Question

Which of these is not correct? (1) Viscosity of a liquid usually decreases with increased temperature. (2) Viscosity of a gas usually increases with increased temperature. (3) Viscosity of a gas usually decreases with increased temperature. (4) Surface tension of a liquid decreases with temperature.

Short answer

Statement (3) is not correct.

Step-by-step solution

- Understand Viscosity Changes in Liquids

Viscosity of a liquid is a measure of its resistance to flow. Generally, as temperature increases, the molecules in a liquid move faster, which reduces viscosity.

- Evaluate Statement (1)

Statement (1) says 'Viscosity of a liquid usually decreases with increased temperature.' This is correct based on the understanding that higher temperatures lower the resistance to flow in liquids.

- Understand Viscosity Changes in Gases

For gases, viscosity increases with temperature because the gas molecules move more vigorously and collide more often.

- Evaluate Statement (2)

Statement (2) says 'Viscosity of a gas usually increases with increased temperature.' This is correct since the behavior of gas viscosity increases with higher temperatures.

- Evaluate Statement (3)

Statement (3) says 'Viscosity of a gas usually decreases with increased temperature.' This contradicts the previously understood principle. Therefore, this statement is incorrect.

- Understand Surface Tension Changes in Liquids

Surface tension in a liquid usually decreases with an increase in temperature because the cohesive forces between liquid molecules are reduced.

- Evaluate Statement (4)

Statement (4) says 'Surface tension of a liquid decreases with temperature.' This aligns with the understanding that higher temperatures reduce cohesive forces, so it is correct.

Key concepts

Viscosity in liquids

Viscosity refers to the resistance of a liquid to flow. Think of honey versus water. Honey is much thicker and flows more slowly compared to water. This difference in flow is due to their viscosities.
In liquids, viscosity generally decreases with an increase in temperature. As the temperature rises, the molecules within the liquid gain more energy. This increased energy causes the molecules to move faster and slip past each other more easily.
For example:

• Imagine trying to pour cold honey and then warm honey. Warm honey flows much faster because its viscosity decreases with heat.

So, in summary, higher temperatures make it easier for the liquid to flow by reducing its viscosity.

Viscosity in gases

Unlike liquids, the behavior of gases in relation to viscosity is quite different. For gases, viscosity actually increases with temperature.
As temperature increases, gas molecules move more vigorously and collide more frequently. These collisions cause resistance to flow, thereby increasing viscosity.
Think of it this way:

• Imagine a room full of bouncing balls. If the balls bounce faster (higher temperature), they collide more often, creating resistance to movement.

Understanding this concept clarifies why statement (2) in the exercise is correct: Gas viscosity increases with temperature.

Surface tension in liquids

Surface tension is the elastic tendency of a liquid's surface that allows it to resist external force. This is what allows small insects to walk on water or small droplets to form.
Surface tension in liquids decreases with an increase in temperature. As temperature rises, the increased molecular activity reduces the cohesive forces that hold the surface molecules together.
A simple demonstration of this:

• If you heat water, you'll observe that its surface tension reduces, making it easier to form bubbles or to spill.

So, higher temperatures lead to decreased surface tension in liquids because the cohesive forces between the molecules are weakened.

Temperature effects on viscosity and surface tension

Temperature significantly impacts both viscosity and surface tension, but in different ways for liquids and gases.
For liquids, increased temperature:

• Decreases viscosity, making it easier for the liquid to flow.
• Reduces surface tension, weakening the cohesive forces between molecules.

For gases, increased temperature:

• Increases viscosity due to more frequent molecular collisions.

Understanding these effects helps explain how substances behave under different temperature conditions, making it easier to predict their behavior in real-world applications.
This knowledge is crucial in fields like chemistry, engineering, and even cooking, where controlling fluid flow and surface tension can be key to success.