Free Evaporation, Boiling Point and Vapour Pressure MCQs with Answers

• 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…

• 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…

• 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…

• 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…

• 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…

• 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…

• 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…

• 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…

• A. Extra heat is spread in the air
• B. High energy molecules continuously leaving the surface of liquid
• C. The forces of attractions become stronger
• D. The vapour pressure of the boiling liquid becomes greater than the external pressure

Explanation: At the boiling point, supplied heat is used as latent heat to overcome intermolecular attractions and convert liquid into vapour, not to…

• A. 1/2sec
• B. 1/3rd
• C. 1/4th
• D. 1/5th

Explanation: A commonly quoted estimate is that about one-third of the solar energy absorbed at Earth's surface drives evaporation, which powers the…

• A. concentrated
• B. dilute
• C. saturated solution
• D. all of the above

Explanation: For a dilute solution of a non-volatile solute, the relative lowering of vapour pressure equals the mole fraction of solute and is…

• A. boiling point elevation
• B. depression in freezing point
• C. lowering of vapour pressure
• D. lowering of osmotic pressure

Explanation: A Beckmann apparatus, particularly its sensitive Beckmann thermometer arrangement, is commonly used to measure the depression in freezing…

• A. a volatile solute in dilute solution
• B. a non-electrolyte & non volatile solute in concentrated solution
• C. a non-electrolyte & non volatile solute in concentrated solute
• D. non volatile solute in a dilute solution

Explanation: Raoult's law gives the relative lowering of vapour pressure for a dilute solution containing a non-volatile solute, and this relation is…

• A. ideal solution
• B. zeotropic solution
• C. azeotropic solution
• D. non-ideal solution

Explanation: A zeotropic mixture changes composition during distillation because its vapour and liquid phases have different compositions.

• A. will be an ideal solution
• B. will be non-ideal solution
• C. will show deviations from Raoults law
• D. both b & c

Explanation: If the volume of a solution equals the sum of the component volumes, the volume change on mixing is zero, which is a property of an ideal…

• A. zeotropic mixture
• B. azeotropic mixture
• C. amphoteric mixture
• D. ideal solution

Explanation: An azeotropic mixture boils and distils at a constant composition because its vapour has the same composition as the liquid.

• A. equal to the mole fraction of solvent
• B. equal to the mole fraction of solute
• C. directly proportional to the mole fraction of solute
• D. both B & C

Explanation: For a solution containing a non-volatile solute, Raoult's law gives (P° − P) / P° = Xsolute, so relative lowering of vapour pressure…

• A. Boiling point of water decrease
• B. Boiling point of water increase
• C. Boiling point of water remains same
• D. None of the above

Explanation: Dissolved salt lowers the vapour pressure of water, so a higher temperature is needed for the solution's vapour pressure to equal…

• A. Surface tension
• B. Aqueous tension
• C. Vapour pressure
• D. Hydraulic pressure

Explanation: The pressure exerted by water vapour is its vapour pressure; when measured in the presence of another gas, it is also commonly called…

• A. Melting
• B. Sublimation
• C. Decomposition
• D. Condensation

Explanation: A substance with very high vapour pressure can pass directly from solid to vapour when heated, provided its vapour pressure exceeds the…