Elements, compounds, mixtures and separation – IGCSE Chemistry exam questions with mark scheme

1Grade 1–34 marks

The diagram shows the particles in four boxes, A, B, C and D.

(a) Give the letter of the box that shows a compound. [1]

(b) Give the letter of the box that shows a mixture. [1]

(d) Box D contains molecules. Explain why D still shows an element. [1]

(a) B(1)
(b) C(1)
(c) A and D (both needed, either order)(1)
(d) (all the molecules / atoms contain) only one type of atom; allow only one element present; ignore 'atoms are bonded together' on its own(1)
Answer: (a) B (b) C (c) A and D (d) It contains only one type of atom.
Examiner tip: In (d) students say 'the atoms are joined' — that is true of compounds too; the mark is for 'only one type of atom'.

2Grade 4–53 marks

A student made two samples of benzoic acid, X and Y. The melting point of pure benzoic acid is 122 °C. The student measured the melting point of each sample.

Sample	Melting point
X	melted over the range 113–119 °C
Y	melted at 122 °C

(a) Which sample is pure? Give two reasons for your answer. [2]

(b) Sea water is a mixture of water and dissolved salts. Suggest how the boiling point of sea water compares with the boiling point of pure water. [1]

(a) Y and it melts at a single / sharp / fixed temperature (X melts over a range); 'Y' alone scores 0(1)
(a) its melting point matches / is the same as that of pure benzoic acid (122 °C); allow X melts below 122 °C so X is impure(1)
(b) sea water boils at a higher temperature (than 100 °C) / above 100 °C; allow boils over a range of temperatures(1)
Answer: (a) Y — it melts at a sharp single temperature which matches the value for pure benzoic acid. (b) Higher than 100 °C.
Examiner tip: Naming Y without a reason scores nothing — give both the sharp melting point and the match to the data value.

3Grade 1–34 marks

Choose a method from the box to obtain each substance. Each method may be used once, more than once or not at all.

crystallisation filtration fractional distillation paper chromatography simple distillation

(a) Pure water from sea water. [1]

(b) Chalk powder from a mixture of chalk and water. (Chalk is insoluble in water.) [1]

(d) Heptane (boiling point 98 °C) from a mixture of the liquids heptane and octane (boiling point 126 °C). [1]

(a) simple distillation; allow distillation; do not accept fractional distillation(1)
(b) filtration(1)
(c) crystallisation; allow evaporation then crystallisation(1)
(d) fractional distillation(1)
Answer: (a) simple distillation (b) filtration (c) crystallisation (d) fractional distillation
Examiner tip: Fractional distillation is for separating miscible liquids with different boiling points; for a solvent from a dissolved solid use simple distillation.

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4Grade 4–53 marks

A student used paper chromatography to identify a yellow food dye. On the chromatogram, the yellow spot had moved 3.9 cm from the baseline. The solvent front had moved 6.5 cm from the baseline.

Dye	J	K	L
R_f value	0.45	0.60	0.75

(a) Calculate the R_f value of the yellow dye. [2]

(b) Use the table to identify the yellow dye. [1]

(a) $R_f = \dfrac{3.9}{6.5}$ (distance moved by spot ÷ distance moved by solvent)(1)
(a) = 0.60; allow 0.6; correct answer with no working scores 2(1)
(b) K; allow ecf from an incorrect R_f value in (a) if it matches a dye in the table(1)
Answer: (a) 0.60 (b) K
Examiner tip: Divide spot distance by solvent distance, never the other way round — an R_f value can never be greater than 1.

5Grade 6–75 marks

A student carried out paper chromatography on three reference dyes, P, Q and R, and on a food colouring, M. The diagram shows the chromatogram.

(a) How many different substances are in M? [1]

(b) Which reference dyes are present in M? [1]

(d) Explain why one spot of M has stayed on the baseline. [1]

(a) 3 / three(1)
(b) P and Q (both needed, no extras)(1)
(c) $R_f = \dfrac{6.0}{7.5}$(1)
(c) = 0.80; allow 0.8; correct answer with no working scores 2(1)
(d) this substance is insoluble in the solvent; allow does not dissolve in the solvent; ignore 'it is heavy' / 'too big'(1)
Answer: (a) 3 (b) P and Q (c) 0.80 (d) It is insoluble in the solvent.
Examiner tip: The spot on the baseline is still a component of M — count it in (a).

6Grade 4–55 marks

A student investigated the colours in a black ink using paper chromatography. This is the method the student used.

Draw a line 1 cm from the bottom of the paper with a black pen.
Put a spot of the ink on the line.
Pour water into a beaker to a depth of 2 cm.
Stand the paper in the beaker and leave until the water has nearly reached the top.

(a) The method contains two mistakes. Identify each mistake and explain how it would affect the result. [4]

(b) The student then removed the paper from the beaker. State what the student should do immediately after removing the paper. [1]

(a) line drawn in pen / ink — should be drawn in pencil(1)
(a) the ink of the line would dissolve / move up the paper / separate and mix with the spots (pencil is insoluble)(1)
(a) water level (2 cm) is above the baseline / spot — the solvent must be below the baseline(1)
(a) the ink spot would dissolve into the water in the beaker (instead of moving up the paper)(1)
(b) mark (the position of) the solvent front (with a pencil); allow draw a line where the water reached(1)
Answer: (a) Line drawn in pen — pen ink would also separate and run; use pencil. Water deeper than the baseline — ink dissolves into the solvent; water must be below the line. (b) Mark the solvent front.
Examiner tip: Each mistake needs its consequence for the second mark — 'use a pencil' alone scores only one.

