Acids, Bases and Salts

QUESTION 1 2 marks Criterion A

Easy

The diagram shows the pH scale.

a. State the pH value that is exactly neutral.

[1]

b. State whether a solution of pH 2 is acidic, neutral, or alkaline.

[1]

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(a)

A pH of 7 is exactly neutral.

(b)

pH 2 is well below 7, so the solution is acidic — in fact, strongly acidic.

QUESTION 2 2 marks Criterion A

Easy

Universal indicator is added to a colourless solution and it turns dark green (a shade between the pure green of neutral and the blue of a moderate alkali).

State the approximate pH of the solution, and whether it is acidic, neutral, or alkaline.

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Universal indicator is green at pH 7 (neutral) and shades towards blue as the pH rises above 7. A dark green colour — between pure green and blue — corresponds to a pH of approximately 8. The solution is therefore weakly alkaline (just above neutral).

QUESTION 3 2 marks Criterion A

Easy

The arrow on the pH scale below marks the pH of Substance X.

a. Is Substance X acidic, neutral, or alkaline?

[1]

b. Would universal indicator turn deep red, orange/yellow, or blue with Substance X? Explain your choice.

[1]

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(a)

Acidic — its pH (4) is below 7.

(b)

Orange/yellow. A pH of 4 is only mildly acidic (not close to pH 0–1), so universal indicator would not turn the deep red seen with a strong acid; it would show the milder orange-yellow colour associated with pH 3–6.

QUESTION 4 3 marks Criterion A

Medium

State the pH range (or value) classed as acidic, neutral, and alkaline, and give one everyday example substance for each.

a. State the pH range for acidic solutions and give an example.

[1]

b. State the pH value for neutral solutions and give an example.

[1]

c. State the pH range for alkaline solutions and give an example.

[1]

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(a)

pH 0–6 (below 7). Example: vinegar (pH ? 3).

(b)

pH = 7 exactly. Example: pure water.

(c)

pH 8–14 (above 7). Example: soap solution (pH ? 9–10).

QUESTION 5 4 marks Criterion A

Medium

Litmus paper and universal indicator are both used to test acidity and alkalinity, but they are not equally useful.

a. Describe what litmus paper shows you about a solution, and state its main limitation.

[2]

b. Explain how universal indicator overcomes this limitation.

[2]

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(a)

Litmus paper only tells you whether a solution is acidic or alkaline: it turns red in acid and blue in alkali (staying purple in neutral solutions). Its limitation is that it cannot tell you how strongly acidic or alkaline a solution is — it gives no pH number, only a two-way acid/alkali result.

(b)

Universal indicator is a mixture of several dyes, so it produces a whole range of distinct colours (red ? orange ? yellow ? green ? blue ? purple) across pH 0–14, instead of just two colours. Matching the colour produced to a reference chart lets you estimate the actual approximate pH value, not just “acid” or “alkali”.

QUESTION 6 3 marks Criterion A

Medium

Explain, in terms of hydrogen ions ($H^+$), why a solution of pH 1 is more strongly acidic than a solution of pH 5.

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The pH scale measures the concentration of dissolved hydrogen ions ($H^+$) in a solution: the higher the concentration of $H^+$ ions, the lower the pH value. A solution of pH 1 has a much greater concentration of $H^+$ ions than a solution of pH 5 — each whole step down the pH scale represents roughly a ten-times increase in $H^+$ concentration — so a pH 1 solution is far more strongly acidic than a pH 5 solution.

QUESTION 7 5 marks Criterion A

Hard

Indicator	Colour in acid	Colour-change range	Colour in alkali
Litmus	Red	around pH 7	Blue
Methyl orange	Red	pH 3.1–4.4 (red ? yellow)	Yellow
Phenolphthalein	Colourless	pH 8.2–10.0 (colourless ? pink)	Pink/magenta

Use the table to answer the questions below about a solution of pH 6.

a. What colour would litmus show in this solution?

[1]

b. What colour would methyl orange show in this solution?

[2]

c. Explain why methyl orange would not be useful for distinguishing a solution of pH 6 from a solution of pH 7, whereas universal indicator would.

[2]

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(a)

Red — pH 6 is below pH 7 (acidic), and litmus turns red in any acidic solution regardless of exactly how acidic it is.

(b)

Yellow — pH 6 is above methyl orange's change range (3.1–4.4), so the indicator has already fully changed to its alkali-side colour, yellow, even though the solution itself is still weakly acidic.

(c)

Methyl orange has already fully changed colour (to yellow) by pH 4.4, so both a pH 6 solution and a pH 7 solution would look identical (yellow) with methyl orange — it gives no further information above pH 4.4. Universal indicator, by contrast, uses a blend of dyes covering the whole 0–14 range, so pH 6 (yellow-green) and pH 7 (green) produce visibly different colours, allowing the two to be told apart.

QUESTION 8 6 marks Criterion B

Medium

A student wants to investigate the pH of five household liquids — lemon juice, milk, tap water, soap solution, and oven cleaner — using universal indicator solution and a colour chart.

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

[2]

b. State two variables the student should control to make the test fair, and explain why for one of them.

[2]

c. Describe a method for carrying out this investigation.

[2]

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(a)

Independent variable: the household liquid being tested (lemon juice, milk, tap water, soap solution, oven cleaner). Dependent variable: the pH value of each liquid, read from the colour produced with universal indicator.

(b)

Control the volume of liquid tested each time (e.g. always $2\,\text{cm}^3$) and the volume of universal indicator added (e.g. always 2 drops). Why control the volume of indicator: adding a different number of drops each time could make the resulting colour more or less intense/diluted, making it harder to match accurately to the colour chart — even for two liquids with the same true pH.

(c)

Using a clean pipette, place $2\,\text{cm}^3$ of each liquid into its own labelled test tube.
Add 2–3 drops of universal indicator solution to each tube.
Gently swirl each tube to mix.
Compare the colour produced to a standard universal indicator colour chart and record the pH value.
Rinse all equipment thoroughly between different liquids to avoid contamination.

QUESTION 9 7 marks Criterion B

Medium

Red cabbage juice can be used as a natural acid–base indicator: it is purple in neutral solutions, turns pink/red in acids, and turns green/yellow in alkalis. A student wants to test whether red cabbage indicator correctly classifies five unknown solutions as acidic, neutral, or alkaline, compared with universal indicator.

a. State a suitable hypothesis for this investigation.

[2]

b. Describe a method to test this hypothesis fairly.

[3]

c. Identify one variable that must be controlled in this investigation, and explain why.

[2]

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(a)

Red cabbage indicator will classify each unknown solution as acidic, neutral, or alkaline in the same way as universal indicator — i.e. it will turn pink/red for any solution universal indicator shows as acidic, purple for any solution shown as neutral, and green/yellow for any solution shown as alkaline.

(b)

Prepare red cabbage indicator by boiling chopped red cabbage in water, then cooling and filtering the purple liquid. Label five test tubes A–E, one for each unknown solution, and add an equal volume (e.g. $2\,\text{cm}^3$) of each unknown to its tube. Add an equal number of drops of cabbage indicator to each tube and record the colour observed. Separately, test the same five unknowns with universal indicator and a colour chart to obtain a “true” pH classification for each, then compare the two sets of results.

