Problem 63 - A Complete Resource Book in Chemistry for JEE Main

Question

The vapour pressure of the liquid depends upon (a) the amount of liquid taken (b) the temperature of the liquid (c) the volume of the vessel (d) both amount and temperature (e) all of these

Step-by-Step Solution

Answer

The correct answer is (b) the temperature of the liquid.

Step 1: Understanding Vapour Pressure

Vapour pressure is the pressure exerted by the vapour in equilibrium with its liquid or solid phase at a particular temperature. It is an intrinsic property of the substance.

Step 2: Analyze Option (a) - Amount of Liquid

The amount of liquid does not affect the vapour pressure. Once equilibrium is reached, vapour pressure depends only on temperature, not on how much liquid is present.

Step 3: Analyze Option (b) - Temperature of the Liquid

Temperature has a direct effect on vapour pressure. As temperature increases, kinetic energy increases, leading to a higher vapour pressure as more molecules escape the liquid phase.

Step 4: Analyze Option (c) - Volume of the Vessel

The volume of the vessel does not impact the vapour pressure as long as there is enough room for evaporation to reach equilibrium. Vapour pressure is not dependent on volume.

Step 5: Analyze Options (d) and (e)

Option (d) is incorrect as the amount does not affect vapour pressure. Option (e) is incorrect because not all factors listed affect vapour pressure.

Key Concepts

Temperature Dependence

Temperature significantly influences a liquid's vapour pressure. As temperature increases, so too does the kinetic energy of the liquid molecules. With more energy, these molecules are more likely to escape the liquid phase and enter the gas phase.
This transformation increases the vapor pressure, which is the pressure exerted by the vapor in equilibrium with its liquid at a given temperature.

• Higher temperature = higher kinetic energy
• More molecules escape to vapor phase
• Resulting in increased vapour pressure

This effect is a fundamental aspect of the behaviour of liquids and is important for understanding phase changes such as boiling.

Equilibrium in Chemistry

In chemistry, equilibrium refers to a state where the concentrations of reactants and products remain constant over time. For vapor pressure, equilibrium occurs when the rate of evaporation of a liquid equals the rate of condensation of its vapour.
At equilibrium, the pressure exerted by the vapour is known as the equilibrium vapour pressure.

• Equilibrium means no net change in the system
• Rate of evaporation = rate of condensation
• Resulting pressure is constant at a given temperature

This balance is crucial because it defines the vapour pressure of a liquid and explains why certain variables, like the amount of liquid, do not affect it once equilibrium is established.

Phase Transitions

Phase transitions occur when a substance changes from one state of matter to another, such as from a liquid to a gas. These changes are influenced by factors like temperature and pressure.
For instance, boiling is a phase transition where a liquid turns into gas, driven by reaching a certain vapor pressure at a specific temperature.

• Vapor pressure helps determine boiling point
• Boiling occurs when vapor pressure equals external pressure
• Temperature increase leads to transitions, explaining boiling and evaporation

Understanding these transitions is essential, as they illustrate how temperature and pressure dictate the physical state of substances and are central to many industrial and natural processes.