Question

Vapour pressure of a liquid increases with (a) decrease in temperature (b) increase in temperature (c) increase in surface area (d) increase in volume.

Short answer

The vapor pressure of a liquid increases with (b) increase in temperature.

Step-by-step solution

Understand the concept of vapor pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase. It increases with temperature because as temperature increases, more molecules have enough kinetic energy to escape from the liquid phase into the vapor phase.

Eliminate incorrect options

The vapor pressure of a liquid is not directly affected by surface area or volume, as these are not factors that affect the equilibrium between the liquid and vapor phases in a closed system. Decrease in temperature generally leads to a decrease in vapor pressure.

Identify the correct option

Since vapor pressure increases with temperature, we can conclude that the correct option is (b) increase in temperature.

Key concepts

Equilibrium

Equilibrium is a fundamental concept in chemistry, physics, and many other scientific fields. It signifies a state of balance where two opposing processes occur at equal rates, leading to a stable system. When we relate this to vapor pressure, equilibrium is achieved when the rate at which molecules evaporate from the liquid surface is equal to the rate at which they condense back into the liquid.

This balance does not imply that no movement is happening, but rather that there is no net change over time. In a closed container, this equilibrium results in a certain vapor pressure, which is characterized by the constancy of its value as long as the system’s temperature remains stable. Understanding this dynamic equilibrium is crucial when studying the behavior of liquids and gases, as well as the conditions that affect their transition between phases.

Kinetic Energy

Kinetic energy is the energy possessed by an object due to its motion. In the context of vapor pressure, it's the kinetic energy of the molecules that determines their state. At higher temperatures, molecules have more kinetic energy and, consequently, move faster.

This increase in molecular motion leads to a higher likelihood of molecules breaking the intermolecular forces that keep them in the liquid phase and escaping into the gas phase, which increases the vapor pressure.

Hence, understanding kinetic energy is essential for explaining why higher temperatures lead to increased vapor pressures. The laws of thermodynamics, which govern the behavior of energy in systems, underline the key relationship between temperature, kinetic energy, and the phase transitions of substances.

Phase Transition

A phase transition is a transformation of a substance from one state of matter to another, such as from a liquid to a gas. This phenomenon is integral to the concept of vapor pressure. When a substance transitions from a liquid to a vapor, we call this process evaporation or vaporization.

The transition requires energy, typically in the form of heat, which increases the kinetic energy of the molecules. When the kinetic energy is sufficient to overcome the intermolecular forces, a molecule breaks free and becomes part of the vapor above the liquid.

The temperature at which the vapor pressure equals atmospheric pressure is the boiling point, which is another crucial phase transition. During this process, the entire liquid turns into vapor. Understanding phase transitions provides insight into how substances change under various temperature and pressure conditions, predicting their behavior in different environments.