The questions are on the crude oil and hydrocarbons test. Take it under real conditions before reading the mark scheme.
Quick answer: Crude oil is a mixture of hydrocarbons separated by fractional distillation into fractions with different chain lengths and boiling points. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 9-10 test on crude oil and hydrocarbons, from separating crude oil to saturated or unsaturated, with the equivalent answers to accept.
Use this marking guide to score your paper out of 20. Award marks for method, balanced equations and correct units, and give credit for a correct alternative wording. Do not penalise an answer for British or American spelling.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C7
Question 1 Separating crude oil (3 marks)
a. Different boiling points.b. Bitumen has very long molecules and a very high boiling point, so it condenses at the hot bottom of the column.
Marking: a. 1 mark. b. 1 mark for long molecules or high boiling point, 1 mark for condensing in the hottest part.
Question 2 Formulae of hydrocarbons (2 marks)
a. C₁₂H₂₆.b. Propene, C₃H₆.
Marking: 1 mark for each correct formula; name and formula both needed in b.
Question 3 Combustion of butane (3 marks)
a. 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O.b. Carbon monoxide.
Marking: a. 1 mark for correct formulae, 1 mark for balancing. b. 1 mark.
Question 4 Bromine water results (3 marks)
a. Liquid Y.b. Alkenes have a C=C double bond. Bromine adds across it, so the orange bromine is used up and the solution becomes colourless. Liquid X is saturated and does not react.
Marking: a. 1 mark. b. 1 mark for C=C double bond, 1 mark for bromine adding so the colour is lost.
Question 5 Cracking (3 marks)
a. C₅H₁₀ (carbons 15 − 10 = 5; hydrogens 32 − 22 = 10).b. A catalyst such as aluminium oxide, or a temperature of about 550 °C.
c. To make shorter alkanes for fuel and alkenes for plastics, because the demand exceeds the supply from distillation.
Marking: 1 mark each.
Question 6 Methane combustion calculation (2 marks)
M(CH₄) = 16; n = 8.0 ÷ 16 = 0.50 mol. CO₂ forms 1 : 1, so 0.50 mol; M = 44. Mass = 0.50 × 44 = 22 g.Marking: 1 mark for 0.50 mol, 1 mark for the final mass with unit.
Question 7 Explain fraction properties (2 marks)
Petrol molecules are short, with weak forces between them, so petrol evaporates easily and mixes with air. The long molecules of fuel oil give a high boiling point and low volatility, so fuel oil is harder to ignite.Marking: 1 mark for shorter chain, 1 mark for evaporating more easily.
Question 8 Saturated or unsaturated (2 marks)
Saturated: only single C–C bonds, the maximum number of hydrogens, for example ethane. Unsaturated: at least one C=C double bond, for example ethene.Marking: 1 mark for each correct explanation with example.