7Grade 6–76 marks

A student has a mixture of two solids: copper(II) sulfate, which is soluble in water, and copper(II) oxide, which is insoluble in water.

Describe how the student could obtain pure, dry crystals of copper(II) sulfate from this mixture. In your answer, explain the purpose of each step. [6]

add the mixture to (warm) water and stir — copper(II) sulfate dissolves but copper(II) oxide does not(1)
filter — copper(II) oxide stays in the filter paper as the residue and copper(II) sulfate solution passes through as the filtrate(1)
heat the filtrate (in an evaporating basin) to evaporate some of the water / until crystals start to form (e.g. on a glass rod) / to the point of crystallisation; reject evaporate / heat to dryness(1)
leave the (saturated) solution to cool so that crystals form(1)
remove the crystals from the remaining solution, e.g. by filtering / decanting(1)
dry the crystals by patting with filter paper / leaving in a warm place / in a desiccator; reject heat strongly(1)
Answer: Dissolve in water, filter to remove copper(II) oxide, heat the filtrate to the point of crystallisation, cool, filter off crystals and dry them.
Examiner tip: Students who boil the solution to dryness lose the 'pure crystals' credit — evaporate only until crystals start to form, then let it cool.

8Grade 4–55 marks

The diagram shows apparatus used to obtain pure water from sea water.

(a) Name the method of separation. [1]

(b) Name the piece of apparatus labelled X. [1]

(d) State the reading on the thermometer while the water is distilling. Give a reason for your answer. [2]

(a) (simple) distillation; do not accept fractional distillation(1)
(b) (Liebig) condenser(1)
(c) so the condenser / jacket fills completely with water; allow so cooling is more efficient / greatest; ignore 'to cool the vapour' alone(1)
(d) 100 °C(1)
(d) it is the boiling point of (pure) water / only water vapour reaches the thermometer; do not accept 'boiling point of sea water'(1)
Answer: (a) simple distillation (b) condenser (c) so the jacket fills completely, giving the best cooling (d) 100 °C — the boiling point of water; only water vapour reaches the thermometer.
Examiner tip: The thermometer is in the vapour at the side arm, so it reads the boiling point of the pure water, not of the sea water in the flask.

9Grade 8–94 marks

A food colouring Z is known to contain some of the dyes A, B, C and D. A scientist ran two chromatograms of Z, one using water as the solvent and one using ethanol. The table shows the R_f values of the pure dyes in each solvent.

Dye	R_f in water	R_f in ethanol
A	0.30	0.55
B	0.70	0.20
C	0.30	0.85
D	0.50	0.20

Using water, Z gave two spots, with R_f values 0.30 and 0.70.
Using ethanol, Z gave two spots, with R_f values 0.20 and 0.55.

(a) Explain why the chromatogram in water alone cannot show which dyes are in Z. [1]

(b) Deduce which dyes are in Z. Explain your reasoning using both chromatograms. [3]

(a) dyes A and C have the same R_f value (0.30) in water, so the 0.30 spot could be either; allow dyes can have the same R_f in one solvent(1)
(b) in water the 0.70 spot shows B is present and/or in ethanol the 0.55 spot shows A is present(1)
(b) C is not present because there is no spot at 0.85 in ethanol and D is not present because there is no spot at 0.50 in water(1)
(b) (conclusion) Z contains A and B only(1)
Answer: (a) A and C both have R_f 0.30 in water. (b) A and B: B gives 0.70 in water, A gives 0.55 in ethanol; C absent (no 0.85 in ethanol), D absent (no 0.50 in water).
Examiner tip: R_f values depend on the solvent — use each solvent to rule dyes out, not just to find matches.

10Grade 8–94 marks

Iron and sulfur are elements. When a mixture of iron powder and sulfur powder is heated, the compound iron(II) sulfide forms.

(a) Give two differences between a mixture of iron and sulfur and the compound iron(II) sulfide. [2]

(b) Iron and sulfur always combine in the ratio 7 g of iron to 4 g of sulfur.
A student heated 7.0 g of iron with 3.2 g of sulfur until no further reaction took place.

Calculate the mass of iron(II) sulfide formed and the mass of the element left unreacted. [2]

(a) any two from: in the compound the elements are chemically combined / bonded (in a mixture they are not); the mixture can be separated by physical means e.g. a magnet removes the iron (the compound cannot); the compound has a fixed composition / proportions (a mixture can have any proportions); the compound has different properties from iron and sulfur (a mixture keeps the properties of its components)(1)
(a) second difference from the list(1)
(b) mass of iron(II) sulfide = 8.8 g (3.2 g S reacts with 3.2 × 7/4 = 5.6 g Fe; 5.6 + 3.2)(1)
(b) 1.4 g of iron left unreacted (element must be named); allow ecf from an incorrect mass of iron reacting(1)
Answer: (a) e.g. elements chemically combined in the compound; iron can be removed from the mixture with a magnet. (b) 8.8 g iron(II) sulfide; 1.4 g iron unreacted.
Examiner tip: Work out which element is in excess first — the sulfur runs out, so only 5.6 g of the iron can react.

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