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 a vapour in equilibrium with its liquid or solid phase at a given temperature.

Step 2: Identify Factors Influencing Vapour Pressure

Vapour pressure is primarily affected by the temperature of the liquid. It is not influenced by the amount of liquid present or the volume of the vessel containing the liquid.

Step 3: Analyze Each Option

(a) The amount of liquid does not affect vapour pressure, so this option is incorrect. (b) Temperature significantly affects vapour pressure as it dictates the energy of particles. (c) The volume of the vessel does not impact vapour pressure when the liquid is in equilibrium; hence, this option is incorrect. (d) Amount and temperature together are not correct since only temperature influences vapour pressure. (e) All of these are incorrect because only temperature affects vapour pressure.

Step 4: Conclusion

The vapour pressure of a liquid depends solely on its temperature. Therefore, the correct answer is (b) the temperature of the liquid.

Key Concepts

Equilibrium

In physics and chemistry, equilibrium refers to a state where opposing forces or influences are balanced. For vapour pressure, it is the condition where the rate of evaporation of the liquid equals the rate of condensation of the vapour. This balance ensures that the concentration of molecules in the vapor phase remains constant unless external conditions change.
Achieving equilibrium is vital for a steady vapour pressure. Without it, the pressure would continually change, making it hard to predict or measure accurately. When a liquid and its vapour are at equilibrium in a closed system, no net change occurs, even though individual molecules may constantly switch between phases.
This balance is dynamic, implying that while the macroscopic properties like vapour pressure remain constant, microscopic events like evaporation and condensation continue. Only shifts in significant environmental factors, like temperature, move the system away from or establish new equilibrium situations.

Temperature Influence

Temperature is a crucial factor in determining vapour pressure. As temperature rises, the kinetic energy of the liquid molecules increases. These fast-moving molecules are more likely to escape the surface and enter the vapor phase. This increase in molecule escape leads to higher vapour pressure.
Essentially, higher temperatures mean more energy, enhancing the ability of molecules to overcome intermolecular forces during phase transitions.

• At a higher temperature, more molecules have enough energy to vaporize.
• The system reaches a new equilibrium with greater vapour pressure.
• Vapour pressure rises exponentially with temperature, reflecting the non-linear nature of this effect.

The relationship between temperature and vapour pressure is fundamentally important for applications ranging from weather systems to industrial processes, as it helps predict how substances behave under different conditions.

Factors Affecting Vapour Pressure

Although temperature is the primary factor affecting vapour pressure, some inherent characteristics of the liquid also play indirect roles. Notably, the liquid's nature and intermolecular forces affect how readily molecules transition to the vapour phase.
Substances with weak intermolecular forces, like volatile liquids (e.g., alcohols), exhibit higher vapour pressures due to easier molecule escape. Conversely, liquids with strong intermolecular forces, such as water or other polar substances, retain molecules more effectively, exhibiting lower vapour pressures at the same temperature.
Other factors such as the surface area of the liquid or the presence of solutes can modulate the rate of evaporation; however, once in equilibrium, they do not affect the actual vapour pressure. Ultimately, while temperature is the dominant factor, understanding these secondary influences is essential for comprehensive insights into the behaviour of different liquids and how environmental conditions affect their vapour pressures.