CIE iGCSE Co-Ordinated Science C12.4 Separation and purification Exam Style Questions Paper 4
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):
• Topic C1.1 — Solids, liquids and gases (Part (a))
• Topic C12.3 — Chromatography (Part (b))
• Topic C12.4 — Separation and purification (Part (c))
▶️ Answer/Explanation
(a)(i) liquid → gas = evaporation/boiling; liquid → solid = freezing/solidification
The forward arrow from liquid to gas represents a change of state called evaporation or boiling.
The reverse arrow from liquid to solid represents freezing (solidification).
(a)(ii) Kinetic energy increases
Heating transfers thermal energy to the gas particles.
This increases their kinetic energy, so they move faster.
(b)(i) Pencil marks are insoluble in the solvent (water)
Pencil (graphite) does not dissolve in the solvent used for chromatography.
Ink would dissolve and run up the paper with the solvent, distorting results.
(b)(ii) Dyes A, B and D
Comparing the spot positions of X with A, B, C and D on the chromatogram.
X matches the heights of spots A, B and D exactly, so these dyes are present in X.
(b)(iii) It is insoluble in the solvent
A substance that stays on the pencil line does not dissolve in the solvent.
Since it cannot move with the solvent front, it remains at the origin.
(b)(iv) \(R_f = 0.65\)
\( R_f = \dfrac{\text{distance moved by spot}}{\text{distance moved by solvent}} \)
\( R_f = \dfrac{2.6}{4.0} = 0.65 \)
(c) Solder melts over a range (220–229°C); tin and silver melt at a fixed point
Pure substances (tin, silver) have one specific, sharp melting point.
Mixtures (solder) melt over a range of temperatures because the different substances interfere with each other’s regular lattice arrangement.
Question
Describe how petroleum is separated by fractional distillation.
Suggest why sulfur impurities are removed from these fuels before the fuels are used.
Describe how a catalytic converter removes nitrogen monoxide from exhaust emissions.
Include a balanced symbol equation in your answer.
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C11.3 — Fuels (Part (a))
• Topic C12.4 — Separation and purification (Part (b))
• Topic C10.2 — Air quality and climate (Part (c))
• Topic C10.2 — Air quality and climate (Part (d))
▶️ Answer/Explanation
(a)(i) Road surfaces
Bitumen, the thickest and least volatile fraction, is used for surfacing roads and roofing.
(a)(ii) 59 kg
Diesel oil makes up 26% of the petroleum sample.
Mass of diesel oil \(= \dfrac{225 \times 26}{100} = 58.5 \approx 59\,\text{kg}\).
(b)
Petroleum is heated, causing the different hydrocarbon fractions to vaporise and rise up the fractionating column.
Fractions are separated according to their boiling point, with fractions of lower boiling point rising higher.
As vapours rise and cool, they condense back to liquid at different heights depending on chain length.
(c)
Sulfur impurities are removed because burning them produces sulfur dioxide, which causes acid rain.
(d)
A catalytic converter contains a hot catalyst that helps nitrogen monoxide react and be removed from exhaust emissions.
This can occur via reaction with carbon monoxide or decomposition of NO itself.
Equation: \(2\text{NO} + 2\text{CO} \rightarrow \text{N}_2 + 2\text{CO}_2\) (or \(2\text{NO} \rightarrow \text{N}_2 + \text{O}_2\)).