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 vapour pressure of the liquid depends on (b) the temperature of the liquid.

Step 1: Understanding Vapour Pressure

Vapour pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. It is primarily dependent on the molecules' ability to escape from a liquid surface.

Step 2: Effect of Amount of Liquid

Vapour pressure is an intensive property, meaning it does not depend on the quantity of the liquid present. Whether you have a lot or a little of the liquid, as long as some liquid is present and it is at the same temperature, the vapour pressure remains the same.

Step 3: Temperature Influence

Temperature has a significant effect on vapour pressure. As temperature increases, more molecules have enough energy to escape from the liquid's surface, increasing the vapour pressure.

Step 4: Volume of the Vessel

The vapour pressure does not depend on the volume of the vessel in which the liquid is contained. It is determined solely by the temperature for a given substance.

Step 5: Conclusion Based on Analysis

From the analysis, vapour pressure depends only on the temperature of the liquid. It is not affected by the amount of liquid, the volume of the vessel, or any combination of these factors.

Key Concepts

Thermodynamic Equilibrium

In the realm of thermodynamics, equilibrium refers to a state where a system's properties are not changing over time. This occurs when processes such as evaporation and condensation happen at equal rates. When a liquid and its vapor reach this balance in a closed container, we say they are in **thermodynamic equilibrium**. The vapor pressure of a liquid is the pressure exerted by the vapor when the liquid and its vapor are in such a balanced state. Notice that for thermodynamic equilibrium, certain conditions must be satisfied:

• The system must be closed.
• The temperature must be constant.
• The rates of processes involved (like evaporation and condensation) must equalize.

All these ensure that no external changes are affecting the system's state, and thus the vapor pressure remains a steady value under these conditions.

Intensive Properties

Intensive properties are fascinating because they do not depend on the amount of the substance present. Instead, they are properties inherently linked to the material itself, remaining constant regardless of size or quantity.

• Examples include temperature, pressure, and density.
• Whether you have a drop of liquid or a barrel full, the temperature and pressure (if in equilibrium) remain the same.

In the case of vapor pressure, it is also an intensive property. This means, as long as the temperature is constant, the vapor pressure does not change even if you double or halve the amount of liquid. Intensive properties help us understand and predict the behavior of substances under different conditions without needing to consider the quantity.

Effect of Temperature on Vapour Pressure

Temperature is the key driver that influences the vapor pressure of a liquid. When temperature rises, it provides energy to the liquid molecules. This energy allows more molecules to overcome the forces that keep them in the liquid form and escape into the vapor phase. This increase in the number of vapor molecules leads to a rise in vapor pressure.
To better visualize this:

• At low temperatures, only a few molecules have enough energy to escape into the vapor phase, resulting in lower vapor pressure.
• At higher temperatures, many more molecules can escape, thus significantly increasing the vapor pressure.

This intrinsic relationship between temperature and vapor pressure is fundamental in understanding how substances behave in different thermal environments. It's an essential concept, especially in fields like meteorology, cooking, and industrial processes where controlling temperature can influence outcomes dramatically.