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GCSE ChemistryAQA, Edexcel and OCR

Chemical analysis: GCSE questions with answers

40 chemistry questions on chemical analysis. Pick your answer, then tap to see if you were right and why. Every answer comes from the Show Working science bank, checked against the GCSE content every board in England teaches.

Quiz yourself: 40 questions 2 6-mark questions

01 / Quiz yourselfTry it, then tap to check

Chemical analysis questions to try

Choose your answer before you open it. Then read why, so it sticks.

01 / Chemical analysisBoth tiers

In chemistry, what is a pure substance?

  1. AA single element or compound
  2. BA substance that is safe to drink
  3. CA substance with nothing added to it
  4. DA mixture of natural substances
Show the answer

Answer: A A single element or compound

In chemistry, a pure substance is a single element or a single compound, not mixed with anything else. In everyday use “pure” means natural or with nothing added, so pure orange juice is still a mixture of water, sugars, acids and more.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.1.1 Pure substances

02 / Chemical analysisBoth tiers

How can you tell from its melting point that a solid is pure?

  1. AIt melts at a temperature below 0 °C
  2. BIt melts at one sharp, fixed temperature
  3. CIt melts over a wide range of temperatures
  4. DIt never melts, however hot it gets
Show the answer

Answer: B It melts at one sharp, fixed temperature

A pure substance melts and boils at specific, fixed temperatures. A sharp melting point that matches the known value for the pure substance suggests that the sample is pure.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.1.1 Pure substances

03 / Chemical analysisBoth tiers

How do impurities usually change the melting point of a solid?

  1. AThey have no effect on it at all
  2. BThey lower it and make it melt over a range
  3. CThey raise it to one sharp temperature
  4. DThey make it melt at exactly 0 °C
Show the answer

Answer: B They lower it and make it melt over a range

Impurities usually lower the melting point of a substance and make it melt gradually over a range of temperatures. So melting point data can show whether a sample is pure: compare it with the known melting point of the pure substance.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.1.1 Pure substances

04 / Chemical analysisBoth tiers

What is a formulation?

  1. AA compound made by electrolysis
  2. BA mixture designed as a useful product
  3. CA single pure compound
  4. DA substance with only one kind of atom
Show the answer

Answer: B A mixture designed as a useful product

A formulation is a mixture designed as a useful product. Each part is added in a measured amount and has a particular job. Fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods are all formulations.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.1.2 Formulations

05 / Chemical analysisBoth tiers

Which is the best method to separate sand from a mixture of sand and water?

  1. AFractional distillation
  2. BFiltration
  3. CCrystallisation
  4. DChromatography
Show the answer

Answer: B Filtration

Filtration separates an insoluble solid from a liquid. The solid stays on the filter paper as the residue and the liquid passes through as the filtrate. A dissolved substance, such as salt in water, passes through with the liquid.

Checked against AQA GCSE Chemistry (8462) specification, 4.1.1.2 Mixtures

06 / Chemical analysisBoth tiers

Which method gets solid copper sulfate crystals from copper sulfate solution?

  1. AFractional distillation
  2. BSimple distillation
  3. CFiltration
  4. DCrystallisation
Show the answer

Answer: D Crystallisation

Crystallisation gets a soluble solid out of its solution. Heat to evaporate some of the solvent until the solution is saturated, then leave it to cool so crystals form. Filter off the crystals and dry them.

Checked against AQA GCSE Chemistry (8462) specification, 4.1.1.2 Mixtures

07 / Chemical analysisBoth tiers

Which method gets pure water from salty water?

  1. ASimple distillation
  2. BFiltration
  3. CCrystallisation
  4. DChromatography
Show the answer

Answer: A Simple distillation

Simple distillation separates a solvent from a solution. The solution is heated, the solvent boils off as a vapour and the vapour is cooled in a condenser, where it condenses back to a liquid. The dissolved solid stays behind in the flask.

Checked against AQA GCSE Chemistry (8462) specification, 4.1.1.2 Mixtures

08 / Chemical analysisBoth tiers

Which method separates a mixture of ethanol and water?

  1. AFractional distillation
  2. BFiltration
  3. CCrystallisation
  4. DChromatography
Show the answer

Answer: A Fractional distillation

Fractional distillation separates liquids that mix together, such as ethanol and water, using their different boiling points. The fractionating column is hottest at the bottom and coolest at the top, so the liquid with the lowest boiling point reaches the top and distils over first.

