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DSE Chemistry Recap – Elements, Compounds and Mixtures | F3 (2026)

Elements, Compounds and Mixtures

DSE Chemistry

F3 (2026)

For use on Sunday, 6 September 2026

01

Classification of Matter

Elements, Compounds and Mixtures

The particle diagrams below represent four different samples. Circles of different colours represent atoms of different elements. Circles that touch are chemically bonded together.

Atom A Atom B Atom C
W X Y Z

Each box represents a separate sample at the particle level.

  1. Classify each sample W, X, Y and Z as an element, a compound or a mixture. Give a reason for each classification.
  2. Which sample contains more than one element but only one pure substance? Explain your answer.
  3. Which sample contains two pure substances, one of which is a compound? Identify the two substances using the symbols A, B and C.
  4. State the difference between:
    1. an atom and a molecule;
    2. a compound and a mixture.
  5. A student claims, “Every substance containing two different types of atoms is a mixture.” Use one of the diagrams to show that this claim is incorrect.
14 marks
02

Separation of Mixtures

Recovering the Components of a Mixture

A dry mixture contains iron filings, sand and sodium chloride. Iron is magnetic, sand is insoluble in water, and sodium chloride is soluble in water. A student is required to separate the mixture and recover all three substances as dry solids.

  1. Explain why the original sample is a mixture rather than a compound.
  2. Describe a complete procedure, in the correct sequence, for separating and recovering dry samples of iron, sand and sodium chloride.
  3. For the filtration step:
    1. identify the residue;
    2. identify the substance dissolved in the filtrate.
  4. Explain why distilled water, rather than a large amount of tap water, should be used during the separation.
  5. State the physical property used in each of the following separation steps:
    1. using a magnet;
    2. dissolving the mixture in water;
    3. filtration;
    4. crystallisation.
  6. A student heats the sodium chloride solution strongly until all the water has evaporated. Suggest one disadvantage of this method compared with crystallisation followed by filtration.
  7. Explain why no new substance is formed during the separation process.
16 marks
03

Comparing Substances

Iron, Sulphur and Iron Sulphide

Iron filings and sulphur powder are mixed in a test tube. Before heating, a magnet can attract the iron filings from the mixture. The mixture is then heated strongly. A black solid, iron(II) sulphide, is formed. The black solid is not attracted by a magnet in the same way as the original iron filings.

  1. Before heating, classify:
    1. iron;
    2. sulphur;
    3. the combination of iron filings and sulphur powder.
  2. After heating, classify iron(II) sulphide as an element, a compound or a mixture. Explain your answer.
  3. Write a word equation for the reaction between iron and sulphur.
  4. The formula of iron(II) sulphide is FeS. State what this formula tells us about the ratio of iron atoms to sulphur atoms in the compound.
  5. Compare the properties of the iron–sulphur mixture with those of iron(II) sulphide.
  6. Explain why a magnet is suitable for separating iron from the unheated mixture but cannot be used to obtain iron from iron(II) sulphide.
  7. [Deleted] A student mixes 7.0 g of iron with 4.0 g of sulphur. After complete reaction, 11.0 g of iron(II) sulphide is formed.
    1. State the law illustrated by these results.
    2. Explain how the results support this law.
  8. Give one difference between the composition of a compound and the composition of a mixture.
16 marks
04

Changes in Matter

Physical and Chemical Changes

Four processes are described below.

Process Description
P Ice melts to form liquid water.
Q A magnesium ribbon burns with a bright white flame and forms a white powder.
R Solid sodium chloride dissolves in water and is later recovered by evaporation.
S Dilute hydrochloric acid is added to calcium carbonate. Effervescence occurs and the solid becomes smaller.
  1. Classify each process P, Q, R and S as a physical change or a chemical change. Give a reason for each classification.
  2. For process Q:
    1. name the white product formed;
    2. write a word equation for the reaction;
    3. state two observations that provide evidence of a chemical change.
  3. For process S:
    1. name the gas produced;
    2. describe a test for the gas and state the positive result;
    3. write a word equation for the reaction.
  4. A student says, “Dissolving must be a chemical change because the solid disappears.” Explain why this statement is incorrect for process R.
  5. Another student says, “Every reversible change is physical, and every irreversible change is chemical.” Explain why reversibility alone is not a completely reliable way to classify a change.
  6. State two general pieces of evidence that may indicate that a chemical reaction has taken place. For each one, explain why it should not be used alone as definite proof.
18 marks

Question 1

    • W is an element. It contains only one type of atom, A.
    • X is a compound. It contains atoms of two different elements, A and B, chemically bonded together. All the particles are identical AB particles.
    • Y is a mixture of two elements. It contains separate A atoms and B atoms that are not chemically bonded together.
    • Z is a mixture. It contains more than one type of particle: AB particles and C2 particles.
  2. Sample X. It contains two elements, A and B, but they are chemically bonded in a fixed arrangement. Since all particles are identical AB particles, X contains only one pure substance.
  3. Sample Z. The two pure substances are:
    • compound AB;
    • element C, present as C2 molecules.
    1. An atom is the smallest particle of an element that retains the chemical properties of that element. A molecule is a group of two or more atoms chemically bonded together.
    2. A compound contains two or more elements chemically combined in a fixed ratio. It can be separated into its elements only by chemical methods. A mixture contains two or more substances that are not chemically combined. Its components can usually be separated by physical methods.
  5. The claim is incorrect. Sample X contains two different types of atoms, A and B, but it is a compound rather than a mixture. The A and B atoms are chemically bonded, and all particles in the sample are identical.
Particle-level test: A pure substance contains only one type of particle. A compound may contain more than one type of atom while still being a pure substance.