(c)

The volume of unknown solution used in each tube must be controlled (kept the same, e.g. $2\,\text{cm}^3$). If different volumes were used, the intensity of the colour produced could vary for reasons unrelated to the solution's actual pH, making the colours harder to compare fairly and possibly leading to an incorrect conclusion about whether cabbage indicator matches universal indicator.

QUESTION 10 8 marks Criterion B

Hard

A student tests the pH of four solutions using a single strip of pH indicator paper. She dips the same strip into Solution A and records the colour, then — without rinsing it or using a new strip — dips the same strip into Solution B, then C, then D, recording a colour each time.

a. Identify the flaw in this method and explain the effect it could have on the results.

[3]

b. Suggest an improved method, explaining how it removes this problem.

[3]

c. Suggest one further improvement to increase the reliability of the results, other than using a fresh strip each time.

[2]

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(a)

The flaw is re-using the same indicator strip for every solution without rinsing or replacing it. Because the strip is not clean between tests, traces of the previous solution remain on it, so each new colour reading is really a mixture of the current solution and left-over residue from earlier ones. This could shift each reading towards the pH of whichever solution was tested just before it — for example, if Solution A is strongly acidic, its residue could make Solution B's reading appear more acidic than it truly is.

(b)

Use a fresh, unused strip of indicator paper for each solution, dipping each new strip only into its own solution (and never re-dipping a used strip). This ensures every reading reflects only the pH of the solution actually being tested, with no contamination carried over from a previous test.

(c)

Repeat the test for each solution two or three times (using a fresh strip for each repeat) and take the most common (or average) colour/pH reading, rather than relying on a single reading — this reduces the effect of any one anomalous reading or misjudged colour match.

QUESTION 11 3 marks Criterion C

Easy

Substance	Lemon juice	Milk	Pure water	Baking soda solution	Oven cleaner
pH value	2	6.5	7	9	13

Use the table to answer the questions below.

a. Which substance is the most acidic?

[1]

b. Which substance is the most alkaline?

[1]

c. Which substance, other than pure water itself, is closest to neutral?

[1]

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(a)

Lemon juice (pH 2) — it has the lowest pH value in the table.

(b)

Oven cleaner (pH 13) — it has the highest pH value in the table.

(c)

Milk (pH 6.5) — of the remaining substances, its pH is closest to 7.

QUESTION 12 4 marks Criterion C

Medium

Trial	1	2	3	4	5
pH reading	6.9	7.0	7.1	6.9	9.8

A student measures the pH of the same sample of tap water five times using a pH meter.

a. Identify the anomalous reading.

[1]

b. Explain how you identified it.

[2]

c. Calculate the mean pH, excluding the anomalous reading.

[1]

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(a)

Trial 5 (pH 9.8) is anomalous.

(b)

The other four readings are all close together, between 6.9 and 7.1 (a spread of only 0.2), consistent with tap water being close to neutral. Trial 5, at 9.8, is far outside this tight cluster and inconsistent with the pattern of the other readings — likely caused by a faulty probe reading, or the meter not being rinsed/calibrated properly before that reading.

(c)

$$ \text{mean} = \frac{6.9+7.0+7.1+6.9}{4} = \frac{27.9}{4} = 6.975 \approx 6.98 $$

QUESTION 13 5 marks Criterion C

Medium

The diagram shows the approximate pH of two unknown solutions, P and Q, found using universal indicator.

a. State whether Solution P is acidic, neutral, or alkaline.

[1]

b. State whether Solution Q is acidic, neutral, or alkaline.

[1]

c. A third solution, R, is exactly halfway between P and Q on the pH scale. State its pH value, and explain whether this means Solution R is “half as acidic” as Solution P.

[3]

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(a)

Acidic (pH 3, below 7).

(b)

Alkaline (pH 11, above 7).

(c)

Halfway between pH 3 and pH 11 is $$ \frac{3+11}{2} = 7 $$ so Solution R has pH 7. This does not mean R is “half as acidic” as P — pH 7 is neutral, not acidic at all. Because each step of 1 on the pH scale represents roughly a ten-times change in $H^+$ ion concentration, being “halfway” in pH units does not correspond to being “half” as concentrated or acidic; R is simply a different type of solution (neutral) rather than a weaker acid.

QUESTION 14 4 marks Criterion C

Medium

Day	Mon	Tue	Wed	Thu	Fri
Rainwater pH	5.6	5.5	5.7	4.1	5.6

A student collects and tests a sample of rainwater each day for a week, near a factory. Normal, unpolluted rainwater is naturally slightly acidic, at about pH 5.6, due to dissolved carbon dioxide.

a. Identify the day with an anomalous reading.

[1]

b. Suggest a reason for this anomalous reading.

[2]

c. Calculate the mean pH for the week, excluding the anomalous reading.

[1]

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(a)

Thursday (pH 4.1).

(b)

The much lower pH suggests that additional acidic pollutant gases (such as sulfur dioxide or nitrogen oxides — for example from the nearby factory burning fossil fuels) dissolved in the rainwater that day, forming a stronger acid than the usual carbon-dioxide-only rainwater. This is sometimes called acid rain.

(c)

$$ \text{mean} = \frac{5.6+5.5+5.7+5.6}{4} = \frac{22.4}{4} = 5.6 $$

QUESTION 15 5 marks Criterion C

Medium

Colour	Red	Orange	Yellow	Green	Blue	Purple
Approx. pH range	0–2	3–4	5–6	7	8–11	12–14

Solution	W	X	Y	Z
Colour observed	Red	Green	Blue	Orange

Four unknown solutions, W, X, Y, and Z, were tested with universal indicator, giving the colours shown. Use the reference chart to classify each solution.

a. State the approximate pH range and acid/alkali classification for Solution W and Solution Y.

[2]

b. State the approximate pH range and classification for Solution X and Solution Z.

[2]

c. Which of the four solutions is closest to being a strong alkali?

[1]

Show complete worked solution

(a)

W (red): approx. pH 0–2 — strongly acidic. Y (blue): approx. pH 8–11 — alkaline.

(b)

X (green): pH 7 — neutral. Z (orange): approx. pH 3–4 — weakly/moderately acidic.

(c)

Solution Y — it is the most alkaline of the four (blue, pH 8–11), although none of the four reach the strong-alkali (purple, pH 12–14) range.

QUESTION 16 6 marks Criterion C

Hard

Trial	1	2	3	4
Meter A (uncalibrated) pH	5.1	6.8	4.5	7.9
Meter B (calibrated) pH	6.5	6.6	6.4	6.5

Two pH meters were used to measure the pH of the same buffer solution (true pH $=6.5$) four times each.

a. Calculate the mean pH reading for each meter.

[2]

b. Comment on the reliability of Meter A compared with Meter B, using the spread of the readings.

[2]

c. Given that the buffer solution's true pH is 6.5, evaluate the accuracy of each meter's mean reading.

[2]

Show complete worked solution

(a)

$$ \text{Meter A mean} = \frac{5.1+6.8+4.5+7.9}{4} = \frac{24.3}{4} = 6.08 \ (\text{3 s.f.}) $$$$ \text{Meter B mean} = \frac{6.5+6.6+6.4+6.5}{4} = \frac{26.0}{4} = 6.5 $$

(b)

Meter A's readings are widely spread (from 4.5 to 7.9, a range of 3.4), showing it is unreliable — repeated measurements of the same solution give very different results. Meter B's readings are tightly clustered (6.4 to 6.6, a range of only 0.2), showing it is far more reliable, giving consistent results each time.

