absorbs light energy from the Sun to drive the reaction.

QUESTION 4 4 marks Criterion A

Medium

The graph shows the temperature of a reaction mixture over time, measured in an insulated cup.

a. Describe what happens to the temperature over the course of the reaction, using the graph.

[1]

b. Is this reaction exothermic or endothermic? Explain using the shape of the graph.

[2]

c. Suggest why the temperature eventually stops rising and levels off.

[1]

Show complete worked solution

(a)

The temperature rises steadily and quite steeply from $18\,^\circ\text{C}$ to a maximum of $40\,^\circ\text{C}$ (by around $t=80\,\text{s}$), then stays constant (flat) for the rest of the time shown.

(b)

Exothermic — the temperature increased, showing that the reaction released heat energy into the surrounding solution.

(c)

The reaction has finished (one of the reactants has been fully used up), so no more heat is being released; the flat section shows no further energy is being transferred to the surroundings.

QUESTION 5 4 marks Criterion A

Medium

The graph shows the temperature of water as solid ammonium nitrate dissolves into it, in an insulated cup.

a. Describe what happens to the temperature over the course of the process, using the graph.

[1]

b. Is this process exothermic or endothermic? Explain using the shape of the graph.

[2]

c. Name one everyday application that uses this type of temperature change.

[1]

Show complete worked solution

(a)

The temperature falls steadily from $20\,^\circ\text{C}$ to $8\,^\circ\text{C}$ (by around $t=60\,\text{s}$), then stays constant (flat) for the rest of the time shown.

(b)

Endothermic — the temperature dropped, showing that the process absorbed heat energy from the surrounding water (and thermometer) rather than releasing it.

(c)

Instant cold packs used to treat sports injuries (they contain ammonium nitrate and water kept separate until needed).

QUESTION 6 4 marks Criterion A

Medium

During any chemical reaction, bonds in the reactants must first break, and new bonds form to make the products.

a. State whether breaking bonds requires energy or releases energy.

[1]

b. State whether forming new bonds requires energy or releases energy.

[1]

c. Using this idea, explain why a reaction is exothermic overall if more energy is released forming new bonds than was needed to break the old ones.

[2]

Show complete worked solution

(a)

Breaking bonds requires (absorbs) energy.

(b)

Forming new bonds releases energy.

(c)

If the energy released when new bonds form is greater than the energy that had to be put in to break the old bonds, there is a net release of energy to the surroundings overall — this net outward energy transfer is exactly what makes a reaction exothermic.

QUESTION 7 5 marks Criterion A

Hard

Time (s)	0	10	20	30	40
Temperature (°C)	22	18	15	13	13

A student mixes solid citric acid with sodium bicarbonate solution and measures the temperature every 10 seconds, as shown.

a. Calculate the total temperature change over the course of the reaction.

[2]

b. State, with a reason, whether this reaction is exothermic or endothermic.

[1]

c. Explain, in terms of energy, why the surrounding solution feels cold to touch by the end of the reaction.

[2]

Show complete worked solution

(a)

$$ 22 - 13 = \textbf{9}\,^\circ\text{C decrease} $$

(b)

Endothermic — the temperature fell, showing the reaction absorbed thermal energy from the solution to proceed, rather than releasing it.

(c)

The reacting chemicals took in (absorbed) heat energy from the water/solution around them in order to react. Since that thermal energy has been removed from the surroundings, the surroundings are left with less thermal energy than before — which is why the solution's temperature, and how it feels to touch, decreases.

QUESTION 8 6 marks Criterion B

Medium

A student wants to investigate how the concentration of hydrochloric acid affects the temperature rise when it reacts with excess magnesium ribbon (an exothermic reaction).

a. State the independent and dependent variables.

[1]

b. State two variables that must be controlled, and explain why for one of them.

[2]

c. Describe a method, including equipment, for measuring the maximum temperature rise fairly for each concentration tested.

[3]

Show complete worked solution

(a)

Independent variable: concentration of hydrochloric acid (mol/dm³). Dependent variable: maximum temperature rise (°C).

(b)

Control the volume of acid used each time and the mass/length of magnesium ribbon (kept in excess). Controlling the volume of acid is important because a larger volume contains more particles able to react, and would release more total heat energy regardless of concentration, making the comparison between concentrations unfair.