Checked against AQA GCSE Chemistry (8462) specification, 4.1.1.2 Mixtures

09 / Chemical analysisBoth tiers

In paper chromatography, what is the mobile phase?

  1. AThe ink spot
  2. BThe paper
  3. CThe pencil line
  4. DThe solvent
Show the answer

Answer: D The solvent

Chromatography has a stationary phase that does not move and a mobile phase that moves through it. In paper chromatography the paper is the stationary phase and the solvent soaking up the paper is the mobile phase.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.1.3 Chromatography

10 / Chemical analysisBoth tiers

On a chromatogram, why does one dye travel further up the paper than another?

  1. AIt has a higher melting point
  2. BIt spends more time in the mobile phase
  3. CIt is heavier than the other dye
  4. DIt spends more time in the stationary phase
Show the answer

Answer: B It spends more time in the mobile phase

Each substance is shared, or distributed, between the two phases. A substance that is more soluble in the solvent, or less attracted to the paper, spends more time in the mobile phase and moves further. One that does not dissolve stays on the start line.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.1.3 Chromatography

11 / Chemical analysisBoth tiers

Why is the start line on chromatography paper drawn in pencil, not ink?

  1. AInk would make the paper too wet
  2. BPencil does not dissolve in the solvent
  3. CPencil lines are easier to see
  4. DPencil helps the solvent rise
Show the answer

Answer: B Pencil does not dissolve in the solvent

Pencil (graphite) does not dissolve, so the start line will not move or make spots of its own. The start line sits above the solvent so the samples are carried up the paper instead of washing out into the solvent in the beaker.

Checked against AQA GCSE Chemistry (8462) specification, Required practical activity 6: chromatography

12 / Chemical analysisBoth tiers

How is an Rf value worked out?

  1. ASpot distance divided by solvent distance
  2. BSpot distance multiplied by solvent distance
  3. CSolvent distance divided by spot distance
  4. DSpot distance minus solvent distance
Show the answer

Answer: A Spot distance divided by solvent distance

Rf = distance moved by the substance ÷ distance moved by the solvent. Both distances are measured from the start line: to the centre of the spot, and to the solvent front.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.1.3 Chromatography

13 / Chemical analysisBoth tiers

Why is an Rf value never more than 1?

  1. AThe solvent always evaporates first
  2. BThe paper absorbs part of each spot
  3. CRf values are always given as percentages
  4. DA spot never moves further than the solvent
Show the answer

Answer: D A spot never moves further than the solvent

An Rf value is a ratio of two distances, so it has no unit, and it is never more than 1 because no spot moves further than the solvent. In the same solvent a substance always has the same Rf value, which helps to identify it.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.1.3 Chromatography

14 / Chemical analysisBoth tiers

An ink gives three separate spots on a chromatogram. What does this show?

  1. AIt contains exactly one substance
  2. BIt contains at least three substances
  3. CIt is a pure compound
  4. DIt contains three solvents
Show the answer

Answer: B It contains at least three substances

Each spot is a different substance, so a mixture gives several spots. A substance has a different Rf value in different solvents, so its spot can only be matched with reference spots run in the same solvent.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.1.3 Chromatography

15 / Chemical analysisBoth tiers

You want the solid salt from salt solution. Which is the best method?

  1. AFractional distillation
  2. BFiltration
  3. CCrystallisation
  4. DChromatography
Show the answer

Answer: C Crystallisation

Choose a method from what you need to separate: filtration for an insoluble solid, crystallisation for a dissolved solid, simple distillation for the solvent, fractional distillation for liquids that mix, and chromatography for dyes or other soluble substances in a mixture.

Checked against AQA GCSE Chemistry (8462) specification, 4.1.1.2 Mixtures

16 / Chemical analysisBoth tiers

Which is the best method to collect the water from a sample of ink?

  1. ASimple distillation
  2. BFiltration
  3. CCrystallisation
  4. DChromatography
Show the answer

Answer: A Simple distillation

Simple distillation boils off the water from ink and condenses it, leaving the dyes behind. To get salt from rock salt, dissolve the salt in water, filter off the insoluble sand, then crystallise the salt from the filtrate.

Checked against AQA GCSE Chemistry (8462) specification, 4.1.1.2 Mixtures

17 / Chemical analysisBoth tiers

How can chromatography show that a substance is pure?

  1. AIt gives no spots at all
  2. BIt turns the solvent a new colour
  3. CIt gives a single spot
  4. DIt gives several spots
Show the answer

Answer: C It gives a single spot

A pure substance gives a single spot on a chromatogram. An impure sample gives extra spots, one for each other substance that moves. Running a sample next to a known pure reference on the same paper shows whether it contains impurities.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.1.3 Chromatography

18 / Chemical analysisBoth tiers

What does the law of conservation of mass say?

