Problem 65 - A Complete Resource Book in Chemistry for JEE Main

Question

When a liquid is heated (a) surface tension increases (b) surface tension is lowered (c) viscosity increases (d) surface tension is not effected

Step-by-Step Solution

Answer

(b) Surface tension is lowered.

Step 1: Understanding Surface Tension in Relation to Temperature

When a liquid is heated, the kinetic energy of its molecules increases. This energy allows the molecules to move more freely, reducing the cohesive forces between them on the surface. Surface tension is the result of these cohesive forces, so we need to determine how increased temperature affects them.

Step 2: Investigating Changes in Surface Tension

As temperature rises, the cohesive forces among the liquid molecules at the surface decrease due to increased molecular movement. This decrease in cohesive forces leads to a decrease in surface tension. Therefore, heating a liquid typically results in a lower surface tension.

Step 3: Conclusion Based on Observations

Based on the investigation, we conclude that heating a liquid decreases its surface tension. Thus, among the given options, the correct choice is that surface tension is lowered when a liquid is heated.

Key Concepts

Viscosity

Viscosity can be thought of as the thickness or resistance a liquid has against flowing. Imagine honey versus water—honey is much thicker, so we say it has higher viscosity. Viscosity depends heavily on the liquid's internal molecular structure and the interactions between these molecules. The more molecules stick together (like in honey), the higher the viscosity.

• High Viscosity: Flows slowly (e.g., honey)
• Low Viscosity: Flows quickly (e.g., water)

When considering the question of whether heating a liquid causes viscosity to increase or decrease, it's important to understand that heating a liquid typically reduces its viscosity. As a liquid heats, its molecules move faster. This increased movement helps to liberate them from the stickiness of their neighbors, thereby allowing the liquid to flow more easily.

Temperature Effects

Temperature has a significant influence on the properties of liquids. An increase in temperature generally increases the kinetic energy of molecules. More kinetic energy means molecules move faster and more freely.
This change in movement often affects different properties of liquids, such as:

• Surface Tension: As temperature increases, surface tension decreases typically because the molecules at the surface can overcome their attraction to each other more easily.
• Viscosity: Similarly, liquids often become less viscous with rising temperatures due to the same increase in molecular movement.
• Color and Bubbles: Not directly related to temperature, but as liquids heat, you might see color changes or bubbles from boiling—these are visible signs of physical changes occurring within.

In short, higher temperatures lead to an increased molecular activity, making liquids behave differently.

Cohesive Forces

Cohesive forces are the attractions that occur between molecules in a substance, keeping them together. Picture molecules holding hands tightly; that's cohesion in action. These forces are crucial in determining properties like surface tension and viscosity of a liquid.
Cohesive forces arise due to:

• Attractive forces between similar molecules.
• Strong intermolecular forces, such as hydrogen bonds or Van der Waals forces.

In the context of liquids, these forces are responsible for keeping the molecules at the surface tightly bound, contributing to the surface tension. When a liquid is heated, the cohesive forces between its molecules generally weaken because the molecules get more energy to move around.
As cohesive forces weaken, surface tension decreases as well, aligning with the original exercise’s conclusion that heating decreases surface tension. Understanding these concepts helps explain why surface tension lowers when temperature rises.