(c)

Meter B's mean (6.5) exactly matches the true pH, so it is both accurate and reliable — it was likely properly calibrated before use. Meter A's mean (6.08) happens to be reasonably close to 6.5, but this is partly coincidental, since its large scatter means individual readings partly cancel out when averaged. Meter A cannot be trusted for any single reading, since one reading alone could be far from the true value — it is unreliable even though its mean is roughly accurate.

QUESTION 17 7 marks Criterion C

Hard

Dilution	Undiluted	1:1	1:3	1:7	1:15
Relative acid concentration	1	0.5	0.25	0.125	0.0625
Measured pH	1.0	1.3	1.6	1.9	2.2

A student repeatedly dilutes a strong acid (each dilution step roughly halves the acid's concentration) and measures the pH after each dilution.

a. Describe the trend shown by the data.

[2]

b. Predict the pH after one further dilution step (halving the concentration again, to 0.03125), explaining your reasoning.

[3]

c. Explain why, however many times the acid is diluted, its pH will never reach or go above pH 7.

[2]

Show complete worked solution

(a)

As the acid is diluted (its concentration halved each time), the measured pH steadily increases — from pH 1.0 when undiluted, rising by roughly 0.3 with each halving of concentration, up to pH 2.2 at the highest dilution tested.

(b)

Following the pattern in the table, pH should rise by roughly another 0.3, giving a predicted pH of about 2.5. This prediction is based on the consistent trend across the previous four data points, where pH increased by approximately 0.3 for each halving of the acid's concentration.

(c)

Diluting an acid only reduces the concentration of $H^+$ ions already present with more water — it never converts the acid into a base or introduces new $OH^-$ ions. As dilution continues, the pH gets closer and closer to 7 (the pH of the water it is being diluted in), but can only approach neutral, never cross past it to become alkaline, since dilution can only ever reduce excess acidity toward that of pure water, not reverse it.

QUESTION 18 4 marks Criterion D

Medium

Swimming pool water is regularly tested with pH indicator kits (or automatic pH sensors), and chemicals are added to keep it within a safe range, typically pH 7.2–7.8.

Discuss one benefit and one drawback of this regular pH testing and adjustment.

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Benefit: Keeping pool water within the safe pH range protects swimmers' health and comfort — water that is too acidic can irritate the eyes and skin and corrode pool equipment, while water that is too alkaline can cause cloudy water and reduce how effectively chlorine disinfects it, allowing harmful bacteria to survive. Regular testing catches pH drift early, before it becomes a health risk.

Drawback: Regular testing and adding chemicals (acid or alkali) to correct the pH costs money and staff time, and requires careful, trained handling, since concentrated pool chemicals are themselves corrosive and hazardous if mishandled or mixed incorrectly. Smaller facilities without the budget for frequent testing risk unsafe water going unnoticed.

QUESTION 19 5 marks Criterion D

Medium

Farmers often test the pH of their soil using indicator kits before planting crops, since most crops grow best within a specific pH range, and add lime (calcium compounds) to correct soil that is too acidic.

Discuss one benefit and one drawback of routinely testing and adjusting soil pH.

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Benefit: Testing soil pH lets farmers identify problems before planting and calculate exactly how much lime (or other treatment) is needed, rather than guessing. Correcting an over-acidic soil to the right pH range improves how easily plant roots can absorb nutrients, which can significantly increase crop yield — supporting food production and farmer income.

Drawback: Soil testing kits and lime treatments cost money and time, which can be a barrier for smaller or lower-income farms, and adding too much lime can overcorrect the soil, making it too alkaline for the intended crop and requiring further correction. Large-scale liming can also affect nearby watercourses if washed off the land by rain, altering the pH of streams and ponds and affecting the organisms living there.

QUESTION 20 6 marks Criterion D

Hard

Environmental agencies increasingly use automatic pH sensors and data loggers placed in rivers to continuously record pH, rather than relying only on occasional manual testing.

Evaluate the impact of this technology, discussing both a benefit and a concern it raises.

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Benefit: Continuous automatic monitoring can detect a sudden drop in river pH — for example from an accidental chemical spill or illegal discharge — within minutes rather than days, allowing a much faster emergency response to protect fish and other aquatic life, which are highly sensitive to rapid pH changes. It also builds up a far more complete long-term record of a river's health than occasional manual sampling ever could.

Concern: The sensors and supporting data infrastructure are expensive to install and maintain across many rivers, meaning smaller or less-funded regions may still be left without this protection, creating unequal environmental monitoring. There is also a risk of over-reliance on automated data — a faulty or poorly maintained sensor could give a false sense of security by failing to flag a real pollution event, so automatic monitoring still needs to be checked and supplemented by regular manual verification.

QUESTION 1 2 marks Criterion A

Easy

State the general word equation for a neutralisation reaction.

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

(An alkali is simply a base that is soluble in water, so this equation also covers acid + alkali reactions.)

QUESTION 2 2 marks Criterion A

Easy

Hydrochloric acid reacts with sodium hydroxide solution.

a. Name the salt produced.

[1]

b. Name the other product of the reaction.

[1]

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(a)

Sodium chloride.

(b)

Water.

QUESTION 3 2 marks Criterion A

Easy

State the two products formed when an acid reacts with a metal carbonate, in addition to a salt.

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$$ \text{acid} + \text{metal carbonate} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide} $$

So, in addition to the salt, the two other products are water and carbon dioxide gas ($CO_2$).

QUESTION 4 4 marks Criterion A

Medium

Metal oxides are bases. When an acid reacts with a metal oxide, a salt and water are formed, just as with a metal hydroxide.

a. Write the word equation for the reaction between sulfuric acid and copper oxide, and name the salt formed.

[2]

b. Write the word equation for the reaction between nitric acid and magnesium oxide, and name the salt formed.

[2]

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(a)

$$ \text{sulfuric acid} + \text{copper oxide} \rightarrow \text{copper sulfate} + \text{water} $$The salt formed is copper sulfate.

(b)

$$ \text{nitric acid} + \text{magnesium oxide} \rightarrow \text{magnesium nitrate} + \text{water} $$The salt formed is magnesium nitrate.

QUESTION 5 4 marks Criterion A

Medium

The name of the salt produced in a neutralisation reaction depends on which acid is used: hydrochloric acid produces chlorides, sulfuric acid produces sulfates, and nitric acid produces nitrates.

a. Name the salt formed when hydrochloric acid reacts with potassium hydroxide.

[1]

b. Name the salt formed when sulfuric acid reacts with sodium hydroxide.

[1]

c. Name the salt formed when nitric acid reacts with calcium carbonate, and state what other two products form alongside it.

[2]

Show complete worked solution

(a)

Potassium chloride.

(b)

Sodium sulfate.

(c)

The salt is calcium nitrate. The other products are water and carbon dioxide gas (since the base here is a carbonate, not just an oxide/hydroxide).

QUESTION 6 5 marks Criterion A

Medium

Symbol equations for neutralisation must be balanced — the same number of each type of atom must appear on both sides.

a. Write the balanced symbol equation for hydrochloric acid reacting with sodium hydroxide.

[2]

b. Sulfuric acid, $H_2SO_4$, reacts with sodium hydroxide, $NaOH$, to form sodium sulfate, $Na_2SO_4$, and water. Write and balance the full symbol equation.

