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MYP 3 · Science

Elements, Compounds and Mixtures

60 questions across 3 sub-topics

Use the Sub-Topic filter above to focus on one.

Atoms, Elements and the Periodic Table (intro) Compounds and Chemical Formulae (basic) Mixtures and Separation Techniques

Atoms, Elements and the Periodic Table (intro) 20 questions

QUESTION 1 3 marks Criterion A
Easy

An atom is made up of three types of subatomic particle.

a. State the electric charge of a proton, a neutron, and an electron.
[1]
b. State whether each of these particles is found in the nucleus, or in the shells (energy levels) surrounding the nucleus.
[1]
c. Which of the three particles has the smallest mass?
[1]
Show complete worked solution
(a)
Proton: +1 (positive). Neutron: 0 (neutral, no charge). Electron: ?1 (negative).
(b)
Protons and neutrons are both found in the nucleus. Electrons are found in shells (energy levels) surrounding the nucleus.
(c)
The electron has a mass so small it is usually considered negligible compared with a proton or a neutron (protons and neutrons have approximately equal, much larger, relative mass).
QUESTION 2 3 marks Criterion A
Easy

A sodium atom has atomic number $11$ and mass number $23$.

a. State the number of protons in this atom.
[1]
b. State the number of electrons in a neutral atom of sodium.
[1]
c. Calculate the number of neutrons in this atom.
[1]
Show complete worked solution
(a)
The atomic number gives the number of protons directly: 11 protons.
(b)
In a neutral atom, the number of electrons equals the number of protons: 11 electrons.
(c)
$$ \text{neutrons} = \text{mass number} - \text{atomic number} = 23 - 11 = 12 $$ 12 neutrons.
QUESTION 3 3 marks Criterion A
Easy

The periodic table is divided into metals and non-metals.

a. State two physical properties typical of metals.
[1]
b. State two physical properties typical of non-metals.
[1]
c. Aluminium is shiny and conducts electricity well. Is aluminium a metal or a non-metal?
[1]
Show complete worked solution
(a)
Metals are typically shiny (lustrous), good conductors of electricity and heat, and malleable (can be hammered into shape) — any two of these.
(b)
Non-metals are typically dull (not shiny), poor conductors of electricity and heat (insulators), and brittle if solid — any two of these.
(c)
Aluminium is a metal — its shine and good electrical conductivity are typical metallic properties.
QUESTION 4 5 marks Criterion A
Medium
Electron arrangement of an atom 11p, 12n outer shell

The diagram shows the electron arrangement of an atom.

a. State the atomic number of this atom, and identify the element.
[2]
b. State the electron arrangement shown in the diagram.
[1]
c. Sodium is in Group 1 of the periodic table. Explain how the electron arrangement in the diagram supports this.
[2]
Show complete worked solution
(a)
The nucleus shows $11$ protons, so the atomic number is 11. This identifies the element as sodium (Na).
(b)
The diagram shows $2$ electrons in the first shell, $8$ in the second shell, and $1$ in the third (outer) shell: $2,8,1$.
(c)
Sodium's outer (third) shell contains just $1$ electron. Elements are placed in Group 1 when they have exactly $1$ electron in their outer shell, so this electron arrangement matches sodium's position in Group 1.
QUESTION 5 4 marks Criterion A
Medium

An atom has the electron arrangement $2,8,6$.

a. State how many electron shells this atom has, and hence which period of the periodic table it is in.
[1]
b. State how many electrons are in the outer shell, and hence which group it is in.
[1]
c. Identify the element, explaining your reasoning.
[2]
Show complete worked solution
(a)
The atom has $3$ electron shells. The period number equals the number of electron shells, so this atom is in Period 3.
(b)
There are $6$ electrons in the outer shell. For groups $13$-$18$, the number of outer electrons is the group number minus $10$, so $6$ outer electrons means this atom is in Group 16.
(c)
The total number of electrons is $2+8+6=16$, which equals the atomic number (and so the number of protons) for a neutral atom. The element with atomic number $16$ is sulfur (S).
QUESTION 6 5 marks Criterion A
Medium

Carbon has two common isotopes: carbon-12 and carbon-14. The atomic number of carbon is $6$.

a. Define the term isotope.
[1]
b. State the number of protons, neutrons, and electrons in an atom of carbon-12.
[2]
c. State the number of protons, neutrons, and electrons in an atom of carbon-14, and state which of these numbers is different from carbon-12.
[2]
Show complete worked solution
(a)
Isotopes are atoms of the same element (same number of protons / same atomic number) that have different numbers of neutrons (and therefore different mass numbers).
(b)
Carbon-12 has mass number $12$ and atomic number $6$: $6$ protons, $12-6=6$ neutrons, $6$ electrons.
(c)
Carbon-14 has mass number $14$ and atomic number $6$: $6$ protons, $14-6=8$ neutrons, $6$ electrons. Only the number of neutrons is different between the two isotopes (protons and electrons are the same, since both are still carbon).
QUESTION 7 6 marks Criterion A
Hard
Groups 1, 2 and 13-18, Periods 2 and 3 1 2 13 14 15 16 17 18 Li Be B C N O F Ne Na Mg Al Si P S Cl Ar reactivity increases halogen alkali metal

The diagram shows part of the periodic table (Periods 2 and 3, Groups 1, 2 and 13–18).

a. State the period and group of the element outlined in blue (Na).
[2]
b. State the number of electrons in the outer shell of the element outlined in red (Cl), and explain how you can work this out from its group number.
[2]
c. The arrow shows that reactivity increases down Group 1. Using ideas about electron shells, explain why potassium (directly below sodium in Group 1) is more reactive than sodium.
[2]
Show complete worked solution
(a)
Sodium (Na) is in the second row shown, which is Period 3, and in the first column shown, which is Group 1.
(b)
Chlorine (Cl) is in Group 17. For elements in Groups $13$-$18$, the number of outer-shell electrons equals the group number minus $10$: $17-10=\mathbf{7}$ outer-shell electrons.
(c)
Potassium has one more electron shell than sodium, so its outer electron is further from the nucleus and more shielded by the extra inner shell of electrons. This means the nucleus attracts potassium's outer electron less strongly, so it is lost more easily in a reaction — making potassium more reactive than sodium.
QUESTION 8 7 marks Criterion B
Medium

A student wants to investigate how density changes across Period 3 of the periodic table (from sodium to sulfur), using small solid samples of each element that can be safely handled.