Question 2

  1. The iron, sand and sodium chloride are not chemically bonded together. They retain their own physical properties, can be present in different proportions and can be separated by physical methods. Therefore, the original sample is a mixture.
  2. A suitable procedure is:
    1. Move a magnet over the dry mixture to attract and remove the iron filings.
    2. Repeat the magnetic separation until no iron filings remain. Collect the iron and keep it dry.
    3. Add a suitable amount of distilled water to the remaining sand and sodium chloride.
    4. Stir the mixture so that the sodium chloride dissolves. The sand remains undissolved.
    5. Filter the mixture. Sand remains as the residue, while sodium chloride solution passes through as the filtrate.
    6. Wash the sand residue with a small amount of distilled water to remove sodium chloride solution from its surface.
    7. Dry the sand between filter papers or in a warm place.
    8. Heat the sodium chloride solution gently to evaporate some water and form a concentrated solution.
    9. Allow the concentrated solution to cool so that sodium chloride crystals form.
    10. Filter off the crystals and dry them between filter papers.
    1. Residue: sand.
    2. Substance dissolved in the filtrate: sodium chloride.
  4. Distilled water does not contain significant dissolved salts, so it does not introduce extra solid impurities. Only a suitable amount should be used. A very large amount would take longer to evaporate and would waste time and energy.
    1. Magnet: difference in magnetic properties.
    2. Dissolving in water: difference in solubility.
    3. Filtration: difference in particle size and insolubility.
    4. Crystallisation: difference in solubility as the solution is concentrated and cooled.
  6. Strong heating to complete dryness may cause the solution to spit, leading to loss of sodium chloride. It may also produce a powder rather than well-formed crystals. Crystallisation allows crystals to form before they are separated from the remaining solution.
  7. No new substance is formed because the identities of iron, sand and sodium chloride do not change. The procedure uses only differences in physical properties to separate the original substances. Therefore, the separation steps are physical changes.
Separation rule: The method selected must depend on a difference in physical properties, such as magnetism, solubility, boiling point or particle size.

Question 3

    1. Iron is an element.
    2. Sulphur is an element.
    3. The unheated combination is a mixture of two elements.
  2. Iron(II) sulphide is a compound. It contains iron and sulphur atoms chemically combined in a fixed ratio. It has properties different from those of iron and sulphur.
  3. Iron + sulphur → iron(II) sulphide
  4. FeS shows that iron and sulphur atoms are present in a 1 : 1 ratio.
  5. In the iron–sulphur mixture, iron and sulphur retain their individual properties. For example, the iron can still be attracted by a magnet. Iron(II) sulphide has its own properties, which are different from those of both iron and sulphur. The iron in FeS can no longer be removed with a magnet.
  6. Before heating, iron remains a separate substance and retains its magnetic property. A magnet can therefore attract and remove it from the mixture. In iron(II) sulphide, iron is chemically combined with sulphur. It is no longer present as separate iron filings, so a physical method such as magnetic separation cannot recover the iron.
    1. The results illustrate the law of conservation of mass.
    2. The total mass of the reactants is:
      7.0 g + 4.0 g = 11.0 g
      This is equal to the mass of iron(II) sulphide formed. Therefore, the total mass remains unchanged during the reaction.
  8. The elements in a compound are chemically combined in a fixed ratio. The substances in a mixture are not chemically combined and can be present in different proportions.
Key distinction: Mixing iron and sulphur produces a mixture. Heating them sufficiently causes a chemical reaction and produces the compound iron(II) sulphide.

Question 4

    • P is a physical change. Only the state changes from solid to liquid. The substance remains water.
    • Q is a chemical change. Magnesium reacts with oxygen and a new substance, magnesium oxide, is formed.
    • R is a physical change. Sodium chloride does not change into a new substance and can be recovered by evaporating the water.
    • S is a chemical change. Calcium carbonate reacts with hydrochloric acid to form new substances, including carbon dioxide.
    1. The white product is magnesium oxide.
    2. Magnesium + oxygen → magnesium oxide
    3. Any two:
      • a bright white light is produced;
      • heat is released;
      • a white powder is formed;
      • the silvery magnesium ribbon changes into a white solid.
    1. The gas is carbon dioxide.
    2. Pass the gas through limewater. A positive result is that the limewater turns milky.
    3. Calcium carbonate + hydrochloric acid
      → calcium chloride + water + carbon dioxide
  4. The statement is incorrect. The sodium chloride solid appears to disappear because its particles separate and spread throughout the water. No new substance is formed. The original sodium chloride can be recovered by evaporating the water, showing that dissolving sodium chloride is a physical change.
  5. Reversibility can be useful evidence, but it is not a complete definition. Some physical changes may be difficult to reverse in practice. For example, a glass object can be broken physically even though restoring its original shape is difficult. Some chemical reactions can also be reversed by another chemical reaction. Therefore, the main question is whether new substances are formed, not simply whether the process is easy to reverse.
  6. Suitable evidence includes:
    • Gas production: bubbles may indicate that a new gaseous substance has formed. However, bubbles can also be produced by boiling, which is a physical change.
    • Colour change: it may indicate that a new substance has formed. However, dilution or mixing coloured substances can also cause a colour change without a chemical reaction.
    • Temperature change: it may show that energy is released or absorbed in a reaction. However, temperature can also change during physical processes such as dissolving or changing state.
    • Formation of a solid: a precipitate may indicate that a new insoluble substance has formed. However, a dissolved substance may also crystallise because water evaporates.
    Any two well-explained examples are acceptable.
Best classification test: In a physical change, the chemical identity of the substance remains unchanged. In a chemical change, one or more new substances are formed.
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