Question

Petroleum is separated into useful fractions by fractional distillation. Separation occurs in a fractionating column. Some properties of three of these fractions are shown.

Fraction	Boiling point range/ $^{0}\text{C}$	Number of carbon atoms in the molecule
1		5-10
2	320-350	16-24
3	120-210

Which statement is correct?
A) Fraction 1 has the higher boiling point range than fraction 2.
B) Fraction 2 is removed from a higher point in the fractionating tower than fraction 1
C) Molecules in fraction 3 have shorter chains than those in fraction 2
D) None of the fractions is liquid at room temperature

Answer 1

To answer this question knowledge of fractional distillation is required. Fractional distillation is a process of taking a chemical mixture and using the difference in their boiling points as the criteria to separate the mixture into different components.

Complete step by step solution:
Fractional distillation separates the mixture into different parts called fractions. In the above options as the range of boiling point is not mentioned for the fraction with 5-10 carbon atoms and on the other hand the fraction with the boiling range 120-210$^{0}\text{C}$ the number of carbon atoms is not mentioned, hence it is difficult to point out which will evaporate first.
Statement 1 which states “Fraction 1 has the higher boiling point range than fraction 2” is incorrect because the number of carbon atoms is lower for Fraction 1 so lower boiling point and faster evaporation on the other hand, fraction 2 with higher number of carbon atoms has higher boiling point.
Statement 2 is also incorrect that fraction 2 is removed from a higher point that fraction 1 in the fractionating column as the one with lower boiling point evaporates faster as is deposited higher in the fractionating column.
Statement 3 says that “Molecules in fraction 3 have shorter chains than those in fraction 2” which is true because molecules with shorter organic chains evaporate more quickly than molecules with longer carbon chains.
Lastly, statement 4 states that “None of the fractions is liquid at room temperature” is also wrong because carbon chains with a number of atoms in the range of 5-10 atoms are liquid at room temperature.

Hence the correct answer is option C.

Note:
Petroleum is a mixture of different hydrocarbons. The petroleum is evaporated and the vapours condense at different temperatures in the fractionating column. The separation is dependent on the molecular weights and the boiling points of the components. Those components with lower molecular weight are evaporated first due to their low boiling point while those components with higher molecular weights are evaporated later due to higher boiling points.