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, including the equipment used, to measure the density of each solid sample.
[3]
Show complete worked solution
(a)
Independent variable: the element tested (its position across Period 3 / atomic number). Dependent variable: the density of the sample.
(b)
Control the temperature at which each measurement is taken, and use the same method to measure volume for every sample (e.g. water displacement). Why control temperature: most materials expand slightly as temperature rises, changing their volume and therefore their density for reasons unrelated to which element is being tested, which would make the comparison unfair.
(c)
  1. Measure the mass of the sample using a balance.
  2. Partly fill a measuring cylinder with water and record the volume.
  3. Carefully lower the sample into the water and record the new volume; the increase is the volume of the sample (water displacement).
  4. Calculate density using $\text{density} = \dfrac{\text{mass}}{\text{volume}}$.
  5. Repeat for each Period 3 element tested, keeping temperature constant.
QUESTION 9 6 marks Criterion B
Medium

A teacher demonstrates the reaction of small, equal-sized pieces of lithium, sodium, and potassium with water, while a student designs an investigation into the trend in reactivity down Group 1.

a. State the independent and dependent variables for this investigation.
[2]
b. State two variables that must be controlled to make this a fair test.
[2]
c. Suggest one hazard of this investigation, and a safety precaution to reduce the risk.
[2]
Show complete worked solution
(a)
Independent variable: the Group 1 metal used (lithium, sodium, or potassium). Dependent variable: a measure of reactivity, such as the temperature rise of the water in a fixed time, or the time taken for the metal to fully react.
(b)
The mass (or size) of the metal piece used each time, and the volume and starting temperature of the water used each time.
(c)
Group 1 metals react vigorously with water and can produce flammable hydrogen gas or spit caustic solution. Precaution: the reaction should be carried out by the teacher only, using a very small piece of metal, behind a safety screen, with safety goggles worn.
QUESTION 10 8 marks Criterion B
Hard

A student measures the melting point of a small sample of a Period 3 element by heating it steadily and strongly with a Bunsen burner, recording the temperature on a thermometer placed in the sample the moment it starts to turn liquid.

a. Identify a source of error in this method that could make the recorded melting point inaccurate.
[2]
b. Suggest an improvement to the method that would reduce this error, and explain how it works.
[3]
c. Explain why repeating the experiment and calculating a mean melting point would make the result more reliable.
[3]
Show complete worked solution
(a)
Heating too quickly and strongly with a direct flame means the sample may not be evenly heated throughout when it starts to melt, and the student's reaction time in noticing the exact moment melting begins could cause the reading to be taken slightly too late — both would tend to give a melting point that reads too high.
(b)
Heat the sample slowly and gently as it approaches the expected melting point (e.g. using a water or oil bath rather than a direct flame), so the temperature rises gradually and evenly throughout the sample. This gives the student much more time to observe and record the exact temperature at which melting begins, rather than heating past it before noticing.
(c)
Repeating the measurement reduces the effect of random error — such as slightly misjudging the exact moment melting starts each time. Any clearly anomalous reading can also be identified and excluded. The mean of several close, repeated readings is a better estimate of the true melting point than relying on a single measurement.
QUESTION 11 3 marks Criterion C
Easy
ElementAtomic numberMass numberProtonsNeutronsElectrons
Helium24222
Oxygen816898
Aluminium1327131413
a. One of the rows in the table contains an error. Identify which element's row is incorrect.
[1]
b. State the correct number of neutrons for this element, showing your working.
[2]
Show complete worked solution
(a)
The oxygen row is incorrect.
(b)
$$ \text{neutrons} = \text{mass number} - \text{atomic number} = 16 - 8 = 8 $$ The table should show 8 neutrons, not $9$.
QUESTION 12 5 marks Criterion C
Medium
ElementNaMgAlSiPSClAr
Group number121314151617?
ElementNaMgAlSiPSClAr
Outer-shell electrons1234567?
a. Describe the pattern between group number and number of outer-shell electrons shown in the tables.
[2]
b. Argon is missing from the tables. Using the pattern, predict its group number and number of outer-shell electrons.
[1]
c. Explain why elements in the same group of the periodic table have similar chemical properties.
[2]
Show complete worked solution
(a)
For Groups $1$ and $2$, the number of outer-shell electrons is equal to the group number. For Groups $13$-$18$, the number of outer-shell electrons is equal to the group number minus $10$.
(b)
Argon is in Group 18, so it has $18-10=\mathbf{8}$ outer-shell electrons.
(c)
Elements in the same group have the same number of outer-shell (valence) electrons. Since chemical reactions mainly involve the outer-shell electrons (being gained, lost, or shared), elements with the same number of them tend to react in similar ways and form similar types of compounds.
QUESTION 13 5 marks Criterion C
Medium
ElementNaMgAlSiPSClAr
Melting point (°C)98650660141444115?101?189
a. State the general trend in melting point from Na to Al (the metals).
[1]
b. Silicon's melting point ($1414\,^\circ\text{C}$) is far higher than any of the metals shown. Suggest why silicon does not follow the simple increasing pattern of the metals.
[2]
c. State the trend in melting point from P to Ar, and suggest why these values are so much lower than the metals'.
[2]
Show complete worked solution
(a)
Melting point increases from sodium to aluminium.
(b)
Silicon is a metalloid with a giant covalent structure, where every atom is joined to its neighbours by strong covalent bonds throughout the whole structure. Breaking this requires far more energy than overcoming the metallic bonding in Na, Mg, and Al, giving silicon a much higher melting point.
(c)
Melting point decreases sharply from P to Ar, with several values below room temperature. These elements exist as small, simple molecules (or, for argon, single atoms) held together by weak forces between molecules, which need very little energy to overcome — unlike the strong metallic or giant covalent bonding of Na to Si.
QUESTION 14 5 marks Criterion C
Medium
MetalTemperature rise of water in 30 s (°C)Time for metal to disappear (s)
Lithium845
Sodium1920
Potassium316
a. Describe the trend in reactivity shown by this data as you go down Group 1.
[2]
b. Calculate the difference in temperature rise between potassium and lithium.
[1]
c. Predict, with a reason, whether rubidium (directly below potassium in Group 1) would react faster or slower than potassium.
[2]
Show complete worked solution
(a)
Reactivity increases down Group 1: the temperature rise gets bigger (from $8$ to $31\,^\circ\text{C}$) and the time for the metal to fully react gets shorter (from $45$ to $6\,\text{s}$) going from lithium to sodium to potassium.
(b)
$$ 31 - 8 = 23\,^\circ\text{C} $$
(c)
Rubidium would react faster (more vigorously) than potassium, because reactivity continues to increase down Group 1 — rubidium has one more electron shell than potassium, so its outer electron is even more easily lost.
QUESTION 15 5 marks Criterion C
Medium

Chlorine exists naturally as a mixture of two isotopes: $75\%$ of chlorine atoms are chlorine-35, and $25\%$ are chlorine-37.