(c)

Measure a fixed volume (e.g. $25\,\text{cm}^3$) of acid into an insulated polystyrene cup with a lid, insert a thermometer through the lid, and record the starting temperature. Add a fixed, excess length of magnesium ribbon, stir gently, and record the highest temperature reached. Calculate the temperature rise (maximum minus starting temperature), and repeat for each concentration, ideally 2–3 times, taking a mean.

QUESTION 9 6 marks Criterion B

Medium

A company wants to find the best insulating material to wrap around a chemical hand warmer (which reacts exothermically) to keep it warm for as long as possible.

a. State the independent and dependent variables.

[1]

b. State one variable that must be controlled, and explain why.

[2]

c. Describe how the student could compare the different insulating materials fairly, including what data would show which material is "best".

[3]

Show complete worked solution

(a)

Independent variable: type of insulating material (e.g. wool, bubble wrap, foam, or no insulation as a control). Dependent variable: temperature of the hand warmer over time (°C).

(b)

The starting mass/amount of the chemical reaction mixture used in each hand warmer must be controlled — a bigger reaction releases more total heat, which would make one material seem better at keeping warmth simply because it started with more heat, not because it insulates better.

(c)

Wrap identical hand warmers in an equal amount of each material, start all the reactions at the same time, and record the temperature of each with a thermometer (or data logger) every minute for $30$ minutes. Plot a temperature-time graph for each material — the material producing the flattest, most gradual temperature drop (staying above a useful temperature, e.g. $40\,^\circ\text{C}$, for the longest) is the best insulator.

QUESTION 10 8 marks Criterion B

Hard

Trial	1	2	3	4	5
Max temperature rise (°C), open beaker	12.0	11.5	8.0	12.2	11.8

A student measures the maximum temperature rise for the reaction of a dilute acid with a metal, repeating the trial five times in an open, uninsulated beaker.

a. Calculate the mean temperature rise, excluding the anomalous result.

[2]

b. Suggest why trial 3 gave a noticeably lower temperature rise than the others, even though the same chemicals and amounts were used.

[3]

c. Suggest a specific change to the equipment that would make results more reliable across repeats, and explain why.

[3]

Show complete worked solution

(a)

$$ \frac{12.0+11.5+12.2+11.8}{4} = \frac{47.5}{4} = \textbf{11.9}\,^\circ\text{C} \ (\text{1 d.p.}) $$

(b)

In trial 3, more heat was likely lost to the surroundings before the maximum temperature was recorded — for example, a delay in reading the thermometer, a draught in the room, or the mixture not being stirred (so the thermometer read a cooler, unmixed patch rather than the true maximum). Using an open, uninsulated beaker means heat loss is not tightly controlled, so it can vary unpredictably between repeats.

(c)

Carry out the reaction in an insulated polystyrene cup with a lid (with a small hole for the thermometer) instead of an open beaker. This reduces, and makes more consistent, the heat lost to the surrounding air during the reaction, so the maximum temperature reached each time should be closer to the true value and far more repeatable between trials.

QUESTION 11 3 marks Criterion C

Easy

Reaction	Starting temp (°C)	Final temp (°C)
1	20	32
2	20	11

Two separate reactions were carried out, and the temperature was recorded before and after each.

a. Calculate the temperature change ($\Delta T$) for each reaction.

[2]

b. Classify each reaction as exothermic or endothermic.

[1]

Show complete worked solution

(a)

Reaction 1: $32-20=+12\,^\circ\text{C}$. Reaction 2: $11-20=-9\,^\circ\text{C}$.

(b)

Reaction 1 is exothermic (temperature rose). Reaction 2 is endothermic (temperature fell).

QUESTION 12 4 marks Criterion C

Medium

Time (s)	0	30	60	90	120	150
Temperature (°C)	21	29	35	38	38	38

A student records the temperature of a reaction mixture in an insulated cup over time.

a. Calculate the total temperature change.

[2]

b. At what time did the reaction finish releasing heat? Explain how you know.

[1]

c. Classify the reaction.

[1]

Show complete worked solution

(a)

$$ 38 - 21 = \textbf{17}\,^\circ\text{C rise} $$

(b)

By $t=90\,\text{s}$ — from that point onward the temperature stays constant at $38\,^\circ\text{C}$ (a flat, zero-gradient section), showing no more heat is being released.

(c)

Exothermic.

QUESTION 13 5 marks Criterion C

Medium

Two trials reacted different masses of magnesium with excess dilute hydrochloric acid in an insulated cup: trial A used $1\,\text{g}$ of magnesium, and trial B used $2\,\text{g}$. The temperature was recorded over time for both, as shown.

a. Which trial released more total energy, A or B? Explain using the graph.

