Crude oil is an important source of organic chemical fuels. It is refined by fractional distillation. Use the information in the table below to answer the questions which follow.

Fraction ........................ Boiling point/°C
A ................................................. 40
B ................................................. 80
C ................................................. 200
D ................................................. 350
E ................................................. above 350

a. For each of the questions that follow, give the letter of the fraction which is most appropriate as an answer. You should also give a reason for your answer in each case.

(i) Which fraction would contain the most volatile substances?

(ii) Which of the fractions would collect at the bottom of the fractionating column?

(iii) Which fraction could be used as a fuel for cars?

(iv) Which fraction would contain the largest molecules?

b. Some of the fractions undergo a further process called cracking to produce further substances. 

(i) Explain what you understand by the term ‘cracking’. What conditions are employed when cracking occurs?

(ii) Write a word and balanced chemical equation to show how octane can be produced by the cracking of C₁₅H₃₂.