a. State what makes these two isotopes atoms of the same element, rather than different elements.
[1]
b. Calculate the relative atomic mass of chlorine, using the abundances given. Give your answer to 1 decimal place.
[3]
c. Explain why the relative atomic mass of chlorine ($35.5$) is not a whole number, even though every individual chlorine atom has a whole-number mass number.
[1]
Show complete worked solution
(a)
They both have the same number of protons ($17$) — the same atomic number — they differ only in the number of neutrons.
(b)
$$ \text{relative atomic mass} = \frac{(75\times35) + (25\times37)}{100} = \frac{2625 + 925}{100} = \frac{3550}{100} $$ Answer: $35.5$
(c)
$35.5$ is a weighted average of the mass numbers of the two isotopes ($35$ and $37$), based on how abundant each one is in nature — it is not the mass of any single atom, which is always a whole number.
QUESTION 16 6 marks Criterion C
Hard
ElementAtomic numberPeriodGroup
Lithium321
Beryllium422
Sodium1131
Magnesium1232
Potassium1941
Calcium2043
a. Identify which row in the table contains an incorrect group number.
[1]
b. State the correct group number for this element, and explain your reasoning using its electron arrangement.
[3]
c. Using the pattern in the table, state the relationship between period number and the number of electron shells in an atom.
[2]
Show complete worked solution
(a)
The calcium row is incorrect.
(b)
Calcium's electron arrangement is $2,8,8,2$, giving $2$ electrons in its outer shell. Elements with $2$ outer-shell electrons belong in Group 2, not Group $3$.
(c)
The period number equals the number of electron shells in an atom — e.g. elements in Period $2$ have $2$ electron shells, and elements in Period $4$ have $4$ electron shells.
QUESTION 17 6 marks Criterion C
Hard
ElementGroupIon formedCharge
Sodium1Na?+1
Magnesium2Mg²?+2
Aluminium13Al³?+3
Oxygen16O²??2
Fluorine17F²??2
a. Identify the row containing an incorrect ion charge, and state the correct charge.
[2]
b. Using the pattern in the table, predict the ion formed by nitrogen (Group 15).
[2]
c. Explain, in terms of electron shells, why Group 1 metals form $+1$ ions but Group 17 non-metals form $-1$ ions.
[2]
Show complete worked solution
(a)
The fluorine row is incorrect. The correct ion is F?, with charge $-1$ (not $-2$).
(b)
For Groups $15$-$17$, ionic charge $= \text{group number} - 18$. For nitrogen: $15-18=-3$, so nitrogen forms the ion N³?.
(c)
Group 1 atoms have just $1$ electron in their outer shell, which is lost to leave a full outer shell underneath, giving one more proton than electron overall — a $+1$ ion. Group 17 atoms have $7$ outer-shell electrons and gain just $1$ more to complete a full outer shell of $8$, giving one more electron than proton overall — a $-1$ ion.
QUESTION 18 5 marks Criterion D
Medium

Certain isotopes are radioactive. Radioactive isotopes have important uses, such as cobalt-60 in cancer radiotherapy and americium-241 in household smoke detectors.

Discuss one benefit and one drawback/concern of using radioactive isotopes in these ways.

Show complete worked solution

Benefit: Radiation from an isotope such as cobalt-60 can be carefully aimed at a tumour to destroy fast-dividing cancer cells, offering an effective, non-invasive treatment that does not require surgery and has saved many lives.

Drawback/concern: Radiation from these isotopes can also damage healthy cells and DNA if exposure is too high or poorly controlled, potentially causing radiation sickness or increasing the risk of new cancers developing later. Radioactive sources and waste must also be stored and disposed of extremely carefully, since they can remain hazardous for a long time, requiring strict safety regulation.

QUESTION 19 5 marks Criterion D
Medium

Lithium is a Group 1 metal used to make rechargeable batteries for electric cars and phones. Demand for lithium has grown rapidly as more electric vehicles are produced.

Discuss one benefit and one drawback of large-scale lithium mining.

Show complete worked solution

Benefit: Lithium-ion batteries store large amounts of energy for their size and can be recharged many times, powering electric vehicles that produce no exhaust emissions. This helps reduce reliance on fossil fuels and can cut air pollution and greenhouse gas emissions from transport.

Drawback: Extracting lithium (for example from underground brine) uses huge quantities of water, which can lower local water tables and harm agriculture and ecosystems in the often dry regions where lithium is mined. Mining can also damage local habitats and produce waste that pollutes soil and water if not carefully managed.

QUESTION 20 6 marks Criterion D
Hard

Uranium (element $92$) is used as fuel in nuclear power stations, where energy released from its atoms generates electricity.

Evaluate the impact of using uranium in nuclear power stations, discussing both a benefit and a concern.

Show complete worked solution

Benefit: A small mass of uranium releases an enormous amount of energy compared with burning the same mass of fossil fuel, and nuclear power stations release no carbon dioxide while generating electricity. This makes them an important source of reliable, large-scale, low-carbon electricity that can help reduce climate change.

Concern: Used uranium fuel becomes highly radioactive waste that stays dangerous for thousands of years and must be stored extremely safely to prevent harm to people and the environment. A serious accident at a nuclear power station could release radioactive material over a wide area, so strict safety systems and regulation are essential — and the high cost of building and eventually safely decommissioning plants raises questions about whether resources might sometimes be better spent on other low-carbon sources, such as wind or solar power.

Compounds and Chemical Formulae (basic) 20 questions

QUESTION 1 4 marks Criterion A
Easy
W X Y Z

The diagrams below use circles to represent atoms (different colours represent different elements; circles joined by a line are atoms chemically bonded together in a molecule).

a. Which diagram (W, X, Y or Z) represents a pure element? Explain your choice.
[1]
b. Which diagram represents a compound? Explain your choice.
[1]
c. Which diagram represents a mixture of two elements, and which represents a mixture of a compound and an element? Explain the difference between them.
[2]
Show complete worked solution
(a)
W — it contains only one colour of atom (one type of element), and none of the atoms are bonded to a different type of atom.
(b)
X — it contains only one type of particle: two different atoms chemically bonded together in a fixed ratio ($1:1$), repeated throughout.
(c)
Y is a mixture of two elements: two different colours of atom are present, but none are bonded together. Z is a mixture of a compound and an element: it contains bonded pairs (the compound) together with separate, unbonded atoms of one of the elements, simply mixed together.
QUESTION 2 3 marks Criterion A
Easy

The formula for glucose is $C_6H_{12}O_6$.

a. State how many atoms of carbon, hydrogen, and oxygen are in one molecule of glucose.
[1]
b. Calculate the total number of atoms in one molecule of glucose.
[1]
c. State whether glucose is an element or a compound, and how you know.
[1]
Show complete worked solution
(a)
$6$ carbon atoms, $12$ hydrogen atoms, and $6$ oxygen atoms.
(b)
$$ 6 + 12 + 6 = 24 \text{ atoms} $$
(c)
Glucose is a compound — it is made of more than one type of element ($C$, $H$, and $O$) chemically combined together.
QUESTION 3 6 marks Criterion A
Medium

Elements combine in fixed ratios based on their valency (combining power): $H=1$, $O=2$, $Mg=2$, $Ca=2$, $Cl=1$, $Al=3$.

a. Use the valencies given to work out the formula for magnesium oxide.
[2]
b. Work out the formula for calcium chloride.
[2]
c. Work out the formula for aluminium oxide.
[2]
Show complete worked solution
(a)
Magnesium ($2$) and oxygen ($2$) have equal valencies, so they combine in a $1:1$ ratio. Formula: $MgO$.
(b)
Calcium has valency $2$, chlorine has valency $1$. Cross the valencies as subscripts: $2$ chlorine atoms are needed for every $1$ calcium atom. Formula: $CaCl_2$.
(c)
Aluminium has valency $3$, oxygen has valency $2$. Crossing the valencies as subscripts gives $2$ aluminium atoms for every $3$ oxygen atoms. Formula: $Al_2O_3$.
QUESTION 4 4 marks Criterion A
Medium

Compounds made from just two elements are usually named using the "-ide" ending.

a. Name the compound $NaCl$, formed between sodium and chlorine.
[1]
b. Name the compound $MgO$.
[1]
c. Name the compound $Al_2O_3$, formed between aluminium and oxygen, and explain the "-ide" naming pattern used for compounds made from just two elements.
[2]
Show complete worked solution
(a)
Sodium chloride.
(b)
Magnesium oxide.
(c)
$Al_2O_3$ is aluminium oxide. When a compound is made from exactly two elements, the ending of the second (usually non-metal) element's name is changed to "-ide" (e.g. chlorine ? chloride, oxygen ? oxide), and this is added after the full name of the first (usually metal) element.
QUESTION 5 4 marks Criterion A
Medium

Magnesium burns in oxygen to form magnesium oxide.

a. Write a word equation for this reaction.
[1]
b. State the law of conservation of mass.
[1]
c. $6\,\text{g}$ of magnesium reacts completely with $4\,\text{g}$ of oxygen. Calculate the mass of magnesium oxide formed.
[2]
Show complete worked solution
(a)
$$ \text{magnesium} + \text{oxygen} \rightarrow \text{magnesium oxide} $$
(b)
Mass cannot be created or destroyed in a chemical reaction — atoms are only rearranged into new substances, so the total mass of the reactants equals the total mass of the products.
(c)
By conservation of mass: $$ 6 + 4 = 10\,\text{g} $$ Answer: $10\,\text{g}$ of magnesium oxide.
QUESTION 6 4 marks Criterion A
Medium

Calcium carbonate decomposes when heated to form calcium oxide and carbon dioxide gas: calcium carbonate $\rightarrow$ calcium oxide + carbon dioxide.

a. $25\,\text{g}$ of calcium carbonate is heated completely and produces $14\,\text{g}$ of calcium oxide. Calculate the mass of carbon dioxide gas released.
[2]
b. If this reaction is carried out in an open container on a balance, explain what would happen to the balance reading during heating, and why.
[2]
Show complete worked solution
(a)
$$ 25 - 14 = 11\,\text{g} $$ Answer: $11\,\text{g}$ of carbon dioxide.
(b)
The balance reading would appear to decrease (lose mass) during heating, because the carbon dioxide gas escapes into the air and is no longer sitting on the balance — even though the total mass (solid $+$ escaped gas) is genuinely conserved overall.
QUESTION 7 6 marks Criterion A
Hard

Methane gas burns in oxygen according to the (unbalanced) equation: $$ CH_4 + O_2 \rightarrow CO_2 + H_2O $$

a. State the number of carbon, hydrogen, and oxygen atoms on the reactant side (left) of this equation, as written.
[2]
b. State the number of carbon, hydrogen, and oxygen atoms on the product side (right) of this equation, as written.
[2]
c. Explain why this equation, as written, does not yet obey the law of conservation of mass, referring to your answers above.
[2]
Show complete worked solution
(a)
Carbon: $1$ (from $CH_4$). Hydrogen: $4$ (from $CH_4$). Oxygen: $2$ (from $O_2$).
(b)
Carbon: $1$ (from $CO_2$). Hydrogen: $2$ (from $H_2O$). Oxygen: $2$ (from $CO_2$) $+\ 1$ (from $H_2O$) $= 3$.
(c)
For mass to be conserved, the same number of each type of atom must appear on both sides of the equation. Here, hydrogen doesn't balance ($4$ on the left, only $2$ on the right) and oxygen doesn't balance ($2$ on the left, $3$ on the right) — so as written, this equation would need extra ($\times 2$) water and oxygen coefficients to properly balance and obey conservation of mass.
QUESTION 8 7 marks Criterion B
Medium

A student wants to investigate whether the law of conservation of mass applies when vinegar (dilute acid) reacts with baking soda (sodium bicarbonate) to produce carbon dioxide gas, comparing an open container with a sealed one.

a. State the independent and dependent variables.
[2]
b. State two variables that should be kept the same (controlled) between the two setups.
[2]
c. Describe a method to compare the mass change in the open and sealed setups.
[3]
Show complete worked solution
(a)
Independent variable: whether the container is open or sealed (closed). Dependent variable: the change in total mass of the reaction (mass before minus mass after), measured on a balance.
(b)
The same mass/volume of vinegar and baking soda used each time, and the same size/type of container (apart from open vs sealed).
(c)
  1. Measure the total mass of the vinegar, baking soda, and container together before mixing, using a balance.
  2. Mix the vinegar and baking soda and allow the reaction to finish completely.
  3. Re-weigh the total mass immediately afterwards, for both the open and the sealed setup.
  4. Calculate the change in mass for each setup, and compare the two results.
QUESTION 9 6 marks Criterion B
Medium

A student investigates whether the mass of carbon dioxide gas released depends on the mass of calcium carbonate (marble chips) reacted with an excess of dilute acid.

a. State the independent and dependent variables.
[2]
b. State two variables that should be controlled.
[2]
c. Predict, with a reason, what would happen to the mass of gas released as the mass of calcium carbonate increases (while the acid stays in excess).
[2]
Show complete worked solution
(a)
Independent variable: the mass of calcium carbonate (marble chips) used. Dependent variable: the mass of gas lost (calculated from mass before minus mass after the reaction).
(b)
The volume and concentration of acid used each time (kept in excess), and the temperature at which the reaction takes place.
(c)
The mass of gas released would increase, because more calcium carbonate means more carbonate particles are available to react and produce carbon dioxide, so proportionally more product forms (up until the point the acid, not the carbonate, becomes the limiting factor).
QUESTION 10 7 marks Criterion B
Hard

A student investigating conservation of mass in an open-container, gas-producing reaction weighs the reactants, then waits several minutes with the container open before starting to record the mass at 1-minute intervals.

a. Identify a problem with this method that would affect the results.
[2]
b. Suggest an improvement to the method that would fix this problem.
[2]
c. The student repeats the (corrected) experiment three times and gets slightly different total mass losses each time. Suggest one reason for this variation, and explain how the student could obtain a more reliable result.
[3]
Show complete worked solution
(a)
Waiting several minutes with the container open before recording the first mass means some gas will have already escaped and been lost, unrecorded, before the first data point is even taken — so the total mass loss calculated from the recorded data would be less than the true total mass lost.
(b)
Weigh the reactants and start timing/recording the mass immediately once they are mixed together ($t=0$), so no gas escapes unrecorded before measurements begin.
(c)
Small differences could arise from slightly different reaction times, room temperature, or exactly when the final balance reading was taken. Taking the mean of the three repeated results (excluding any clearly anomalous one) gives a more reliable estimate of the true mass loss than relying on a single trial.
QUESTION 11 3 marks Criterion C
Easy
ExperimentMass of magnesium ribbon before (g)Mass of magnesium oxide after burning (g)
10.240.40
20.480.80
30.721.16
a. Explain why the mass increases when magnesium burns, even though nothing appears to be added.
[1]
b. Using the pattern in Experiments 1 and 2, identify which result appears anomalous, and explain why.
[2]
Show complete worked solution
(a)
Magnesium combines with oxygen from the air during burning, and this oxygen's mass is added to the solid, forming magnesium oxide — nothing is created, atoms from the air have simply combined with the magnesium.
(b)
Experiments $1$ and $2$ both show a mass ratio of about $0.40/0.24=1.67$ and $0.80/0.48=1.67$. Following this pattern, Experiment $3$ should give $0.72\times1.67\approx1.20\,\text{g}$, but only $1.16\,\text{g}$ was obtained — so Experiment 3 is anomalous (slightly low, possibly due to incomplete reaction or product lost).
QUESTION 12 5 marks Criterion C
Medium
Mass of copper before (g)Mass of copper oxide after (g)
2.02.5
a. Calculate the mass of oxygen that combined with the copper.
[1]
b. Using relative atomic masses ($Cu=64$, $O=16$), calculate the relative formula mass of copper oxide, $CuO$.
[2]
c. Using your answers, explain whether the ratio of masses in this experiment is consistent with the formula $CuO$.
[2]
Show complete worked solution
(a)
$$ 2.5 - 2.0 = 0.5\,\text{g} $$
(b)
$$ 64 + 16 = 80 $$
(c)
The formula $CuO$ means $Cu$ and $O$ combine in a $1:1$ atom ratio, i.e. a mass ratio of $64:16 = 4:1$. In the experiment, $2.0\,\text{g}$ of copper gained $0.5\,\text{g}$ of oxygen, a ratio of $2.0:0.5=4:1$ — this matches the ratio predicted by the formula $CuO$, so the experiment is consistent with this formula.
QUESTION 13 4 marks Criterion C
Medium

Relative atomic masses: $H=1$, $C=12$, $O=16$, $Ca=40$.

a. Calculate the relative formula mass of water, $H_2O$.
[1]
b. Calculate the relative formula mass of carbon dioxide, $CO_2$.
[1]
c. Calculate the relative formula mass of calcium carbonate, $CaCO_3$.
[2]
Show complete worked solution
(a)
$$ (2\times1) + 16 = 18 $$
(b)
$$ 12 + (2\times16) = 44 $$
(c)
$$ 40 + 12 + (3\times16) = 40+12+48 = 100 $$
QUESTION 14 5 marks Criterion C
Medium

Magnesium oxide, $MgO$, has a relative formula mass of $40$ ($Mg=24$, $O=16$).

a. Calculate the percentage of magnesium in magnesium oxide by mass.
[2]
b. Calculate the percentage of oxygen in magnesium oxide by mass.
[1]
c. A sample of magnesium oxide has a mass of $15\,\text{g}$. Calculate the mass of magnesium it contains.
[2]
Show complete worked solution
(a)
$$ \frac{24}{40}\times100 = 60\% $$
(b)
$$ 100\% - 60\% = 40\% $$
(c)
$$ 60\% \text{ of } 15 = 0.60 \times 15 = 9\,\text{g} $$
QUESTION 15 5 marks Criterion C
Hard
Trial12345
Mass loss (g)0.880.910.851.340.89
a. Identify the anomalous result.
[1]
b. Calculate the mean mass loss, excluding the anomalous result.
[2]
c. Suggest a possible reason for the anomalous reading.
[2]
Show complete worked solution
(a)
Trial $4$ ($1.34\,\text{g}$) is anomalous.
(b)
$$ \frac{0.88+0.91+0.85+0.89}{4} = \frac{3.53}{4} = 0.8825 \approx 0.88\,\text{g} $$
(c)
Possible reasons include: extra acid or carbonate accidentally used in that trial, a longer time left before reweighing (allowing more gas to escape), or a misreading/recording error on the balance.
QUESTION 16 6 marks Criterion C
Hard

A student is told a white solid is pure calcium carbonate ($CaCO_3$, $M_r=100$). They heat $5.0\,\text{g}$ of the solid completely, and it leaves $2.6\,\text{g}$ of solid calcium oxide residue ($CaO$, $M_r=56$) after the carbon dioxide has escaped.

a. Calculate the expected mass of calcium oxide if the $5.0\,\text{g}$ sample were $100\%$ pure calcium carbonate. (Use: expected mass of $CaO$ = mass of $CaCO_3$ $\times \dfrac{56}{100}$.)
[2]
b. Compare this expected mass to the actual mass obtained ($2.6\,\text{g}$), and use this to evaluate whether the sample was likely pure calcium carbonate.
[2]
c. Suggest one reason (other than impurity in the original sample) why the actual mass could be lower than the expected mass.
[2]
Show complete worked solution
(a)
$$ 5.0 \times \frac{56}{100} = 2.8\,\text{g} $$
(b)
The actual mass ($2.6\,\text{g}$) is slightly less than the expected mass ($2.8\,\text{g}$). This suggests the original sample probably was not $100\%$ pure calcium carbonate (an impurity that didn't decompose the same way would reduce the calcium oxide yield), or that some product was lost during the experiment.
(c)
Some of the fine calcium oxide powder could have been lost — for example stuck to the container, blown away, or spilt during heating or transfer — or the sample may not have been heated for long enough/hot enough to fully decompose.
QUESTION 17 3 marks Criterion C
Easy
ChangeType
Melting ice into waterPhysical
Rusting of ironChemical
Dissolving salt in waterPhysical
Burning woodPhysical
a. Identify which row in the table has been classified incorrectly.
[1]
b. State the correct classification for this change, and give one piece of evidence that supports it being this type of change.
[2]
Show complete worked solution
(a)
The "burning wood" row is incorrect.
(b)
Burning wood is a chemical change. Evidence: it produces new substances (ash, smoke, and gases) that are very different from wood and cannot easily be turned back into wood — the formation of new substances is a key sign of a chemical change, unlike a physical change.
QUESTION 18 5 marks Criterion D
Medium

Ammonia ($NH_3$) is a compound used to manufacture nitrogen-based fertilizers, which farmers spread on fields to help crops grow.

Discuss one benefit and one drawback of using nitrogen-based fertilizers made from ammonia.

Show complete worked solution

Benefit: Nitrogen fertilizers supply plants with the nitrogen compounds they need to make proteins and chlorophyll, significantly increasing crop yields. This helps farmers grow enough food to feed the world's growing population on limited farmland.

Drawback: Excess fertilizer that isn't absorbed by plants can be washed off fields by rain into rivers and lakes (runoff), causing eutrophication — the extra nutrients make algae grow rapidly, blocking light, and when the algae die and decompose, the process uses up oxygen in the water, which can kill fish and other aquatic life.

QUESTION 19 5 marks Criterion D
Medium

Plastics are compounds called polymers, made mostly from carbon and hydrogen atoms and manufactured from crude oil. They are used to make many everyday products, from packaging to medical equipment.

Discuss one benefit and one drawback of the widespread use of plastic.

Show complete worked solution

Benefit: Plastics are cheap, lightweight, waterproof, and can be moulded into almost any shape, making them extremely useful for hygienic, single-use medical equipment (e.g. syringes) and for protecting food during transport and storage, which reduces food waste and spoilage.

Drawback: Most plastics are not biodegradable and take hundreds of years to break down naturally, so plastic waste that ends up in landfill or the ocean builds up over time. It harms wildlife (e.g. animals mistaking plastic for food or becoming entangled) and gradually breaks down into microplastics that are now found throughout the environment and food chain.

QUESTION 20 6 marks Criterion D
Hard

Carbon dioxide ($CO_2$) is a compound essential for photosynthesis, and is also used industrially (e.g. in fizzy drinks and fire extinguishers). It is also a greenhouse gas. Atmospheric $CO_2$ concentration has risen from about $280$ parts per million (ppm) before industrialisation to over $420$ ppm today, largely due to burning fossil fuels.

Evaluate the impact of carbon dioxide, discussing both a benefit and a concern, using the data given.

Show complete worked solution

Benefit: Carbon dioxide is essential for photosynthesis, the process plants use to make their own food and release oxygen, so almost all life on Earth ultimately depends on it. It also has direct industrial uses, such as carbonating drinks and in fire extinguishers, where it starves fires of oxygen to put them out.

Concern: $CO_2$ is a greenhouse gas, meaning it traps heat in the atmosphere. The rise in concentration from about $280$ ppm to over $420$ ppm shown by the data — an increase of roughly $50\%$ — is strongly linked to increased burning of fossil fuels since industrialisation, and this build-up is a major driver of global climate change, including rising average temperatures, melting ice, and more extreme weather. This is why reducing $CO_2$ emissions, for example by switching to renewable energy, has become such an urgent global priority.

Mixtures and Separation Techniques 20 questions

QUESTION 1 4 marks Criterion A
Easy

Mixtures and compounds are both made of more than one substance combined, but they are different.

a. State two ways a mixture is different from a compound.
[2]
b. Classify each of the following as a mixture or a compound: air; pure water; salt water; carbon dioxide.
[2]
Show complete worked solution
(a)
In a mixture, the substances are not chemically bonded together (just physically mixed), can be present in any proportion, and can be separated again using physical methods. In a compound, the elements are chemically bonded in a fixed ratio, and can only be separated back into elements by a chemical reaction.
(b)
Air: mixture (mainly nitrogen and oxygen gases, not chemically bonded). Pure water: compound ($H_2O$, chemically bonded). Salt water: mixture (salt dissolved in water, not chemically combined). Carbon dioxide: compound ($CO_2$, chemically bonded).
QUESTION 2 4 marks Criterion A
Easy

State the most suitable technique to separate each of the following mixtures.

a. Sand from water.
[1]
b. Salt from salt water (recovering the dry salt).
[1]
c. Iron filings from a mixture with sulfur powder.
[1]
d. The different coloured dyes that make up a black ink.
[1]
Show complete worked solution
(a)
Filtration.
(b)
Evaporation (crystallisation).
(c)
Magnetism (using a magnet).
(d)
Chromatography.
QUESTION 3 4 marks Criterion A
Medium
funnel filter paper residue filtrate beaker

The diagram shows the apparatus used to separate a mixture of sand and water.

a. State the name of the solid collected in the filter paper (the substance that cannot pass through).
[1]
b. State the name of the liquid that passes through the filter paper and collects in the beaker below.
[1]
c. Explain, in terms of particle size, why the sand is trapped by the filter paper but the water passes through.
[2]
Show complete worked solution
(a)
The residue (the sand).
(b)
The filtrate (the water).
(c)
The filter paper has very tiny holes (pores). The sand particles are too large to fit through these holes, so they are trapped and stay behind as residue. The water particles are much smaller than the holes, so they pass straight through as filtrate.
QUESTION 4 5 marks Criterion A
Medium
water out water in heat thermometer condenser distillate flask

The diagram shows simple distillation apparatus, used to separate a solvent (e.g. water) from a dissolved solid (e.g. salt) in a solution, or to separate two liquids with different boiling points.

a. State the property of the substances in the mixture that simple distillation relies on to separate them.
[1]
b. Describe what happens to the liquid in the flask as it is heated, and what happens in the condenser.
[2]
c. Explain why the thermometer is positioned at the top of the flask, level with the side-arm, rather than in the liquid itself.
[2]
Show complete worked solution
(a)
The difference in boiling points of the substances.
(b)
The substance with the lower boiling point evaporates first as the flask is heated, forming a vapour. This vapour travels into the condenser, where it is cooled by the surrounding cold water and condenses back into a liquid, which is collected in the beaker as the distillate. The dissolved solid (or higher-boiling liquid) stays behind in the flask.
(c)
The thermometer needs to measure the temperature of the vapour leaving the flask (which shows the boiling point of whatever is currently being collected), not the temperature of the liquid mixture remaining behind, which can be different (and would not indicate when a substance is actually boiling off).
QUESTION 5 4 marks Criterion A
Medium

Different separation techniques rely on different physical properties of the substances in a mixture.

a. State the property that filtration depends on.
[1]
b. State the property that chromatography depends on.
[1]
c. State the property that fractional distillation depends on when separating a mixture of liquids (e.g. crude oil).
[1]
d. A mixture contains iron powder and sulfur powder, but no filter paper, water, or heat is available. Which single technique could still separate them, and why?
[1]
Show complete worked solution
(a)
Particle size — insoluble solid particles are too big to pass through the tiny pores in the filter paper.
(b)
How soluble each substance is in the solvent, and how strongly it is attracted to (sticks to) the paper — substances that are more soluble and less attracted to the paper travel further up it.
(c)
Differences in the boiling points of the liquids in the mixture.
(d)
Magnetism — using a magnet. Iron is magnetic and sulfur is not, so a magnet will attract and lift out the iron, leaving the sulfur behind.
QUESTION 6 3 marks Criterion A
Medium

In paper chromatography, the baseline is drawn in pencil, and must be positioned above the level of the solvent in the beaker.