[3]

Show complete worked solution

(a)

$$ HCl + NaOH \rightarrow NaCl + H_2O $$This equation is already balanced: 1 $H$, 1 $Cl$, 1 $Na$, 1 $O$ on each side.

(b)

Sodium sulfate contains 2 sodium atoms, so 2 $NaOH$ are needed, which in turn produce 2 $H_2O$:$$ H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O $$Check: left side has 4 $H$ (2 from $H_2SO_4$ + 2 from $2NaOH$), 1 $S$, 6 $O$ (4 + 2), 2 $Na$; right side has 2 $Na$, 1 $S$, 4 $O$ (from sulfate) + 2 $O$ (from $2H_2O$) $=6\,O$, and 4 $H$ (from $2H_2O$). Balanced.

QUESTION 7 6 marks Criterion A

Hard

Hydrochloric acid reacts with calcium carbonate (found in limestone/marble chips) to produce calcium chloride, water, and carbon dioxide gas.

a. Write and balance the full symbol equation for this reaction. (Calcium chloride is $CaCl_2$.)

[3]

b. Describe a simple chemical test to confirm that the gas produced is carbon dioxide, including the expected positive result.

[3]

Show complete worked solution

(a)

Since calcium chloride, $CaCl_2$, contains 2 chlorine atoms, 2 $HCl$ are needed:$$ 2HCl + CaCO_3 \rightarrow CaCl_2 + H_2O + CO_2 $$Check: left side has 2 $H$, 2 $Cl$, 1 $Ca$, 1 $C$, 3 $O$; right side has 2 $Cl$, 1 $Ca$ (from $CaCl_2$), 2 $H$ + 1 $O$ (from $H_2O$), 1 $C$ + 2 $O$ (from $CO_2$) — that's 1 $C$ and $1+2=3\,O$ in total. Balanced.

(b)

Bubble the gas produced through limewater (a colourless solution of calcium hydroxide). If the gas is carbon dioxide, the limewater will turn cloudy/milky — this is the standard positive test for $CO_2$, caused by the formation of insoluble calcium carbonate.

QUESTION 8 6 marks Criterion B

Medium

A student wants to find the volume of dilute hydrochloric acid needed to exactly neutralise $25\,\text{cm}^3$ of sodium hydroxide solution, using an indicator to show when neutralisation is complete.

a. State the independent and dependent variables.

[2]

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

[2]

c. Describe a method for this investigation, including the equipment used.

[2]

Show complete worked solution

(a)

Independent variable: the volume of hydrochloric acid added. Dependent variable: the colour of the indicator (used to judge when neutralisation is reached, and hence the volume of acid needed).

(b)

Control the volume of sodium hydroxide solution used each time (e.g. always $25\,\text{cm}^3$) and its concentration. Why control the volume: using a different starting volume of alkali would need a different volume of acid to neutralise it even at the same concentrations, making results from different trials impossible to compare fairly.

(c)

Using a pipette, measure $25\,\text{cm}^3$ of sodium hydroxide solution into a conical flask and add a few drops of indicator (e.g. phenolphthalein).
Fill a burette with dilute hydrochloric acid and record the starting volume.
Add the acid to the flask slowly, swirling constantly, and add it drop by drop as the colour starts to change.
Stop as soon as the indicator changes colour permanently (the end point), and record the final burette reading.
Calculate the volume of acid added (final reading ? starting reading).

QUESTION 9 7 marks Criterion B

Medium

Neutralisation reactions are exothermic (they release heat energy). A student wants to investigate the temperature rise produced when a fixed volume of hydrochloric acid is neutralised by sodium hydroxide solution.

a. State the independent and dependent variables.

[2]

b. Describe a method to measure the temperature rise accurately and fairly.

[3]

c. Explain why the reaction mixture should be insulated (e.g. using a polystyrene cup with a lid) during this investigation.

[2]

Show complete worked solution

(a)

Independent variable: (for example) the concentration of the sodium hydroxide solution used. Dependent variable: the maximum temperature rise of the mixture.

(b)

Measure a fixed volume of hydrochloric acid (e.g. $25\,\text{cm}^3$) into an insulated cup (e.g. a polystyrene cup) and record its starting temperature with a thermometer. Add a fixed volume of sodium hydroxide solution, stir continuously, and record the highest temperature reached. Repeat for each concentration of sodium hydroxide being tested, keeping the acid volume, acid concentration, and alkali volume the same each time.

(c)

Insulating the mixture reduces heat energy loss to the surroundings. Without insulation, some of the heat released by the neutralisation reaction would escape into the air before the maximum temperature could be measured, making the recorded temperature rise lower than the true amount of heat actually released — reducing the accuracy of the results.

QUESTION 10 8 marks Criterion B

Hard

Trial	1	2	3
Titre volume (cm³)	18.4	24.9	17.6

A student carries out a titration to find the volume of acid needed to neutralise a fixed volume of alkali. She does not swirl the conical flask while adding acid from the burette, adds the acid quickly rather than dropwise near the end point, and does not repeat the titration to check for consistency (the table above shows all three attempts she happened to make regardless).

a. Identify two flaws in this method and explain the effect each would have on the results.

[3]

b. Explain why the titre volumes in the table are not reliable, and describe how the method should be improved.

[3]

c. Explain how the student should use a set of concordant results to calculate her final answer.

[2]

Show complete worked solution

(a)

Not swirling the flask: the acid and alkali would not mix thoroughly and instantly as it is added, so the indicator could show a local colour change near where the acid enters before the whole solution has actually reached the end point — causing the student to stop too early (an under-estimated titre). Adding the acid quickly rather than dropwise near the end point: it becomes very easy to overshoot the true neutralisation point, adding more acid than actually needed and recording a titre that is too high.

(b)

The three titre volumes (18.4, 24.9, and 17.6 $\text{cm}^3$) vary far too much to be trusted — reliable titration results should be “concordant”, meaning repeat titres agree closely (typically within $0.1$–$0.2\,\text{cm}^3$ of each other). The method should be improved by swirling constantly, adding acid dropwise once the colour starts to change, and repeating the titration until at least two concordant results are obtained.

(c)

She should take the mean of the concordant titres only (the readings that agree closely with each other), discarding any rough or clearly inconsistent trial, since this gives the most reliable estimate of the true volume of acid needed.

QUESTION 11 3 marks Criterion C

Easy

Volume of NaOH added (cm³)	0	5	10	15	20
Indicator colour	Red	Red	Red	Green	Purple

A student adds sodium hydroxide solution, drop by drop, to a fixed volume of hydrochloric acid containing universal indicator, and records the colour after adding each $5\,\text{cm}^3$.

a. Between which two volumes does neutralisation occur?

[1]

b. Explain how the colour data shows this.

[2]

Show complete worked solution

(a)

Between $10\,\text{cm}^3$ and $15\,\text{cm}^3$ of sodium hydroxide added.

(b)

The indicator is red (acidic) up to $10\,\text{cm}^3$, then green by $15\,\text{cm}^3$. Since universal indicator turns green at pH 7 (neutral), the solution must have passed through neutral somewhere in the $10$–$15\,\text{cm}^3$ range — after this point the mixture continues turning purple (alkaline) as more sodium hydroxide is added beyond the amount needed for neutralisation.