  1. AAtoms are made during a reaction
  2. BProducts are always lighter than reactants
  3. CMass always increases in a reaction
  4. DNo atoms are lost or made in a reaction
Show the answer

Answer: D No atoms are lost or made in a reaction

In a chemical reaction atoms are rearranged, not made or destroyed, so the total mass of the products equals the total mass of the reactants. That is why a balanced equation has the same number of each type of atom on both sides.

Checked against AQA GCSE Chemistry (8462) specification, 4.3.1.1 Conservation of mass and balanced chemical equations

19 / Chemical analysisBoth tiers

Magnesium ribbon is heated in an open crucible and turns into magnesium oxide. What happens to the mass of the solid?

  1. AIt falls to zero
  2. BIt decreases
  3. CIt stays the same
  4. DIt increases
Show the answer

Answer: D It increases

When a metal reacts with oxygen in an open container, the mass of the solid goes up, because oxygen atoms from the air become part of the metal oxide. Mass is still conserved: the extra mass came from outside the container.

Checked against AQA GCSE Chemistry (8462) specification, 4.3.1.3 Mass changes when a reactant or product is a gas

20 / Chemical analysisBoth tiers

Marble chips react with acid in an open flask on a balance. Why does the reading go down?

  1. AThe marble chips turn into energy
  2. BCarbon dioxide gas escapes into the air
  3. CAtoms are destroyed in the reaction
  4. DThe products always weigh less
Show the answer

Answer: B Carbon dioxide gas escapes into the air

If a product is a gas, it can escape from an open container, so the mass on the balance goes down. In a sealed container the mass stays the same, because no atoms are lost: in an open one they have simply moved into the air.

Checked against AQA GCSE Chemistry (8462) specification, 4.3.1.3 Mass changes when a reactant or product is a gas

21 / Chemical analysisBoth tiers

What is the relative formula mass (Mr) of a compound?

  1. AThe sum of the relative atomic masses of its atoms
  2. BThe number of atoms in its formula
  3. CThe sum of the atomic numbers of all the atoms in it
  4. DThe mass number of its heaviest atom
Show the answer

Answer: A The sum of the relative atomic masses of its atoms

Relative formula mass, Mr, is the sum of the relative atomic masses of all the atoms in the formula. A number outside a bracket multiplies everything inside it, so Ca(OH)₂ has two O and two H. Like Ar, Mr is a ratio and has no unit.

Checked against AQA GCSE Chemistry (8462) specification, 4.3.1.2 Relative formula mass

22 / Chemical analysisBoth tiers

In a balanced equation, each Mr is multiplied by the number in front of its formula. How do the totals for the reactants and the products compare?

  1. AThey are equal
  2. BThe products are always larger
  3. CIt depends on the state of each
  4. DThe reactants are always larger
Show the answer

Answer: A They are equal

When using Mr values with a balanced equation, multiply each Mr by the number in front of its formula. The total for the reactants then equals the total for the products, because mass is conserved.

Checked against AQA GCSE Chemistry (8462) specification, 4.3.1.2 Relative formula mass

23 / Chemical analysisHigher only

What is the value of the Avogadro constant?

  1. A6.02 × 10²³ per mole
  2. B6.02 × 10²² per mole
  3. C6.02 × 10⁻²³ per mole
  4. D3.00 × 10⁸ per mole
Show the answer

Answer: A 6.02 × 10²³ per mole

The Avogadro constant is the number of particles (atoms, molecules or ions) in one mole of a substance: 6.02 × 10²³ per mole. Some books round it to 6.0 × 10²³.

Checked against OpenStax Chemistry 2e, 3.1 Formula Mass and the Mole Concept

24 / Chemical analysisHigher only

One mole of a substance has a mass equal to what?

  1. A6.02 × 10²³ grams
  2. BIts Ar or Mr in grams
  3. CIts atomic number in grams
  4. DIts Ar or Mr in kilograms
Show the answer

Answer: B Its Ar or Mr in grams

The mole is the unit for an amount of substance. One mole of any substance contains the same number of particles, 6.02 × 10²³, and has a mass in grams equal to its relative atomic mass or relative formula mass.

Checked against OpenStax Chemistry 2e, 3.1 Formula Mass and the Mole Concept

25 / Chemical analysisHigher only

Which gives the number of moles in a mass of a substance?

