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

(c) liquid molecules become more energetic

Step 1: Understand Evaporation

Evaporation is the process where liquid molecules at the surface gain enough energy to transform into the gas phase. This occurs at any temperature below the boiling point.

Step 2: Analyze Temperature Changes

During evaporation, the molecules that escape are those with higher energy. As these energetic molecules leave, the average energy of the remaining molecules decreases. This is why the temperature of the liquid drops.

Step 3: Assess Molecular Energy

Although the temperature of the liquid decreases overall, the molecules that escape have absorbed energy to break free from the liquid's surface, making them more energetic compared to before they escaped.

Step 4: Evaluate the Options

Given that evaporation results in a temperature decrease of the liquid while escaping molecules become more energetic, option (c) 'liquid molecules become more energetic' best describes the process as these molecules have gained energy.

Key Concepts

Temperature change during evaporation

When evaporation occurs, you might observe that the temperature of the remaining liquid tends to drop. But why does this happen? Let's dive into the science of it! When a liquid is exposed to the surrounding environment, some of its surface molecules escape into the air. This phenomenon happens because these molecules have acquired enough energy to break free from the liquid's grip.
As a result, the more energetic molecules leave first, taking away energy, in the form of heat, with them. Without these high-energy molecules, the liquid that remains has a lower average energy, thus causing a decrease in temperature. Simply put, evaporation cools the liquid since the most energetic (and warmest) molecules are the ones that escape into the gaseous state.

• This process is useful in various day-to-day applications, such as sweating, which cools us down during hot weather.

Kinetic energy of molecules in evaporation

Have you ever thought about why some molecules in a liquid can escape into the air during evaporation? It's all about kinetic energy! Molecules in a liquid are constantly moving and jostling around, each with its own amount of kinetic energy.
Not all molecules possess the same energy, and those with higher energy move faster. During evaporation, the molecules at the surface with the greatest kinetic energy are the ones that have enough force to overcome the inter-molecular attractions holding them in the liquid.
Once they break free, these molecules have transitioned to a gaseous state, carrying away with them more kinetic energy compared to those that remain in the liquid. This is why evaporation is a selective process, primarily driven by the energetic molecules at the surface.

• This selective escape influences the cooling effect we discussed previously since the remaining liquid has less kinetic energy on average.

Phase transition of liquids to gas

Evaporation is a fascinating phase transition where liquid transforms into gas, occurring at temperatures below boiling. This isn't the same as boiling, which we often associate with bubbles and heat. Instead, evaporation is a gradual process that can take place in everyday situations, like a puddle drying up or clothes hanging on a line.
For a liquid to become gas, energy must be provided for molecules to separate from each other and the surface. This is why we sometimes leave wet towels in warmer, breezy places, as extra heat and air flow help the evaporation process along.
As the molecules leave the liquid and turn into gas, they're not confined to the space of their container anymore—they become free molecules in the air! This transition plays a crucial role in the water cycle, governing how water evaporates from oceans, rivers, and even leaves, depositing moisture into the atmosphere.

• Understanding this phase transition helps explain why evaporation is crucial in both natural and industrial processes.