QUESTION 12 4 marks Criterion C

Medium

Trial	1	2	3
Titre volume (cm³)	22.5	22.4	22.6

A student titrates hydrochloric acid against $25\,\text{cm}^3$ of sodium hydroxide solution three times.

a. State whether these three results are concordant (in close agreement).

[1]

b. Calculate the mean titre volume.

[2]

c. Explain why performing the titration three times, rather than once, improves the reliability of the result.

[1]

Show complete worked solution

(a)

Yes — all three titres are within $0.2\,\text{cm}^3$ of each other, so they are concordant.

(b)

$$ \text{mean} = \frac{22.5+22.4+22.6}{3} = \frac{67.5}{3} = 22.5\,\text{cm}^3 $$

(c)

Repeating the titration allows anomalous results to be identified and lets a mean be calculated from consistent (concordant) readings, reducing the effect of any small errors in judging the exact end point in a single trial.

QUESTION 13 5 marks Criterion C

Medium

The chart shows the temperature rise recorded when $25\,\text{cm}^3$ of three different acids, each of the same concentration, were neutralised with $25\,\text{cm}^3$ of sodium hydroxide solution of the same concentration.

a. Which acid–alkali pair released the most heat energy?

[1]

b. Compare the temperature rise for hydrochloric acid and sulfuric acid with sodium hydroxide.

[2]

c. Suggest a reason why ethanoic acid ($CH_3COOH$, a weak acid) produces a noticeably smaller temperature rise than the two strong acids.

[2]

Show complete worked solution

(a)

$H_2SO_4 + NaOH$ — it produced the largest temperature rise ($8.6\,°C$).

(b)

The temperature rises for $HCl + NaOH$ ($8.5\,°C$) and $H_2SO_4 + NaOH$ ($8.6\,°C$) are very close — only $0.1\,°C$ apart — showing that neutralising a strong acid with a strong alkali releases a very similar amount of heat energy, regardless of which strong acid is used.

(c)

Ethanoic acid is a weak acid, meaning it only partially splits up (ionises) into $H^+$ ions in solution, unlike the strong acids $HCl$ and $H_2SO_4$, which ionise almost completely. Because fewer free $H^+$ ions are available to react at any one time, and some energy is used ionising the remaining acid molecules as the reaction proceeds, less heat energy is released overall, giving a smaller temperature rise.

QUESTION 14 5 marks Criterion C

Medium

Mass of calcium carbonate used (g)	1	2	3	4
Mass of $CO_2$ gas produced (g)	0.44	0.88	1.32	1.32

A student reacts increasing masses of calcium carbonate with a fixed volume and concentration of excess dilute hydrochloric acid each time, and measures the mass of carbon dioxide gas produced.

a. Describe the pattern shown between 1 g and 3 g of calcium carbonate.

[2]

b. State the independent variable in this investigation.

[1]

c. Explain why the mass of $CO_2$ produced does not increase further between 3 g and 4 g of calcium carbonate.

[2]

Show complete worked solution

(a)

Between 1 g and 3 g of calcium carbonate, the mass of $CO_2$ produced increases proportionally — it doubles from $0.44\,\text{g}$ to $0.88\,\text{g}$ when the carbonate mass doubles from 1 g to 2 g, and reaches $1.32\,\text{g}$ (three times $0.44$) at 3 g.

(b)

The mass of calcium carbonate used.

(c)

By 3 g of calcium carbonate, all of the fixed amount of hydrochloric acid has been used up — the acid is now the limiting reactant. Adding a 4th gram of calcium carbonate has no more acid left to react with, so no additional $CO_2$ can be produced, and the mass levels off at $1.32\,\text{g}$.

QUESTION 15 4 marks Criterion C

Medium

Trial	1	2	3
Titre volume (cm³)	15.2	15.3	19.8

A student's titration results are shown above.

a. Identify the anomalous result.

[1]

b. Explain how you identified it.

[1]

c. Calculate the best estimate of the true titre, using only the concordant results.

[2]

Show complete worked solution

(a)

Trial 3 ($19.8\,\text{cm}^3$) is anomalous.

(b)

Trials 1 and 2 agree closely ($15.2$ and $15.3\,\text{cm}^3$, within $0.1\,\text{cm}^3$), but Trial 3 is over $4\,\text{cm}^3$ higher than both — well outside the close agreement of the other two, suggesting the end point was overshot in that trial.

(c)

$$ \text{mean} = \frac{15.2+15.3}{2} = \frac{30.5}{2} = 15.25\,\text{cm}^3 $$

QUESTION 16 6 marks Criterion C

Hard

Trial	1	2	3	4
Titre volume (cm³)	21.30	21.25	21.60	21.28

A student titrates the same acid–alkali pair four times.

a. Identify which of the four results should be excluded from the mean, and explain why.

[2]

b. Calculate the mean titre using only the concordant results.

[2]

c. Evaluate the overall reliability of this set of titration data.

[2]

Show complete worked solution

(a)

Trial 3 ($21.60\,\text{cm}^3$) should be excluded. The other three results (21.30, 21.25, 21.28) all lie within $0.05\,\text{cm}^3$ of each other — closely concordant — while Trial 3 is around $0.3$–$0.35\,\text{cm}^3$ higher, well outside this tight agreement.

(b)

$$ \text{mean} = \frac{21.30+21.25+21.28}{3} = \frac{63.83}{3} = 21.28\,\text{cm}^3 \ (\text{to 2 d.p.}) $$

(c)

Three of the four results are highly concordant (within $0.05\,\text{cm}^3$), which suggests the student's technique was mostly careful and consistent, giving a reliable mean of $21.28\,\text{cm}^3$. However, the fact that one trial (Trial 3) deviated noticeably shows the method was not perfectly controlled every time — for example, the end point may have been overshot in that one trial — so further repeats would help confirm that $21.28\,\text{cm}^3$ is indeed the true value rather than a coincidence of the three closest readings.

QUESTION 17 7 marks Criterion C

Hard

Antacid tablet	P	Q	R
Volume of acid neutralised (cm³)	42	58	55
Tablet mass (g)	0.6	0.6	1.2

A student adds indigestion tablets P, Q, and R to separate flasks of the same acid (with indicator), adding acid dropwise until the indicator shows neutral, and records the volume of acid each tablet neutralises. Tablet R has double the mass of tablets P and Q.

a. Based only on the volumes in the table, which tablet appears to neutralise the most acid?

[2]

b. Explain why comparing the tablets on volume of acid neutralised alone may not fairly show which tablet is “best” at neutralising acid.

[3]

c. Calculate the volume of acid neutralised per gram of tablet for P, Q, and R, and use this to identify the most effective tablet.

[2]

Show complete worked solution

(a)

Tablet Q neutralises the most acid ($58\,\text{cm}^3$), compared with P ($42\,\text{cm}^3$) and R ($55\,\text{cm}^3$).

(b)

Tablet R has double the mass of tablets P and Q, so it would be expected to neutralise more acid simply because it contains more antacid substance overall — not necessarily because it is chemically more effective. A fair comparison should be based on acid neutralised per gram of tablet, not the raw volume, since the tablets are not equal in size.

(c)

$$ P: \frac{42}{0.6}=70\,\text{cm}^3/\text{g} \qquad Q: \frac{58}{0.6}=96.7\,\text{cm}^3/\text{g} \qquad R: \frac{55}{1.2}=45.8\,\text{cm}^3/\text{g} $$Per gram, Tablet Q is the most effective — it neutralises far more acid for its mass than either P or R.