[2]

b. Explain, in terms of particles, why using more magnesium produces a bigger temperature rise.

[1]

c. Predict, with reasoning, what the maximum temperature would be for $3\,\text{g}$ of magnesium (assuming excess acid).

[2]

Show complete worked solution

(a)

Trial B released more energy — its temperature rise ($20\to44\,^\circ\text{C}$, $\Delta T=24\,^\circ\text{C}$) is much larger than trial A's ($20\to32\,^\circ\text{C}$, $\Delta T=12\,^\circ\text{C}$), and a larger temperature rise in the same volume of acid means more heat energy was transferred to the surroundings.

(b)

More magnesium means more magnesium atoms available to react (with acid in excess), so more chemical bonds are broken and reformed overall — releasing proportionally more total energy.

(c)

The temperature rise roughly doubles as the mass doubles ($12\,^\circ\text{C}$ for $1\,\text{g}$, $24\,^\circ\text{C}$ for $2\,\text{g}$), i.e. about $12\,^\circ\text{C}$ per gram. For $3\,\text{g}$: $\Delta T \approx 3\times12=36\,^\circ\text{C}$, giving a predicted maximum temperature of about $20+36=\textbf{56}\,^\circ\text{C}$ — though this pattern may not continue exactly if the acid starts to become the limiting reactant.

QUESTION 14 4 marks Criterion C

Medium

Salt dissolved	Starting temp (°C)	Final temp (°C)
Ammonium nitrate	21	9
Ammonium chloride	21	15
Sodium chloride	21	20

A student dissolves equal masses of three different salts in equal volumes of water, recording the temperature change for each.

a. Calculate $\Delta T$ for each salt.

[2]

b. Which salt's dissolving process is most strongly endothermic? Explain using your answers.

[2]

Show complete worked solution

(a)

Ammonium nitrate: $9-21=-12\,^\circ\text{C}$. Ammonium chloride: $15-21=-6\,^\circ\text{C}$. Sodium chloride: $20-21=-1\,^\circ\text{C}$.

(b)

Ammonium nitrate — it produced the largest temperature drop ($12\,^\circ\text{C}$), showing it absorbed the most heat energy from the water as it dissolved, making it the most strongly endothermic of the three.

QUESTION 15 4 marks Criterion C

Medium

A student adds solid sodium hydroxide to water in an insulated cup. The temperature rises from $19\,^\circ\text{C}$ to $31\,^\circ\text{C}$ over 60 seconds, then stays constant.

Using this data, explain what type of process this is, and describe the shape you would expect if this were plotted as a temperature-time graph.

Show complete worked solution

This is an exothermic process — dissolving sodium hydroxide in water releases heat energy into the surrounding water, causing the temperature to rise. Plotted as a temperature-time graph, the line would rise steadily/steeply from $(0\,\text{s}, 19\,^\circ\text{C})$ up to about $(60\,\text{s}, 31\,^\circ\text{C})$ as heat is released, then become a flat horizontal line from $60\,\text{s}$ onward, since no further heat is being released once the sodium hydroxide has fully dissolved.

QUESTION 16 7 marks Criterion C

Hard

Time (s)	0	20	40	60	80	100
Temperature (°C)	20	26	31	35	30	38

A student records the temperature of an exothermic reaction mixture over time in an insulated cup.

a. Identify the anomalous reading, explaining your reasoning based on the trend.

[2]

b. Suggest a likely cause of this anomalous reading.

[2]

c. Describe how the student should treat this anomalous point, and calculate the total temperature change for the reaction ignoring it.

[3]

Show complete worked solution

(a)

$t=80\,\text{s}$ ($30\,^\circ\text{C}$) is anomalous — the temperature should keep rising steadily, following the increasing pattern of the previous readings ($20,26,31,35$), but $30\,^\circ\text{C}$ is a drop rather than a further rise, before the temperature continues up to $38\,^\circ\text{C}$ at $t=100\,\text{s}$.

(b)

Most likely the thermometer was misread at that point, or read too early — before the mixture was fully swirled/mixed (giving a cooler pocket of liquid) — a momentary reading error, rather than the reaction actually cooling down and then reheating.

