21 past paper MCQs on Evaporation, Vapour Pressure and Boiling Point, from the Liquids unit of MDCAT Chemistry. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.
1.Vapour pressure is independent of which factor? (MDCAT 2020)
Correct answer: (d) surface area of the liquid
A wider surface evaporates more molecules but condenses just as many, so the equilibrium vapour pressure is unchanged.
2.Which of the following factors does not affect the vapour pressure of a liquid? (MDCAT 2022)
Correct answer: (a) amount of liquid
Vapour pressure is an intensive property; a larger volume of the same liquid at the same temperature has the same vapour pressure.
3.In order to boil water at $110^\circ$C, the external pressure should be: (MDCAT 2024)
Correct answer: (b) 760-1200 torr
Above $100^\circ$C the vapour pressure of water is above one atmosphere, so the external pressure must be above 760 torr.
4.The boiling point of a liquid is the temperature at which: (MDCAT 2024)
Correct answer: (c) the vapour pressure equals the atmospheric pressure
Bubbles can form in the bulk only once the vapour pressure has risen to the pressure above the liquid.
5.Which one of the following has higher vapour pressure? (UHS MDCAT 2025)
Correct answer: (b) acetaldehyde
Acetaldehyde boils at about $20^{\circ}\mathrm{C}$, the lowest of the four, so its vapour pressure is the highest.
6.Which factor does not affect the rate of evaporation? (NUMS MDCAT 2025)
Correct answer: (a) Amount of liquid
Evaporation is a surface process, so its rate depends on the area, the temperature and the forces between molecules, but not on how much liquid there is.
7.Which of the following is least volatile? (SIBA MDCAT 2025)
Correct answer: (a) Glycerol
Glycerol has three hydroxyl groups and so the strongest hydrogen bonding of the four.
8.In order to boil water at 110°C, external pressure should be (UHS MDCAT 2024)
Correct answer: (b) 760 - 1200 torr
Water boils above $100^{\circ}\mathrm{C}$ only when the pressure on it is above one atmosphere.
9.Boiling point of a liquid is a temperature at which: (UHS MDCAT 2024)
Correct answer: (c) Vapour pressure equals the atmospheric pressure
A liquid boils when its vapour pressure has risen to the pressure above it.
10.Molar heat of vaporization of water is (KMU MDCAT 2024)
Correct answer: (d) 40.7 kJ/mol
It takes $40.7\,\mathrm{kJ}$ to turn a mole of water at $100^{\circ}\mathrm{C}$ into steam.
11.Distillation under very reduced pressure is ____ distillation (KMU MDCAT 2024)
Correct answer: (d) Vacuum
Lowering the pressure lowers the boiling point, which is how heat sensitive liquids are distilled.
12.If external pressure is reduced to half, boiling point of ethanol will be: (NUMS MDCAT 2024)
Correct answer: (b) Less than 78°C
A liquid boils when its vapour pressure reaches the pressure above it, so less pressure means a lower boiling point.
13.Which of the following factor does not affect the magnitude of vapor pressure? (UHS MDCAT 2023)
Correct answer: (a) amount of liquid
Vapour pressure is a property of the liquid at a given temperature, whatever quantity of it there is.
14.Vapor pressure is independent of which factor? (NUMS MDCAT 2023)
Correct answer: (d) Surface area of liquid
Vapour pressure is a property of the liquid at a given temperature, whatever area is exposed.
15.Which of the following molecules will show a higher rate of evaporation? (NUMS MDCAT 2022)
Correct answer: (a) Acetone
Acetone has the weakest intermolecular forces of the four, so it evaporates fastest.
16.In which of the following will water evaporate earlier? (PMC Practice 2021)
Correct answer: (b) Saucepan
Evaporation is a surface process, and the saucepan offers the widest surface.
17.While calculating vapor pressure of a liquid in manometer, the column of mercury facing the vapors of a liquid is _____ due to pressure on surface of liquid in flask. (PMC Practice 2021)
Correct answer: (b) Depressed
The vapour presses on the mercury and pushes that column down.
18.In a pressure cooker, the boiling point of water is _________. (PMC Practice 2021)
Correct answer: (a) Raised than the normal
The pressure inside is above atmospheric, so the water must be hotter before its vapour pressure matches it.
19.Vapor pressure is measured by calculating the difference in liquid pressure and? (PMC Practice 2021)
Correct answer: (c) Atmospheric pressure
The vapour pressure is the difference between the pressure with the liquid present and the atmospheric pressure.
20.Which of the following has the lowest vapour pressure at 20 degrees Centigrade? (PMC MDCAT 2020)
Correct answer: (d) Water
Water is hydrogen bonded, so far fewer of its molecules escape than of the others.
21.Freezing point will also be defined as that temperature at which its solid and liquid phases have the same: (UHS MDCAT 2016)
Correct answer: (c) Vapour pressure
At the freezing point the solid and the liquid are in equilibrium, so their vapour pressures are equal.