a. Explain why pencil, not ink, is used to draw the baseline.
[1]
b. Explain why the baseline must be drawn above the level of the solvent in the beaker before the paper is placed in.
[2]
Show complete worked solution
(a)
Pencil marks are made of graphite/carbon, which is insoluble in the solvent, so it will not dissolve and interfere with the sample. Ink, however, would dissolve and run, mixing with the sample being separated.
(b)
If the baseline (and sample spot) were below the solvent level, the sample would dissolve straight into the solvent in the beaker and wash away, instead of being carried up the paper by the rising solvent — so no separation would occur.
QUESTION 7 6 marks Criterion A
Hard

A student has a mixture of sand, salt, and water all mixed together, and needs to separate it into three pure samples: dry sand, dry salt, and pure water.

a. Describe the first step needed to separate the mixture, and name the technique used.
[2]
b. Describe how you would obtain dry salt from the filtrate (salt solution), and explain why this technique works.
[2]
c. Explain how you could instead collect the water itself as a pure liquid, rather than letting it evaporate away, and name this technique.
[2]
Show complete worked solution
(a)
Filter the mixture. Since sand is insoluble in water but salt is not, this traps the sand as the residue in the filter paper, while the salt solution (salty water) passes through as the filtrate.
(b)
Evaporate the water from the salt solution by heating it, leaving the dissolved salt behind as dry crystals. This works because water has a much lower boiling point than salt, so it evaporates away while the salt does not.
(c)
Use simple distillation instead of open evaporation: heat the salt solution, and pass the water vapour given off through a condenser, where it cools and condenses back into a liquid, collected separately as pure water. The salt (with a much higher boiling point) stays behind in the flask.
QUESTION 8 7 marks Criterion B
Medium

A student wants to investigate how the temperature of a salt solution affects the time it takes to fully evaporate, leaving dry salt crystals behind.

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 needed.
[3]
Show complete worked solution
(a)
Independent variable: the temperature the solution is heated at. Dependent variable: the time taken for all the visible water to evaporate, leaving dry salt.
(b)
The same volume/concentration of salt solution used each time, and the same type/size of evaporating dish. Why control volume: a larger volume of solution would naturally take longer to fully evaporate regardless of temperature, making the comparison between temperatures unfair.
(c)
  1. Measure $20\,\text{cm}^3$ of salt solution into an evaporating dish using a measuring cylinder.
  2. Place the dish on a tripod and gauze over a Bunsen burner (or on a hotplate) set to the first test temperature, and start a stopwatch.
  3. Heat until all the visible liquid has evaporated and only dry salt crystals remain, and record the time taken.
  4. Repeat using the same volume of solution at several different temperatures, repeating each temperature at least twice for reliability.
QUESTION 9 6 marks Criterion B
Medium

A student wants to investigate whether the pore size of filter paper affects how quickly a muddy water mixture can be filtered.

a. State the independent and dependent variables.
[2]
b. State two variables that should be controlled.
[2]
c. Suggest how the results could be used to decide which filter paper is most suitable for cleaning drinking water on a large scale, referring to more than just speed.
[2]
Show complete worked solution
(a)
Independent variable: the type/pore size of filter paper used. Dependent variable: the time taken for a fixed volume of the muddy mixture to filter through.
(b)
The same volume and concentration of muddy water mixture used each time, and the same size of funnel and method of adding the mixture.
(c)
The fastest filter paper isn't necessarily the best choice — the water's clarity (whether fine particles still pass through) should also be checked for each paper. A paper that filters very quickly but lets more fine particles through would produce lower-quality water, so testing both speed and clarity together would show which filter paper balances the two best.
QUESTION 10 7 marks Criterion B
Hard

A student performs chromatography to test which coloured dyes are in a sample of ink. They draw the baseline dot on the paper using a black fountain pen (ink), place the paper in a beaker with the solvent covering the baseline dot, and leave it uncovered without a lid.

a. Identify two errors in this method.
[2]
b. Suggest a corrected method that avoids both errors.
[3]
c. Explain why the beaker should be covered with a lid during the experiment.
[2]
Show complete worked solution
(a)
(1) Using ink for the baseline — the ink mark itself will dissolve and travel up the paper, mixing with and interfering with the sample being tested. (2) Placing the paper so the solvent covers the baseline dot — the sample would simply wash off into the solvent in the beaker rather than being carried up the paper, so no separation would happen.
(b)
Draw the baseline using a pencil, a small distance above the bottom of the paper. Place a small spot of the ink sample onto the pencil baseline and allow it to dry. Lower the paper into the beaker so that the solvent level is below the baseline/spot (only the very bottom of the paper touches the solvent), and cover the beaker with a lid.
(c)
A lid keeps the atmosphere inside the beaker saturated with solvent vapour, preventing the solvent from evaporating too quickly from the paper as it rises. This gives a clearer, more even, and more reliable separation of the dyes.
QUESTION 11 3 marks Criterion C
Easy
Temperature (°C)020406080
Solubility (g per 100 g water)2035506560
a. Describe the trend shown by the data, ignoring the anomalous value.
[1]
b. Identify the anomalous value in the table, and state what its value should be if the pattern had continued.
[2]
Show complete worked solution
(a)
Solubility increases steadily as temperature increases — by a constant $15\,\text{g}$ for every $20\,^\circ\text{C}$ rise.
(b)
The value at $80\,^\circ\text{C}$ ($60\,\text{g}$) is anomalous. Following the pattern ($+15\,\text{g}$ per $20\,^\circ\text{C}$), it should be about $80\,\text{g}$.
QUESTION 12 5 marks Criterion C
Medium
Chromatogram (dye spots X and Y) 0 cm 1 2 3 4 5 6 7 8 cm baseline solvent front X (3.2 cm) Y (5.6 cm)

The chromatogram shows the distances travelled by two dye spots, X and Y, from the baseline, and the distance travelled by the solvent front.

