Problem 61 - Master Resource Book in JEE Main Physics

Question

The surface tension of a liquid at its boiling point (a) becomes zero (b) becomes infinity (c) is equal to the value at room temperature (d) is half to the value at the room temperature

Step-by-Step Solution

Answer

The surface tension of a liquid at its boiling point becomes zero.

Step 1: Understanding the Concept

Surface tension is a property of a liquid that describes the elastic tendency of its surface, allowing it to resist an external force. At the boiling point, the liquid molecules have enough kinetic energy to escape into the vapor phase, reducing this tendency.

Step 2: Analyzing the Boiling Point

At the boiling point, a liquid's molecules have increased energy, which means they break free more easily from the surface of the liquid. This leads to the conclusion that the surface tension decreases with increasing temperature, especially reaching a minimum or zero at the boiling point.

Step 3: Selecting the Correct Choice

Based on the changes in molecular kinetic energy and behavior at the boiling point, the surface tension of a liquid tends to become zero. Thus, the correct answer is option (a).

Key Concepts

Boiling Point

The boiling point is a significant hallmark for understanding the behavior of liquids. It is the temperature at which a liquid's vapor pressure equals the atmospheric pressure. Simply put, this is when bubbles of vapor begin to form within the liquid, rather than just on the surface. During this process, a lot of heat energy is absorbed.

At this stage, the molecules inside the liquid gain enough energy to break free from the intermolecular forces holding them together, triggering a transformation from liquid to gas. This is why a liquid boils.

• The boiling point can vary based on the type of liquid and the pressure in the surrounding environment.
• An increase in atmospheric pressure raises the boiling point, while a decrease lowers it.

The concept of boiling is crucial to various applications, like cooking and engineering processes. It's essential for understanding how different conditions can affect the state of substances.

Molecular Kinetic Energy

Molecular kinetic energy is the energy possessed by molecules due to their motion. In liquids, molecules are constantly moving, rotating, vibrating, and sliding past each other. The kinetic energy of these molecules is significantly influenced by temperature.

As the temperature of a liquid increases, so does the kinetic energy of its molecules. This higher energy results in faster movement and increases the tendency of molecules to overcome the forces that hold them together. Due to this increased movement, molecules can escape into a gaseous state more easily at higher temperatures.

• Rising kinetic energy leads to an increase in molecular agitation, causing bonds within the liquid to break.
• When the kinetic energy is sufficient to overcome the intermolecular forces, the liquid boils and the molecules enter the vapor phase.

Understanding molecular kinetic energy is essential for explaining changes in physical states and the effects of heat on materials.

Temperature Effect on Liquids

The temperature has a profound effect on liquids, dictating the behavior of the molecules within them. As the temperature of a liquid increases, two major phenomena occur related to surface tension and kinetic energy.

Firstly, increasing temperature lowers surface tension. As heat energy is introduced, molecules at the surface gain kinetic energy and start escaping into the air, leading to a decrease in the cohesive forces that were holding the molecules together on the surface. Consequently, surface tension weakens.

• This is why hot liquids appear more "fluid" or "runny" compared to their colder counterparts.
• At the boiling point, the surface tension can become zero as the liquid transforms into vapor.

Secondly, the addition of heat elevates the molecular kinetic energy as discussed earlier. This overall enhances molecular motion and activity within the liquid. Understanding these effects is vital for grasping the basics of thermodynamics and fluid dynamics.