Q. 4.27 - An Introduction to Thermal Physics

Question

In table 4.1, why does the entropy of water increase with increasing temperature, while the entropy of steam decreases with increasing temperature?

Step-by-Step Solution

Answer

Because the overall change in temperature outweighs the change in volume, entropy for water molecules rises.

In the case of steam, the overall change in volume drops more than the temperature increase, and so the entropy decreases.

Step 1. Introduction

There are 3 basic parameters that would be affecting the entropy conditions of water and steam accordingly. 

The basic behaviour of entropy is that it will increase with increase in randomness or the disorderness among the molecules.

Step 2. Explanation for water

The kinetic energy of particles increases as the temperature rises, and as a result, particle motion increases, causing the water molecules to move more quickly, increasing the entropy.

Along with the increase in temperature, the pressure of the liquid particles increases, resulting in a change in volume. However, because the overall change in temperature outweighs the change in volume, the chaos among molecules becomes even more. Water molecules have a higher entropy as a result of this.

As given in the data the respective pressure values are also to be considered.

Step 3. Explanation for steam

The increase in temperature is the first parameter, followed by the increase in pressure. Because the total number of particles does not rise, the volume of steam particles begins to decrease. For steam particles, unlike water, the decrease in overall volume takes over the parameter of temperature increase.

However, the overall change in volume decreases more than the temperature increase, resulting in a decrease in entropy.