Problem 64 - A Complete Resource Book in Chemistry for JEE Main

Question

When a liquid is evaporated (a) temperature of liquid decreases (b) temperature of the liquid increases (c) liquid molecules become more energetic (d) both (b) and (c)

Step-by-Step Solution

Answer

Option (c) is correct: liquid molecules become more energetic.

Step 1: Understand Evaporation

Evaporation is a process where molecules at the surface of a liquid gain sufficient energy to overcome the attractive forces of the liquid and convert into the gas phase. This occurs at any temperature when molecules collide with enough energy.

Step 2: Analyze Temperature Change

When molecules evaporate, they take away heat energy from the liquid, causing the temperature of the remaining liquid to decrease. Therefore, the temperature of the liquid does not increase during evaporation.

Step 3: Consider Molecular Energy

The molecules that escape into the gas phase have more kinetic energy compared to the molecules that remain in the liquid. Hence, the liquid molecules that evaporate become more energetic.

Step 4: Determine Correct Answer

Combining the insights from the above steps, we know that during evaporation: the liquid temperature decreases and the evaporated molecules are more energetic. Therefore, option (c) is correct.

Key Concepts

Molecular Energy

Molecular energy refers to the energy possessed by molecules, primarily in the form of kinetic energy. In the context of evaporation, molecular energy plays a vital role in determining whether a molecule can escape from the liquid phase into the gas phase.
When a liquid evaporates, molecules at its surface require sufficient energy to overcome the intermolecular forces holding them within the liquid. This energy is typically provided by heat or as a result of molecular collisions. As molecules absorb energy and gain speed, their kinetic energy increases.

Importantly, only those molecules with enough energy can break free to become gas. This select group of molecules is crucial to understanding why some evaporate while others remain. As a result, the molecules that manage to evaporate are more energetic compared to the average molecules in the liquid, leading to the phenomenon of increased molecular energy among the evaporated molecules.

Temperature Change

Temperature change is a key factor during the evaporation process. Understanding this concept offers insights into the dynamics that take place in a liquid undergoing phase transition.
The core principle is related to how heat energy interacts with molecules at the liquid's surface. When these molecules gain enough energy to evaporate, they take that energy with them as they leave the liquid. This process is often referred to as evaporative cooling.

Evaporative cooling results in a decrease in temperature for the remaining liquid. This decrease occurs because the more energetic molecules, which were contributing to the higher average kinetic energy (and thus temperature) of the liquid, have left. Consequently, the remaining liquid has a lower average kinetic energy and a reduced temperature.

• The higher energy molecules leave, cooling the liquid.
• The average kinetic energy and temperature of the liquid decrease.
• This is why liquids feel cooler during evaporation.

Phase Transition

A phase transition describes the process of a substance changing from one state of matter to another. In the case of evaporation, it is a transition from the liquid phase to the gas phase. This transition requires energy to overcome the intermolecular forces keeping molecules in the liquid state.
Evaporation is a type of phase transition that can occur at any temperature. Unlike boiling, which happens at a specific temperature, evaporation depends on individual molecular energy. This feature allows molecules with enough kinetic energy to escape into the gaseous phase, even at lower temperatures.

This transition is crucial in the context of evaporation because it explains how and why molecules leave the liquid phase:

• Molecules must overcome intermolecular forces to transition to gas.
• The energy for this transition comes from the liquid and is taken away when molecules evaporate.
• Phase transitions are important for understanding energy transfer in evaporation.