QUESTION 18 4 marks Criterion D

Medium

Farmers commonly spread powdered lime (calcium oxide or calcium hydroxide) onto fields where the soil has become too acidic for crops to grow well, using a neutralisation reaction between the lime and the acidic soil.

Discuss one benefit and one drawback of this practice.

Show complete worked solution

Benefit: Adding lime neutralises excess acid in the soil, raising its pH into the range most crops need to grow well. This improves how easily plant roots can take up nutrients from the soil, increasing crop yield and supporting farmers' income and food production for the wider population.

Drawback: Applying too much lime can overcorrect the soil, making it too alkaline for the crop being grown, which can be just as harmful as excess acidity and may require further, costly correction. Lime is also relatively heavy and must be transported and spread across large areas, using fuel and machinery, and excess lime washed off the land by rain can raise the pH of nearby streams, affecting the plants and animals living there.

QUESTION 19 5 marks Criterion D

Medium

Indigestion (antacid) tablets contain a mild base, such as calcium carbonate or magnesium hydroxide, which neutralises excess hydrochloric acid in the stomach to relieve discomfort.

Discuss one benefit and one drawback of using antacid tablets.

Show complete worked solution

Benefit: Antacid tablets are a cheap, fast, and widely available way to relieve the pain and discomfort of excess stomach acid (heartburn/indigestion), working within minutes by directly neutralising the acid through a simple acid–base reaction. They do not usually require a doctor's prescription, making relief accessible to almost anyone.

Drawback: Antacids only relieve the symptom (excess acid) rather than treating any underlying cause, so relying on them frequently could mask a more serious digestive condition that needs proper medical attention. Overuse can also cause side effects, such as constipation or diarrhoea depending on the antacid used, or interfere with the body's natural, mildly acidic stomach environment that helps kill harmful bacteria in food.

QUESTION 20 6 marks Criterion D

Hard

Some industrial processes produce acidic waste water. Before this water is released into rivers, it is often treated with lime (calcium hydroxide) in a neutralisation reaction to raise its pH closer to neutral.

Evaluate the impact of this practice, discussing both a benefit and a concern it raises.

Show complete worked solution

Benefit: Neutralising acidic waste water before release protects rivers and the organisms living in them — many fish, insects, and plants can only survive within a narrow pH range, so releasing strongly acidic water untreated could kill large numbers of them and damage the whole ecosystem. Treating the waste water also helps factories comply with environmental regulations that limit how acidic (or alkaline) discharged water is allowed to be.

Concern: Neutralising large volumes of waste water requires buying, transporting, and safely storing significant quantities of lime, adding an ongoing cost to the industry, which may be passed on to consumers or lead some companies to try to cut corners. The neutralisation reaction also produces a solid, insoluble sludge (containing the salt formed and any other dissolved impurities) that must itself be safely collected and disposed of, rather than simply being released into the environment, creating a further waste-management problem.

QUESTION 1 2 marks Criterion A

Easy

Give two examples of common household acids, and state the specific acid each one contains.

Show complete worked solution

Vinegar contains acetic acid (ethanoic acid). Lemon/citrus fruit juice contains citric acid. (Other acceptable examples: fizzy drinks contain carbonic acid; the stomach contains hydrochloric acid.)

QUESTION 2 2 marks Criterion A

Easy

Give two examples of common household bases/alkalis, and state one physical property that alkaline solutions typically have.

Show complete worked solution

Examples: soap, baking soda (sodium hydrogencarbonate) solution, oven cleaner, and toothpaste are all alkaline. Alkaline solutions typically feel soapy or slippery to the touch.

QUESTION 3 2 marks Criterion A

Easy

Many household chemicals, such as drain cleaner and bleach, carry a hazard label reading “corrosive”.

a. State what the “corrosive” hazard label means.

[1]

b. Suggest one safety precaution that should be taken when using a corrosive household product.

[1]

Show complete worked solution

(a)

It means the substance can chemically attack and destroy living tissue (such as skin and eyes) and materials such as metal on contact.

(b)

Wear protective gloves and/or eye protection, and avoid direct skin contact — or ensure good ventilation and keep the product away from children.

QUESTION 4 4 marks Criterion A

Medium

Substance	Milk	Soap solution	Orange juice	Oven cleaner
pH value	6.5	9.5	3.5	13

Use the table to classify each substance.

a. Classify milk and orange juice as acidic, neutral, or alkaline.

[2]

b. Classify soap solution and oven cleaner as acidic, neutral, or alkaline.

[2]

Show complete worked solution

(a)

Milk (pH 6.5): weakly acidic (just below 7). Orange juice (pH 3.5): acidic.

(b)

Soap solution (pH 9.5): alkaline. Oven cleaner (pH 13): strongly alkaline.

QUESTION 5 4 marks Criterion A

Medium

The human stomach produces hydrochloric acid, giving it a pH of approximately 1.5–2.

a. Suggest two biological roles that this acid plays in digestion.

[2]

b. The stomach lining produces a layer of alkaline mucus to protect itself. Explain why this is necessary.

[2]

Show complete worked solution

(a)

It helps to break down food (particularly proteins, by activating digestive enzymes such as pepsin) and it helps to kill harmful bacteria and microorganisms that are swallowed with food, protecting the body from infection.

(b)

Without this protective mucus layer, the strongly acidic stomach acid (pH 1.5–2) would gradually damage/corrode the stomach's own lining, since the acid is strong enough to break down proteins and tissue. The alkaline mucus neutralises the acid immediately next to the stomach wall, protecting it from this damage.

QUESTION 6 4 marks Criterion A

Medium

State four safety precautions that should be followed when handling acids and alkalis, whether in a school laboratory or at home.

Show complete worked solution

Any four of, for example: wear eye protection (goggles) at all times; wear gloves when handling concentrated or corrosive substances; never taste or directly smell chemicals; add concentrated acid to water (not water to acid) if diluting, to avoid violent splashing; store acids and alkalis in clearly labelled, sealed containers, away from each other and out of reach of children; and know the location of an eyewash station/running water in case of a splash to the skin or eyes.

QUESTION 7 5 marks Criterion A

Hard

Burning fossil fuels in power stations and vehicles releases gases such as sulfur dioxide ($SO_2$) and nitrogen oxides into the atmosphere. These gases dissolve in water vapour in clouds to form acid rain.

a. State the approximate pH of normal, unpolluted rainwater, and explain why it is not exactly pH 7.

[2]

b. Explain, in terms of the gases involved, why acid rain has a noticeably lower pH than normal rainwater.

[3]

Show complete worked solution

(a)

Normal rainwater has a pH of about 5.6, slightly acidic rather than neutral, because carbon dioxide from the air naturally dissolves in it, forming a weak acid (carbonic acid).

(b)

Sulfur dioxide and nitrogen oxides released by burning fossil fuels dissolve in water droplets in clouds and react with the water to form stronger acids (such as sulfuric acid and nitric acid) than the weak carbonic acid formed by dissolved $CO_2$ alone. This adds a much higher concentration of extra $H^+$ ions to the rainwater, lowering its pH well below the normal 5.6 — acid rain can have a pH as low as 4 or below.

