Most of the compounds in crude oil are hydrocarbons.
Hydrocarbons with the smallest molecules are very volatile.
 
Describe and explain how petrol is separated from the mixture of hydrocarbons in crude oil.

Use the diagram and your knowledge to answer this question.		(6 marks)

Most of the compounds in crude oil are hydrocarbons.
Hydrocarbons with the smallest molecules are very volatile.
 
Describe and explain how petrol is separated from the mixture of hydrocarbons in crude oil.

Use the diagram and your knowledge to answer this question.		(6 marks)

Marks awarded for this answer will be determined by the Quality of Written Communication (QWC) as well as the standard of the scientific response. Examiners should also refer to the information in the Marking guidance.
0 marksNo relevant content.
Level 1 (1-2 marks)There is a statement that crude oil is heated or that substances are cooled. However there is little detail and any description may be confused or inaccurate.

Level 2 (3-4 marks)There is some description of heating / evaporating crude oil and either fractions have different boiling points or there is an indication of a temperature difference in the column.
Level 3 (5-6 marks)There is a reasonable explanation of how petrol is or fractions are separated from crude oil using evaporating and condensing.

If cracking is given as a preliminary or subsequent process to fractional distillation then ignore.
However, if cracking / catalyst is given as part of the process, maximum is level 2.
Examples of chemistry points made in the response could include:
•        Some / most of the hydrocarbons (or petrol) evaporate / form vapours or           gases
•        When some of / a fraction of the hydrocarbons (or petrol) cool to their           boiling point they condense
•         Hydrocarbons (or petrol) that have (relatively) low boiling points and are            collected near the top of the fractionating column or hydrocarbons with           (relatively) high boiling points are collected near the bottom of the fractionating           column
•         The process is fractional distillation
•         Heat the crude oil / mixture of hydrocarbons or crude oil / mixture is heated to            about 350°C
•         Some of the hydrocarbons remain as liquids
•         Liquids flow to the bottom of the fractionating column
•         Vapours / gases rise up the fractionating column
•         Vapours / gases cool as they rise up the fractionating column
•        The condensed fraction (or petrol) separates from the vapours / gases and            flows out through a pipe 
•         Some of the hydrocarbons remain as vapours / gases
•         Some vapours / gases rise out of the top of the fractionating column 
•        There is a temperature gradient in the fractionating column or the fractionating            column is cool at the top and hot at the bottom
6