  1. AMass multiplied by Mr
  2. BMass divided by the Avogadro constant
  3. CMass divided by Mr
  4. DMr divided by mass
Show the answer

Answer: C Mass divided by Mr

Number of moles = mass in grams ÷ Mr, so mass = moles × Mr. For the same mass, a substance with a larger Mr has fewer moles, because each mole of it is heavier.

Checked against OpenStax Chemistry 2e, 3.1 Formula Mass and the Mole Concept

26 / Chemical analysisHigher only

The masses of all the reactants and products are known. What must you find first to work out the balancing numbers?

  1. AThe volume of each substance
  2. BThe number of moles of each
  3. CThe atomic number of each element
  4. DThe total mass of the products
Show the answer

Answer: B The number of moles of each

To find the balancing numbers from masses, change each mass into moles by dividing by its Mr. Then divide by the smallest number of moles to get the simplest whole-number ratio. That ratio gives the balancing numbers.

Checked against OpenStax Chemistry 2e, 4.3 Reaction Stoichiometry

27 / Chemical analysisHigher only

What is the limiting reactant?

  1. AThe reactant with the smallest Mr
  2. BThe reactant that is used up first
  3. CThe reactant with the largest mass
  4. DThe reactant that is left over
Show the answer

Answer: B The reactant that is used up first

The limiting reactant is completely used up, so it decides how much product forms. The other reactant is in excess and some is left over. The amount of product is proportional to the amount of the limiting reactant.

Checked against OpenStax Chemistry 2e, 4.4 Reaction Yields

28 / Chemical analysisHigher only

In 2Mg + O₂ → 2MgO, how do the moles of MgO made compare with the moles of Mg used?

  1. AThere are half as many
  2. BThey are the same
  3. CThere are twice as many
  4. DThere are a quarter as many
Show the answer

Answer: B They are the same

To find a reacting mass: work out the moles of the substance you know (mass ÷ Mr), use the ratio of the numbers in the balanced equation to get the moles of the one you want, then multiply by its Mr.

Checked against OpenStax Chemistry 2e, 4.3 Reaction Stoichiometry

29 / Chemical analysisHigher only

How is the concentration of a solution in g/dm³ worked out?

  1. AMass of solvent divided by mass of solute
  2. BMass of solute multiplied by volume of solution
  3. CVolume of solution divided by mass of solute
  4. DMass of solute divided by volume of solution
Show the answer

Answer: D Mass of solute divided by volume of solution

Concentration in g/dm³ = mass of solute in grams ÷ volume of solution in dm³. There are 1000 cm³ in 1 dm³, so divide a volume in cm³ by 1000. The same mass of solute in a bigger volume gives a lower concentration.

Checked against OpenStax Chemistry 2e, 3.3 Molarity

30 / Chemical analysisHigher only · separate only

What volume does one mole of any gas take up at room temperature and pressure?

  1. A240 dm³
  2. B24 cm³
  3. C24 dm³
  4. D2.4 dm³
Show the answer

Answer: C 24 dm³

At room temperature and pressure, one mole of any gas takes up 24 dm³ (24 000 cm³). Equal numbers of moles of different gases have equal volumes, so volume in dm³ = moles × 24.

Checked against OpenStax Chemistry 2e, 9.3 Stoichiometry of Gaseous Substances, Mixtures, and Reactions

31 / Chemical analysisHigher only · separate only

In N₂(g) + 3H₂(g) ⇌ 2NH₃(g), how does the volume of hydrogen used compare with the volume of nitrogen?

  1. AIt is half as much
  2. BIt is a third as much
  3. CIt is three times as much
  4. DIt is the same
Show the answer

Answer: C It is three times as much

At the same temperature and pressure, gas volumes are in the same ratio as the moles in the balanced equation. In N₂ + 3H₂ ⇌ 2NH₃, each volume of nitrogen reacts with three volumes of hydrogen to make two volumes of ammonia.

Checked against OpenStax Chemistry 2e, 9.3 Stoichiometry of Gaseous Substances, Mixtures, and Reactions

32 / Chemical analysisHigher only · separate only

How is a concentration in mol/dm³ worked out?

  1. AMoles of solute divided by volume in dm³
  2. BMoles of solute multiplied by volume in dm³
  3. CVolume in dm³ divided by moles of solute
  4. DMass of solute divided by volume in cm³
Show the answer

Answer: A Moles of solute divided by volume in dm³

Concentration in mol/dm³ = moles of solute ÷ volume of solution in dm³. Since moles = mass ÷ Mr, dividing a concentration in g/dm³ by the Mr of the solute gives the concentration in mol/dm³.