QUESTION 8 6 marks Criterion B

Medium

A student wants to investigate whether different fruit juices (orange, lemon, apple, grape) differ in acidity, using universal indicator solution and a colour chart.

a. State the independent and dependent variables.

[2]

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

[2]

c. Describe a method for this investigation.

[2]

Show complete worked solution

(a)

Independent variable: the type of fruit juice tested. Dependent variable: the pH of each juice, read using universal indicator and a colour chart.

(b)

Control the volume of juice tested (e.g. always $5\,\text{cm}^3$) and the number of drops of indicator added. Why control the volume of juice: using different volumes could affect how intense/diluted the resulting colour appears, making it harder to compare colours fairly against the chart, even between juices of the same true pH.

(c)

Pour $5\,\text{cm}^3$ of each juice into a separate labelled test tube.
Add the same number of drops (e.g. 3) of universal indicator solution to each tube and swirl gently.
Compare the resulting colour to a standard colour chart and record the pH for each juice.
Repeat each juice at least twice more and take a mean/most common pH reading, to check the result is reliable.

QUESTION 9 7 marks Criterion B

Medium

A student wants to investigate how temperature affects the rate at which an antacid (indigestion) tablet neutralises a fixed volume of dilute hydrochloric acid, which models stomach acid.

a. State the independent and dependent variables.

[2]

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

[2]

c. Describe a method for this investigation, including how the acid would be heated to different temperatures.

[3]

Show complete worked solution

(a)

Independent variable: the temperature of the hydrochloric acid. Dependent variable: the time taken for the tablet to fully react/dissolve (or the time for the indicator to change colour, showing neutralisation is complete).

(b)

Control the volume and concentration of acid used and the mass/size of tablet used each time. Why control tablet mass: a larger tablet contains more antacid substance and would take longer to fully react regardless of temperature, making it impossible to tell whether any difference in reaction time was due to temperature or to tablet size.

(c)

Measure a fixed volume of hydrochloric acid (e.g. $50\,\text{cm}^3$) into a beaker and heat it in a water bath to the required temperature (e.g. $20°C$, $30°C$, $40°C$, $50°C$), checking with a thermometer.
Once the acid reaches the target temperature, add one whole antacid tablet and start a stopwatch immediately.
Time how long it takes for the tablet to completely react/dissolve.
Repeat at each temperature (using a fresh tablet and fresh acid each time) and take a mean time, then compare across the different temperatures tested.

QUESTION 10 8 marks Criterion B

Hard

Fruit juice	Orange	Lemon	Apple
Volume tested (cm³)	5	3	8
Time for indicator to fully change colour (s)	12	6	20

A student investigates which fruit juice is most acidic by adding a few drops of each juice to a fixed volume of dilute alkali with indicator, and timing how long it takes for the colour to fully change. However, as shown in the table, she used a different volume of each juice, and only tested each juice once. She judged “fully changed” by eye each time.

a. Identify two flaws in this method, and explain the effect each would have on the results.

[3]

b. Suggest an improved method that would let the juices be compared fairly.

[3]

c. Suggest one way to make judging the colour change less subjective (less dependent on the student's own judgement).

[2]

Show complete worked solution

(a)

Using different volumes of each juice: a juice tested with a larger volume (like apple, $8\,\text{cm}^3$) has more total acid added overall, which could make it appear to change the colour faster/more completely even if it is actually less acidic per $\text{cm}^3$ than a juice tested with a smaller volume — this makes the comparison unfair. Testing each juice only once: a single result could be affected by chance (e.g. a slightly inaccurate reaction time, or a misjudged colour), so there is no way to check whether the result is reliable or a one-off.

(b)

Use the same, fixed volume of each juice (e.g. always $5\,\text{cm}^3$) added to the same fixed volume of alkali and indicator each time, and repeat each juice at least three times, taking a mean time for each juice. This ensures any difference in time is caused only by the juice's acidity, not by differing amounts of juice, and checks that the results are reliable rather than one-off readings.

(c)

Use a pH meter or colorimeter/data logger to measure the exact pH or light absorbance electronically, instead of judging "fully changed" by eye — this removes human judgement from deciding the end point and gives a more precise, repeatable measurement.

QUESTION 11 3 marks Criterion C

Easy

The chart shows the pH of several household substances.

a. Which substance shown is the most acidic?

[1]

b. Which substance shown is the most alkaline?

[1]

c. List the substances that are acidic (pH below 7).

[1]

Show complete worked solution

(a)

Lemon juice (pH 2) — it has the lowest pH.

(b)

Oven cleaner (pH 13) — it has the highest pH.

(c)

Lemon juice, vinegar, and milk — all have a pH below 7.

QUESTION 12 4 marks Criterion C

Medium

Antacid brand	X	Y	Z
Volume of acid neutralised (cm³)	35	48	40

A student compares three brands of antacid tablet, X, Y, and Z, each of identical mass, by measuring the volume of standard acid each tablet can neutralise.

a. Which brand neutralises the most acid?

[1]

b. Calculate how much more acid brand Y neutralises than brand X, as a percentage of brand X's value.

[2]

c. Since the tablets are of identical mass, explain why this is a fair comparison of their effectiveness.

[1]

Show complete worked solution

(a)

Brand Y ($48\,\text{cm}^3$).

(b)

$$ \text{difference} = 48 - 35 = 13\,\text{cm}^3 $$$$ \text{percentage} = \frac{13}{35}\times100 = 37.1\% \ (\text{3 s.f.}) $$

(c)

Because all three tablets have the same mass, any difference in the volume of acid neutralised must be due to a genuine difference in how much active antacid each brand contains (or how effective it is), rather than simply one tablet being bigger than another.

QUESTION 13 5 marks Criterion C

Medium

The chart shows the mean time taken for the same fixed volume of four fruit juices to fully change the colour of an indicator in a fixed volume of dilute alkali (a faster colour change suggests a more acidic juice).

a. Which juice appears to be the most acidic?

[1]

b. Which juice appears to be the least acidic, and by how many seconds slower is it than the fastest juice?

[2]

c. Explain why a shorter time for the colour change suggests a juice is more acidic.

[2]

Show complete worked solution

(a)

Lemon juice — it changed the indicator's colour fastest (5 s).

(b)

Grape juice appears least acidic, taking the longest (18 s). It is $18 - 5 = 13\,\text{s}$ slower than lemon juice.

(c)

A shorter time means the acid reacted with (neutralised) the fixed amount of alkali more quickly, which happens when there is a higher concentration of $H^+$ ions available to react straight away — a property of a more strongly acidic solution. A less acidic juice has a lower $H^+$ concentration, so it takes longer to neutralise the same amount of alkali.

QUESTION 14 4 marks Criterion C

Medium

City	A	B	C	D
Rainwater pH	5.6	4.3	5.5	5.7

Rainwater pH was measured in four cities on the same day. Normal, unpolluted rainwater has a pH of about 5.6.

a. Identify the city with the anomalous (unusually low) reading.

[1]

b. Suggest a reason for this city's unusually low reading.

[2]

c. Calculate the mean rainwater pH for the three other cities (A, C, and D).

[1]

Show complete worked solution

(a)

City B (pH 4.3).

(b)

City B's rainwater is noticeably more acidic than normal rainwater and the other three cities, suggesting higher levels of air pollution there — for example from more traffic or nearby factories/power stations releasing sulfur dioxide and nitrogen oxides, which dissolve in rain to form stronger acids (acid rain).