(c)

She should not force a smooth curve through this point — instead, she should draw a best-fit curve connecting the other five consistent points, and treat the anomalous point as an outlier to exclude (or re-measure). Total temperature change ignoring the anomaly: $$ 38 - 20 = \textbf{18}\,^\circ\text{C rise} $$

QUESTION 17 8 marks Criterion C

Hard

Reaction X (exothermic)	Trial 1	Trial 2	Trial 3	Trial 4	Trial 5
?T (°C)	14	15	13	15	28

Reaction Y (endothermic)	Trial 1	Trial 2	Trial 3	Trial 4	Trial 5
?T, magnitude of drop (°C)	6	7	6.5	7	6.5

A student repeats two different reactions five times each, recording the temperature change every time.

a. Calculate the mean $\Delta T$ for reaction X, first identifying and excluding any anomalous result, and for reaction Y (using all five results).

[3]

b. Compare the reliability of the two data sets, referring to the spread/consistency of the repeated values.

[2]

c. Using your calculated means, explain which reaction transferred more energy per trial, and state whether each is exothermic or endothermic.

[3]

Show complete worked solution

(a)

Reaction X: $28\,^\circ\text{C}$ is anomalous (roughly double the other four consistent results of $13$–$15\,^\circ\text{C}$). Mean of the remaining four: $$ \frac{14+15+13+15}{4}=\frac{57}{4}=14.25\,^\circ\text{C} $$ Reaction Y: $$ \frac{6+7+6.5+7+6.5}{5}=\frac{33}{5}=6.6\,^\circ\text{C} $$

(b)

Reaction Y's data is more reliable — its five values are tightly clustered (all between $6$ and $7\,^\circ\text{C}$, a spread of just $1\,^\circ\text{C}$), whereas reaction X's data (before removing the anomaly) is far more scattered, with one value ($28\,^\circ\text{C}$) well outside the range of the rest, suggesting reaction X's measurements were less controlled or repeatable.

(c)

Reaction X transferred more energy per trial on average (mean $\Delta T$ of $14.25\,^\circ\text{C}$, compared with Y's $6.6\,^\circ\text{C}$), since a larger temperature change in the same volume/mass of mixture corresponds to more energy transferred. Reaction X is exothermic (temperature rose, energy released to the surroundings), while reaction Y is endothermic (temperature fell, energy absorbed from the surroundings).

QUESTION 18 5 marks Criterion D

Medium

Instant cold packs use an endothermic reaction (dissolving ammonium nitrate in water) to provide quick cooling for sports injuries, without needing a freezer.

Discuss one benefit and one drawback of this technology.

Show complete worked solution

Benefit: Cold packs are portable and provide instant cooling without needing refrigeration or electricity, making them extremely useful for treating injuries quickly in remote locations, such as sports fields or while hiking, where ice is not available.

Drawback: Most instant cold packs are single-use only — once the endothermic reaction has taken place, they cannot be reused like a reusable ice pack. This creates ongoing plastic and chemical waste, especially if used frequently, and there is a risk if the inner pouch ruptures, exposing the chemicals to skin or eyes.

QUESTION 19 5 marks Criterion D

Medium

Self-heating food and drink cans use an exothermic reaction (such as calcium oxide reacting with water) to warm their contents without a stove or electricity.

Discuss one benefit and one drawback of this technology.

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Benefit: Self-heating cans provide convenient hot food or drinks without needing a stove, kettle, or electricity supply, making them especially useful for camping, disaster relief efforts, or military use where normal cooking facilities aren't available.

Drawback: These cans require more complex packaging with separate compartments for the chemicals and the food, increasing production cost and making the packaging harder to recycle than a standard can. There is also a safety concern if the packaging fails, since the exothermic reaction could cause burns.

QUESTION 20 6 marks Criterion D

Hard

Power stations rely on the exothermic combustion of fuels (such as coal or gas) to release large amounts of usable energy for generating electricity.

Evaluate the impact of using exothermic combustion reactions in this way, discussing both a benefit and a concern.

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Benefit: Harnessing the huge amount of energy released by exothermic combustion has allowed societies to generate electricity reliably on a massive scale, powering everything from lighting and hospitals to communication and industry — driving enormous improvements in quality of life and technology over the last two centuries.

Concern: Burning fossil fuels to release this energy also releases large quantities of carbon dioxide, a greenhouse gas that is a major driver of climate change, along with other pollutants that can harm air quality and human health. This shows that while exothermic reactions provide essential energy, relying on combustion of fossil fuels for it carries serious long-term environmental costs.

Signs of a Chemical Reaction 20 questions

Word Equations 20 questions

Conservation of Mass 20 questions

Exothermic and Endothermic Reactions 20 questions