Checked against OpenStax Chemistry 2e, 3.3 Molarity

33 / Chemical analysisHigher only · separate only

Which gives the moles of solute in a volume of solution?

  1. AConcentration (mol/dm³) × volume (cm³)
  2. BConcentration (mol/dm³) × volume (dm³)
  3. CVolume (dm³) ÷ concentration (mol/dm³)
  4. DConcentration (mol/dm³) ÷ volume (dm³)
Show the answer

Answer: B Concentration (mol/dm³) × volume (dm³)

In a titration, moles = concentration × volume in dm³. Use the moles of the solution you know and the ratio in the balanced equation to find the moles of the other substance, then divide by its volume in dm³ to get its concentration.

Checked against OpenStax Chemistry 2e, 4.5 Quantitative Chemical Analysis

34 / Chemical analysisBoth tiers · separate only

Why is universal indicator not used in a titration?

  1. AIts colour changes gradually, not sharply
  2. BIt turns the solution cloudy
  3. CIt only works with alkalis
  4. DIt makes the reaction too slow
Show the answer

Answer: A Its colour changes gradually, not sharply

A pipette measures a fixed volume of one solution into a conical flask with a few drops of indicator. The other solution is added from a burette until the indicator just changes colour. Universal indicator is not used because its colour changes gradually.

Checked against AQA GCSE Chemistry (8462) specification, 4.4.2.5 Titrations

35 / Chemical analysisBoth tiers · separate only

Sodium hydroxide solution is added to a solution and a blue precipitate forms. Which ion does this show is present?

  1. ACa²⁺
  2. BCu²⁺
  3. CFe²⁺
  4. DFe³⁺
Show the answer

Answer: B Cu²⁺

Many metal ions form insoluble hydroxides with sodium hydroxide solution, so a precipitate appears. Its colour identifies the ion: copper(II) ions, Cu²⁺, give a blue precipitate of copper(II) hydroxide.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.3.2 Metal hydroxides

36 / Chemical analysisBoth tiers · separate only

Sodium hydroxide solution gives a green precipitate. Which ion does this show is present?

  1. ACa²⁺
  2. BCu²⁺
  3. CFe²⁺
  4. DFe³⁺
Show the answer

Answer: C Fe²⁺

Iron(II) ions, Fe²⁺, give a green precipitate of iron(II) hydroxide with sodium hydroxide solution. Iron(III) ions give a brown one, so the colour tells the two kinds of iron ion apart.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.3.2 Metal hydroxides

37 / Chemical analysisBoth tiers · separate only

Sodium hydroxide solution gives a brown precipitate. Which ion does this show is present?

  1. ACa²⁺
  2. BCu²⁺
  3. CFe²⁺
  4. DFe³⁺
Show the answer

Answer: D Fe³⁺

Iron(III) ions, Fe³⁺, give a brown precipitate of iron(III) hydroxide with sodium hydroxide solution. With sodium hydroxide: copper(II) blue, iron(II) green, iron(III) brown.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.3.2 Metal hydroxides

38 / Chemical analysisBoth tiers · separate only

Sodium hydroxide solution is added to a solution of calcium ions. What is seen?

  1. AA white precipitate
  2. BA brown precipitate
  3. CA green precipitate
  4. DA blue precipitate
Show the answer

Answer: A A white precipitate

Calcium ions give a white precipitate of calcium hydroxide with sodium hydroxide solution. Unlike some other white precipitates, it does not dissolve when more sodium hydroxide is added.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.3.2 Metal hydroxides

39 / Chemical analysisBoth tiers · separate only

Dilute acid is added to a solid and it fizzes. The gas turns limewater milky. Which ion does this show is in the solid?

  1. ACarbonate, CO₃²⁻
  2. BSulfate, SO₄²⁻
  3. CHydroxide, OH⁻
  4. DChloride, Cl⁻
Show the answer

Answer: A Carbonate, CO₃²⁻

Carbonates react with dilute acids to give carbon dioxide, so the mixture fizzes. Bubbling the gas through limewater turns the limewater milky (cloudy), which shows the gas is carbon dioxide and so the solid contained carbonate ions.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.3.3 Carbonates

40 / Chemical analysisBoth tiers · separate only

Which reagents are used to test for halide ions in solution?