a. State the formula for calculating the Rf value of a spot.
[1]
b. Calculate the Rf value for spot X.
[2]
c. Calculate the Rf value for spot Y.
[2]
Show complete worked solution
(a)
$$ R_f = \frac{\text{distance travelled by spot}}{\text{distance travelled by solvent front}} $$ (both measured from the baseline)
(b)
$$ R_f = \frac{3.2}{8.0} = 0.40 $$
(c)
$$ R_f = \frac{5.6}{8.0} = 0.70 $$
QUESTION 13 5 marks Criterion C
Medium
Time (min)024681012
Temperature (°C)20457090100100100
a. Describe what happens to the temperature between $t=8$ min and $t=12$ min.
[1]
b. Explain, in terms of what is happening to the liquid, why the temperature stops rising at this point even though heating continues.
[2]
c. State the boiling point of the substance shown by this data, and identify what it most likely is.
[2]
Show complete worked solution
(a)
The temperature stays constant (flat) at $100\,^\circ\text{C}$.
(b)
The liquid has reached its boiling point ($100\,^\circ\text{C}$). All the extra heat energy supplied is now being used to change the liquid into vapour (boiling), rather than to raise its temperature further, so the temperature stays constant while the liquid boils away.
(c)
$100\,^\circ\text{C}$ — this is the boiling point of pure water, so the liquid is most likely (pure) water.
QUESTION 14 5 marks Criterion C
Medium
Volume of seawater evaporated (cm³)100200300400
Mass of dry salt collected (g)3.57.010.513.5
a. Describe the relationship between the volume of seawater evaporated and the mass of salt collected, ignoring the anomalous result.
[1]
b. Identify the anomalous result, and state what its value should be if the pattern had continued exactly.
[2]
c. Use the non-anomalous data to calculate the concentration of salt in this seawater sample, in g per 100 cm³.
[2]
Show complete worked solution
(a)
The mass of salt collected is directly proportional to the volume of seawater evaporated — it increases by about $3.5\,\text{g}$ for every extra $100\,\text{cm}^3$.
(b)
The $400\,\text{cm}^3$ result ($13.5\,\text{g}$) is anomalous. Based on the pattern ($3.5\,\text{g}$ per $100\,\text{cm}^3$), it should be $14.0\,\text{g}$.
(c)
Using the $300\,\text{cm}^3$ result: $$ \frac{10.5}{300}\times100 = 3.5\,\text{g per }100\,\text{cm}^3 $$ (consistent with the other, non-anomalous rows).
QUESTION 15 5 marks Criterion C
Hard
DyeReference dye AReference dye BReference dye CUnknown ink spot 1Unknown ink spot 2
Rf value0.250.500.750.260.74
a. Using the reference Rf values, identify which two reference dyes are most likely present in the unknown ink.
[2]
b. Explain why Rf values, rather than just how far a spot travelled, are used to identify substances by chromatography.
[2]
c. State one condition that must be kept the same for Rf values to be validly compared between the reference dyes and the unknown ink.
[1]
Show complete worked solution
(a)
Dye A (Rf $0.25$, very close to unknown spot 1's $0.26$) and Dye C (Rf $0.75$, very close to unknown spot 2's $0.74$).
(b)
Rf values are a ratio (spot distance ÷ solvent-front distance), so they stay the same for a given substance in a given solvent regardless of exactly how far the solvent itself travelled (e.g. a different paper size or run time). This makes Rf values a reliable way to compare and identify substances between different experiments, unlike raw distances, which would only match if the solvent happened to travel exactly the same distance each time.
(c)
The same solvent (and the same type of paper, at the same temperature) must be used for both the reference dyes and the unknown sample.
QUESTION 16 5 marks Criterion C
Hard
Trial12345
Time to filter 50 cm³ (s)4245447143
a. Identify the anomalous result.
[1]
b. Calculate the mean filtration time, excluding the anomalous result.
[2]
c. Suggest a possible cause of the anomalous result, and explain how repeating the trial improves the reliability of the conclusion.
[2]
Show complete worked solution
(a)
Trial $4$ ($71\,\text{s}$).
(b)
$$ \frac{42+45+44+43}{4} = \frac{174}{4} = 43.5\,\text{s} $$
(c)
The filter paper may have partly torn or become blocked, or leftover residue from a previous trial slowed that one trial down. Repeating trials, identifying and excluding an anomaly like this, and then averaging the consistent results gives a mean that better represents the true, typical filtration time rather than being skewed by one unusual result.
QUESTION 17 3 marks Criterion C
Easy
MixtureBest separation technique
Sand and waterFiltration
Salt dissolved in water (recovering the salt)Evaporation
Iron filings and sandMagnetism
Different coloured dyes in a sweetDistillation
a. Identify which row pairs the mixture with an incorrect separation technique.
[1]
b. State the correct technique for separating different coloured dyes, and briefly explain why distillation would not work well for this.
[2]
Show complete worked solution
(a)
The "different coloured dyes" row (paired with "distillation").
(b)
The correct technique is chromatography. Distillation separates substances based on differences in boiling point, but dyes dissolved together generally don't have distinctly different, easily separable boiling points suited to this (and many would decompose before boiling) — chromatography instead separates them based on differing solubility, which works well for dyes.
QUESTION 18 5 marks Criterion D
Medium

Some countries with limited fresh water, but access to the sea, use large desalination plants that use distillation to turn seawater into drinking water.

Discuss one benefit and one drawback of using distillation for desalination on a large scale.

Show complete worked solution

Benefit: Distillation reliably removes salt and other dissolved substances from seawater, producing safe drinking water for regions without enough natural fresh water sources (such as many desert or drought-affected countries), which can be essential for supporting the population's health and agriculture.

Drawback: Distillation requires heating huge volumes of seawater to boiling point, which uses a very large amount of energy, making desalination expensive and, if that energy comes from burning fossil fuels, contributing to greenhouse gas emissions and climate change. The leftover concentrated salty waste (brine) also needs careful disposal, as pumping large amounts back into the sea can harm marine ecosystems near the plant.

QUESTION 19 5 marks Criterion D
Medium

Water treatment plants use filtration (alongside other steps) to remove solid particles and some microorganisms from river or lake water before it is supplied as drinking water to homes.

Discuss one benefit and one drawback/limitation of relying on filtration to help provide clean drinking water.

Show complete worked solution

Benefit: Filtration removes suspended solid particles, dirt, and some larger microorganisms from water cheaply and effectively, making water clearer and safer. This has dramatically reduced the spread of waterborne diseases in places where treated tap water is available, saving many lives.

Drawback/limitation: Filtration alone cannot remove substances that are fully dissolved in water, or very small microorganisms and viruses that can pass straight through the filter — so it must be combined with other treatments, such as adding chlorine, to make water fully safe to drink. In places without funding or infrastructure for a full treatment process, people may still be left with unsafe drinking water.

QUESTION 20 6 marks Criterion D
Hard

Old electronic devices (e-waste) contain valuable metals, such as copper and gold, mixed together with plastics and other materials. Recycling plants use a combination of separation techniques (including magnetism and processes based on density) to recover these metals so they can be reused.

Evaluate the impact of recycling metals from e-waste in this way, discussing both a benefit and a concern.

Show complete worked solution

Benefit: Recovering metals like copper and gold from old devices reduces the need to mine new metal ores from the ground, conserving a limited natural resource and avoiding some of the habitat destruction, energy use, and pollution associated with mining. It also reduces the amount of e-waste sent to landfill, where toxic substances inside old electronics could otherwise leak into the soil and water.

Concern: E-waste recycling, if not carried out carefully and safely, can itself release harmful substances — some older electronics contain toxic materials such as lead or mercury, and the processes used to separate and extract metals (especially in unregulated settings) can expose workers to these substances or release them into the environment. This means e-waste recycling needs to be carried out in properly regulated, safe facilities, so that its environmental and health benefits aren't outweighed by new risks created during the recycling process itself.