Free Evaporation, Boiling Point and Vapour Pressure MCQs with Answers

1. Evaporation of a liquid causes cooling because

• A. the surface of the liquid contracts
• B. cold air replaces the escaping vapour
• C. the vapour formed is colder than the liquid
• D. the fastest moving molecules escape, lowering the average kinetic energy of those left behind

Explanation: Only molecules with above average kinetic energy can break free from the surface, so the average energy of the remaining liquid falls and the temperature drops with it. This is why sweating cools the body and why water in an earthen pot stays cool. Evaporation happens at all temperatures, unlike boiling, which occurs only at one specific temperature for a given pressure.

2. The vapour pressure of a liquid is the pressure exerted by its vapour when

• A. the liquid has completely evaporated
• B. the vapour is in dynamic equilibrium with the liquid in a closed container
• C. the liquid is boiling in an open vessel
• D. the container is completely empty of air

Explanation: In a closed vessel evaporation and condensation eventually occur at the same rate, and the constant pressure of vapour at that point is the equilibrium vapour pressure of the liquid at that temperature. It depends only on the liquid and the temperature, never on the volume of the container or the amount of liquid present. In an open vessel equilibrium is never reached because vapour escapes.

3. A liquid boils when its vapour pressure becomes equal to

• A. zero
• B. its surface tension
• C. the external atmospheric pressure
• D. the vapour pressure of water

Explanation: Bubbles of vapour can only form and rise through the bulk of the liquid when the vapour pressure inside them matches the pressure pushing down from outside. Because atmospheric pressure falls with altitude, water boils below 100 degrees Celsius on a mountain, which is why food takes longer to cook there. A pressure cooker uses the same relationship in reverse.

4. Which of the following liquids would be expected to have the highest vapour pressure at room temperature?

• A. Water
• B. Glycerol
• C. Ether
• D. Mercury

Explanation: Ether has weak dispersion forces between its molecules, so a large fraction escape the surface easily and it is highly volatile, which is why it evaporates almost as soon as it is spilt. Water and glycerol are held by extensive hydrogen bonding and mercury by metallic bonding, all of which are far stronger. Vapour pressure and boiling point always run in opposite directions.

5. The energy required to convert one mole of a liquid into vapour at its boiling point is called the

• A. molar heat of fusion
• B. molar heat of vaporisation
• C. lattice energy
• D. activation energy

Explanation: Molar heat of vaporisation measures the energy needed to overcome the intermolecular forces completely, so it is large for hydrogen bonded liquids such as water at about 40.7 kJ per mole. Heat of fusion is the smaller quantity needed to melt a solid, because melting loosens the lattice without separating the particles. Neither is related to lattice energy, which concerns ionic solids.

6. As the temperature of a liquid rises, its vapour pressure

• A. decreases
• B. remains constant
• C. increases
• D. first increases and then decreases

Explanation: Higher temperature means a larger fraction of molecules have enough kinetic energy to escape the surface, so more vapour is present at equilibrium and the pressure rises, following an exponential rather than a straight line relationship. The boiling point is simply the temperature at which this rising curve meets the external pressure. This is why a liquid in a sealed container can burst it when heated.

7. A pressure cooker cooks food faster because increasing the pressure inside

• A. lowers the boiling point of water
• B. removes all the air from the vessel
• C. increases the amount of steam produced
• D. raises the boiling point of water above 100 degrees Celsius

Explanation: A liquid boils when its vapour pressure equals the external pressure, so raising that external pressure means a higher temperature is needed, and water inside a pressure cooker can reach about 120 degrees Celsius. The food cooks faster because the reaction rate rises with temperature, not because there is more steam. At high altitude the reverse problem arises and cooking takes longer.

8. Which one of the following has higher vapour pressure?

• A. Acetone
• B. Acetaldehyde
• C. Isopentane
• D. Benzene

Explanation: This is the option marked correct in the official UHS answer key. Isopentane is a branched non polar hydrocarbon whose only intermolecular attractions are weak dispersion forces, and branching reduces the contact area further, so it evaporates readily. Note that acetaldehyde, boiling at about 21 degrees Celsius, is in fact the more volatile of the two in standard data, so this is a question where the official key is worth knowing alongside the underlying principle that weak intermolecular forces mean high vapour pressure.