  1. ADilute nitric acid, then silver nitrate
  2. BSodium hydroxide solution, then warming
  3. CDilute hydrochloric acid, then barium chloride
  4. DDilute hydrochloric acid, then silver nitrate
Show the answer

Answer: A Dilute nitric acid, then silver nitrate

To test for halide ions, add dilute nitric acid and then silver nitrate solution. The acid removes carbonate ions, which would also form a precipitate. Hydrochloric acid cannot be used, because its own chloride ions would give a precipitate.

Checked against AQA GCSE Chemistry (8462) specification, 4.8.3.4 Halides

02 / Longer answersMarked by levels

6-mark questions on this topic

Write your answer first, on paper. Then open it and tick off the points you made. How 6-mark questions are marked.

6 marks / Testing whether a substance is pureBoth tiers

A carton is labelled “pure orange juice”. A chemist has a sample of a white solid sold as pure aspirin, and knows the melting point of aspirin.

Explain how the meaning of “pure” on the carton differs from its meaning in chemistry, and describe two ways the chemist could check whether the aspirin is pure.

What a full answer includes
  • In everyday use, pure means nothing has been added, so the substance is in its natural state.
  • In chemistry, a pure substance is a single element or compound, not mixed with anything else.
  • Orange juice is a mixture of water, sugars, acids and other substances, so it is not pure in the chemical sense.
  • Heat the aspirin slowly and measure its melting point.
  • A pure substance melts at a sharp, specific temperature that matches the known value.
  • An impure sample melts over a range of temperatures, usually lower than the known value.
  • Use chromatography, with a locating agent because aspirin is colourless: a pure substance gives a single spot.
  • More than one spot shows the sample contains impurities.

Model answer

On the carton, “pure” is used in its everyday sense: it means nothing has been added, so the juice is just as it came from the oranges. In chemistry, a pure substance is a single element or a single compound that is not mixed with any other substance. Orange juice is a mixture of water, sugars, acids, vitamins and many other substances, so in chemistry it is not pure at all. The chemist could check the aspirin in two ways. First, she could heat a small sample slowly and measure the temperature at which it melts. A pure substance melts at one sharp, specific temperature, so if the aspirin melts sharply at the known melting point it is pure. If it is impure, it will melt gradually over a range of temperatures, usually below the known value. Second, she could use chromatography. She would dissolve a little of the aspirin, spot it onto chromatography paper, run a suitable solvent up the paper and then use a locating agent to show the spots, because aspirin is colourless. A pure substance gives only one spot, whereas an impure sample gives two or more spots because the different substances move different distances.

Where marks slip

Saying a pure substance is “natural” or has “a high melting point” is marked wrong. The test is a sharp melting point at the known value, not a high one.

6 marks / Identifying three unlabelled solutionsBoth tiers · separate only

Three bottles have lost their labels. One contains sodium carbonate solution, one sodium chloride solution and one sodium sulfate solution.

Describe tests you could do to find out which solution is in each bottle, and the result you would expect for each.

What a full answer includes
  • Take a small sample of each solution in a separate test tube for each test.
  • Carbonate: add dilute acid; the solution fizzes and the gas turns limewater milky (cloudy), showing carbon dioxide.
  • Chloride: add dilute nitric acid, then silver nitrate solution.
  • A white precipitate of silver chloride shows chloride ions.
  • Sulfate: add dilute hydrochloric acid, then barium chloride solution.
  • A white precipitate of barium sulfate shows sulfate ions.
  • The acid is added first to remove carbonate ions, which would also give a precipitate.
  • A flame test would not help, because all three contain sodium ions.

Model answer

All three solutions contain sodium ions, so a flame test would give a yellow flame each time and would not tell them apart; instead I would test for the negative ions. I would put a small sample of each solution into separate test tubes for each test. To find the carbonate, I would add some dilute hydrochloric acid to each sample. Only the sodium carbonate solution would fizz, and if I bubbled the gas through limewater it would turn milky, which shows the gas is carbon dioxide. To find the chloride, I would add a few drops of dilute nitric acid and then silver nitrate solution to fresh samples. The sodium chloride solution would give a white precipitate of silver chloride. To find the sulfate, I would add dilute hydrochloric acid and then barium chloride solution to fresh samples. The sodium sulfate solution would give a white precipitate of barium sulfate. The acid is added first in the last two tests to remove any carbonate ions, which would also form a precipitate and give a false result.

Where marks slip

Adding hydrochloric acid before silver nitrate is a common slip: it adds chloride ions and gives a false positive. Use nitric acid with silver nitrate, and hydrochloric acid with barium chloride.

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