How the physical properties of hydrocarbons change with molecule size
4 marks · NormalThe table below shows properties of different hydrocarbon fractions obtained from crude oil.
| Fraction | Approx. number of C atoms | Boiling point (°C) | Colour | Flow (viscosity) | Ease of burning |
|---|---|---|---|---|---|
| Refinery gas | 3 | -20 | Colourless | Very runny | Burns very easily |
| Kerosene | 12 | 200 | Light yellow | Fairly runny | Burns easily |
| Fuel oil | 20 | 340 | Dark brown | Thick | Does not burn easily |
Which fraction is the most viscous? Give a reason for your answer.
State how boiling point, colour, and ease of burning change as you go from refinery gas to fuel oil.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Fuel oil. It is described as 'thick', which means it is most viscous.
(b) From refinery gas to fuel oil, boiling point increases, colour gets darker, and ease of burning decreases.
Common mistakes: Mixing up fractions
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies fuel oil as the most viscous fraction.
B2 (a) · 1 mark · Gives a correct reason (e.g., described as 'thick', has most carbon atoms).
B3 (b) · 1 mark · States that boiling point increases and colour darkens from refinery gas to fuel oil.
B4 (b) · 1 mark · States that ease of burning decreases (burns less easily) from refinery gas to fuel oil.
Worked solution
a) Fuel oil is the most viscous fraction because it is thick and contains the most carbon atoms.
b) As you go from refinery gas to fuel oil, the boiling point increases, the colour becomes darker, and the ease of burning decreases (fuel oil does not burn easily).