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

Surface tension is a physical property of liquids, influenced by the intermolecular forces at the surface of the liquid. It measures the elastic tendency of a liquid, which makes it acquire the least surface area possible. Typically, a higher temperature weakens the intermolecular forces.

Step 2: Effect of Temperature on Intermolecular Forces

When a liquid is heated, the kinetic energy of the molecules increases. This increase in energy causes the molecules to move more vigorously, weakening the cohesive forces between the molecules at the surface.

Step 3: Analyzing the Direct Effect on Surface Tension

As the intermolecular forces weaken due to the increase in temperature, the surface tension of the liquid decreases. This is because surface tension is directly related to the strength of these forces.

Step 4: Conclusion from Options Given

Based on the understanding that heating a liquid generally decreases its surface tension due to weaker intermolecular forces, we identify that option (b) 'surface tension is lowered' is the correct choice.

Key Concepts

Intermolecular Forces

Intermolecular forces play a crucial role in determining the physical properties of liquids and solids. These forces are the attractive forces between molecules, which hold them together. They can be thought of as the "glue" that keeps molecules from flying apart. There are several kinds of intermolecular forces, but the most common are:

• **Dipole-dipole forces**: These occur between polar molecules, where positive and negative dipoles from different molecules attract each other.
• **London dispersion forces**: Present in all molecules, these are weak attractions between temporary induced dipoles in molecules.
• **Hydrogen bonds**: A particularly strong type of dipole-dipole interaction, occurring when hydrogen is covalently bonded to electronegative atoms like nitrogen, oxygen, or fluorine.

Surface tension arises because of the intermolecular forces at the liquid surface, creating a kind of "film" that makes the liquid more resistant to external forces. This property is why small insects can walk on water and why liquid droplets form spheres.
When we heat a liquid, the kinetic energy of its molecules increases, leading to a reduction in these cohesive intermolecular forces.

Effect of Temperature

Temperature significantly influences the behavior of liquids. As temperature increases, particles in a substance move faster and become more agitated. This increase in kinetic energy affects how molecules interact with each other in a liquid. The three main impacts of heating a liquid include:

• **Increased kinetic energy**: Molecules move more vigorously, causing the bonds between them to weaken.
• **Decreased intermolecular attraction**: As molecules move faster, they are less attracted to each other, which decreases forces like surface tension.
• **Lower viscosity**: When heated, liquids flow more easily because molecules can move past each other with less resistance.

By understanding these effects, we can predict that surface tension will decrease with a rise in temperature. This is because as the molecules at the surface move faster and further apart, the bonds between them are not as strong, leading to a drop in surface tension.

Viscosity

Viscosity is the measure of a fluid's resistance to flow. It's essentially how "thick" or "thin" a liquid is. For example, honey has a high viscosity because it flows slowly, whereas water has a low viscosity, which allows it to flow quickly. Several factors determine the viscosity of a liquid:

• **Intermolecular forces**: Stronger intermolecular forces generally mean higher viscosity, as molecules stick together more, hindering flow.
• **Temperature**: As the temperature of a liquid increases, its viscosity usually decreases. This is because the added heat gives molecules more energy to overcome intermolecular forces.

When a liquid heats up, it becomes less viscous. This is because the increased molecular motion allows particles to slide past each other with less friction. Consequently, the flow improves, making the liquid smoother in its movement. This explains why heating up substances like oil or syrup results in easier pouring and spreading.