(c)

$$ \text{mean} = \frac{5.6+5.5+5.7}{3} = \frac{16.8}{3} = 5.6 $$

QUESTION 15 5 marks Criterion C

Medium

Site	Trial 1	Trial 2	Trial 3
Tap pH (Site 1)	7.0	7.1	6.9
Tap pH (Site 2)	7.0	8.4	7.1

A student measures the pH of tap water at two different sites in a town, three times each.

a. Calculate the mean pH for Site 1.

[2]

b. Identify the anomalous reading at Site 2.

[1]

c. Evaluate whether the student has enough evidence to conclude that Site 2's tap water is generally more alkaline than Site 1's.

[2]

Show complete worked solution

(a)

$$ \text{mean} = \frac{7.0+7.1+6.9}{3} = \frac{21.0}{3} = 7.0 $$

(b)

Trial 2 at Site 2 (pH 8.4).

(c)

No — excluding the anomalous Trial 2, Site 2's other two readings (7.0 and 7.1) are almost identical to Site 1's readings (7.0, 7.1, 6.9), both close to neutral. The one high reading at Site 2 (8.4) is not enough on its own to conclude the site's water is generally more alkaline; it should be re-tested and, if it happens again, investigated further (e.g. for possible contamination) rather than treated as proof of a real, ongoing difference between the two sites.

QUESTION 16 6 marks Criterion C

Hard

Lime added (g per kg soil)	0	2	4	6	8
Soil pH	4.8	5.4	6.0	6.6	7.6

A gardener adds increasing amounts of lime to samples of the same acidic soil and measures the resulting pH. Most vegetable crops grow best in soil with a pH close to 6.5.

a. Describe the trend shown by the data between 0 g and 6 g of lime.

[2]

b. Using the data, estimate the mass of lime (per kg of soil) needed to bring this soil closest to the ideal pH of 6.5 for vegetables.

[2]

c. Explain why the gardener should be careful not to add significantly more than this amount of lime.

[2]

Show complete worked solution

(a)

As the mass of lime added increases from 0 g to 6 g, the soil pH rises steadily and fairly evenly — by about $0.6$ for every $2\,\text{g}$ of lime added, from pH 4.8 (0 g) up to pH 6.6 (6 g).

(b)

The data shows pH 6.6 at $6\,\text{g}$ of lime, which is already very close to 6.5 — only $0.1$ above. Based on the trend, a mass slightly less than $6\,\text{g}$ (roughly $5$–$6\,\text{g}$) per kg of soil would bring the pH closest to 6.5.

(c)

Beyond 6 g, pH rises sharply to 7.6 at 8 g — a jump of a full 1.0 pH unit for only 2 more grams, much larger than earlier steps. This shows that adding too much lime can easily overcorrect the soil, making it too alkaline (above the ideal 6.5) for the crop being grown, which could harm plant growth just as much as the original acidity did.

QUESTION 17 7 marks Criterion C

Hard

Cleaning product	Bleach	Vinegar spray	Baking soda paste
pH	13	3	8.5
Scrubbing time used (s)	10	60	45
Stain removed?	Yes	Yes	Partially

A student tests three cleaning products on identical stains and records the results shown, concluding: “A higher pH makes a cleaning product more effective at removing stains, since bleach (pH 13) fully removed the stain, while baking soda (pH 8.5) only partially removed it.”

a. Identify a variable in this investigation that was not controlled, and explain the problem this causes.

[2]

b. Evaluate whether the student's conclusion (“a higher pH makes a product more effective”) is fully supported by the data.

[3]

c. Suggest one change to the method that would make the comparison of the three products fairer.

[2]

Show complete worked solution

(a)

The scrubbing time used with each product was not controlled — bleach was used with only 10 s of scrubbing, but vinegar spray was scrubbed for 60 s and baking soda for 45 s. This means any difference in stain removal could be caused by the amount of scrubbing rather than (or as well as) the product's pH, so pH cannot be identified as the only factor affecting the results.

(b)

The conclusion is not well supported. Vinegar spray, which has a low pH (3, acidic — the opposite end of the scale from bleach), also fully removed its stain, even though it needed more scrubbing time. If pH alone determined effectiveness, an acidic product should not work as well as a strongly alkaline one, yet here both bleach (very alkaline) and vinegar (very acidic) succeeded, while the mid-range baking soda (pH 8.5) only partially worked. This suggests that how far a product's pH is from neutral, or another factor (such as scrubbing time, or the specific chemical, not just pH), may matter more than simply having a high pH.

(c)

Control the scrubbing time (and scrubbing force) so it is identical for all three products — e.g. always scrubbing for exactly 30 s with the same tool — so that any difference in stain removal can be attributed only to the cleaning product itself.

QUESTION 18 4 marks Criterion D

Medium

Bleach, a strongly alkaline chemical, is widely used as a household cleaner and disinfectant.

Discuss one benefit and one drawback of using bleach in the home.

Show complete worked solution

Benefit: Bleach is highly effective at killing bacteria, viruses, and mould, and at removing stains, making it a cheap and powerful way to keep kitchens, bathrooms, and other surfaces clean and hygienic, helping to prevent the spread of illness in the home.

Drawback: Bleach is corrosive and can burn or irritate skin and eyes on contact, and it releases harmful, potentially dangerous fumes if accidentally mixed with certain other cleaning products (such as those containing ammonia or acids). It must therefore be stored safely away from children and used with care (e.g. gloves, ventilation), and excess bleach entering waterways can also harm aquatic organisms.

QUESTION 19 5 marks Criterion D

Medium

Most electricity is still generated by burning fossil fuels, which releases sulfur dioxide and nitrogen oxides that cause acid rain, damaging forests, lakes, and stone buildings.

Discuss one benefit and one drawback of continuing to generate electricity this way.

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Benefit: Fossil-fuel power stations can generate large, reliable amounts of electricity on demand, using well-established, relatively low-cost technology and existing infrastructure, which continues to provide affordable and dependable power for homes, hospitals, schools, and industry.

Drawback: Burning fossil fuels releases acidic gases that form acid rain, which lowers the pH of soil and lakes (harming plants, fish, and other organisms) and gradually corrodes stone and metal buildings and monuments over time. It also contributes to air pollution and climate change, meaning the environmental cost of this convenient electricity extends well beyond the immediate area around the power station.

QUESTION 20 6 marks Criterion D

Hard

Chemical fertilisers, some of which are acidic or leave acidic residues in the soil over time, are widely used in farming to boost crop yields.

Evaluate the impact of using chemical fertilisers, discussing both a benefit and a concern it raises for the environment.

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Benefit: Chemical fertilisers supply plants with nutrients (such as nitrogen, phosphorus, and potassium) quickly and in a precisely controllable amount, allowing farmers to grow significantly more food on the same area of land than would otherwise be possible. This supports feeding a growing global population and can make farming more economically viable.

Concern: Repeated use of certain fertilisers can gradually lower soil pH over years, making the soil more acidic and less suitable for crops unless it is regularly treated with lime to correct it — an ongoing extra cost. Excess fertiliser not taken up by crops can also be washed by rain into rivers and lakes (a process called run-off), where it can alter the water's chemistry and cause excessive growth of algae, which then depletes oxygen in the water as it decomposes, harming fish and other aquatic life.

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