Simple recall questions are usually worth one mark. They often have command words like 'Give...', 'State...', 'Name..' or 'Identify...'. Some questions may ask you to state two things, rather than just one, and will be worth two marks.
Structured questions, with command words such as 'Describe...' or 'Explain...', will be worth two or more marks:
More complex structured questions will be worth three or four marks. They include questions with complex descriptions and explanations, questions in which you need to compare and contrast two different things, or calculations with several stages.
The mark schemes given here may show answers as bullet points. This is to show clearly how a mark can be obtained. However, it is important that your answer is written in a logical manner, the points are linked to each other, and are relevant to the question.
Edexcel questions courtesy of Pearson Education Ltd.
Describe how carbon dioxide is released into today's atmosphere. [2 marks]
One from the following for  each:
The Earth's earliest atmosphere was very different from the Earth's atmosphere today.
a) There is much less water vapour in the Earth's atmosphere today than in the Earth's earliest atmosphere.
Explain how the amount of water vapour decreased.
b) When plants first started to grow on the Earth they caused the composition of the atmosphere to change.
Describe how the composition of the atmosphere changed as a result of plants growing. [2 marks]
a)Any two from:
b) Any two from:
Fuel oil can contain sulfur as an impurity.
Explain how burning this impurity can cause problems in the environment. [3 marks]
Three from the following:
In the oil industry some fuel oil fraction is converted into petrol.
This is done by heating the fuel oil fraction to thermally decompose it and produce smaller molecules.
a) State the name given to this process. [1 mark]
b) Give two reasons why it is necessary to carry out this process to make more petrol. [2 marks]
a) Cracking 
b) Any two from: