Short Answer

Temperature remains constant during a change of state because the heat supplied or removed is used to break or form the intermolecular forces between particles, not to increase their kinetic energy. This hidden heat is called latent heat. Since the energy goes into changing the arrangement of particles, the thermometer shows no change in temperature.

For example, when ice melts at 0°C or water boils at 100°C, the temperature stays the same until the entire substance has changed its state. Only after the change is complete does the temperature start rising or falling again.

Detailed Explanation :

Temperature Remaining Constant During Change of State

The change of state refers to the transformation of matter from solid to liquid, liquid to gas, or the reverse processes. During these changes, temperature behaves differently from normal heating or cooling. Unlike regular conditions where temperature increases or decreases with heat, during a state change the temperature remains constant. This stable temperature is observed during melting, boiling, condensation, freezing, sublimation, and deposition.

The reason behind this is the involvement of latent heat, a special kind of heat energy that does not raise temperature but is used to change the structure of matter. Understanding why temperature remains constant helps explain many natural and daily-life processes like melting ice, boiling water, and cloud formation.

Why Temperature Does Not Change

1. Heat Energy Is Used to Break Intermolecular Forces

In solids and liquids, particles are held together by forces of attraction. When a solid melts or a liquid boils:

• Heat energy is supplied
• This energy does not increase the movement of particles
• Instead, it breaks the bonds between particles

Because the heat is used for breaking forces rather than increasing particle speed, the temperature does not rise.

Example:

When ice melts at 0°C, all heat supplied breaks the bonds between water molecules. The thermometer stays at 0°C until the entire ice becomes water.

2. Heat Energy Does Not Increase Kinetic Energy

Temperature is a measure of kinetic energy—the speed of particle movement.
During a state change:

• Kinetic energy remains the same
• Particle movement does not increase
• Only potential energy increases as bonds break

Since kinetic energy stays constant, temperature also stays constant.

3. Latent Heat Is Absorbed or Released

The heat used during state change is called latent heat.

Two kinds are important:

• Latent heat of fusion: During melting and freezing
• Latent heat of vaporization: During boiling and condensation

This energy is “hidden” because it does not show up as a temperature change.

Example:

Water stays at 100°C until all of it becomes steam because the heat is used as latent heat of vaporization.

4. Equal Energy Used Throughout the Process

The amount of energy needed to break bonds remains the same during the whole state change. That is why:

• Melting keeps the temperature constant until all solid becomes liquid
• Boiling keeps the temperature constant until all liquid becomes gas
• Freezing and condensation keep temperature constant until the whole substance changes state

The state change completes only after enough latent heat is absorbed or released.

Temperature Behavior in Different State Changes

1. Melting (Solid → Liquid)

• Heat breaks strong bonds
• Temperature stays constant at melting point
• Example: Ice remains at 0°C until fully melted

2. Freezing (Liquid → Solid)

• Heat is released as bonds form
• Temperature stays constant until all liquid becomes solid
• Example: Water stays at 0°C during freezing

3. Boiling (Liquid → Gas)

• Heat breaks forces between particles to form vapor
• Temperature remains at boiling point
• Example: Water stays at 100°C until all water becomes steam

4. Condensation (Gas → Liquid)

• Heat is released
• Temperature stays the same as particles form bonds
• Example: Steam condenses at 100°C

5. Sublimation and Deposition

• Temperature remains constant during both processes
• Energy is absorbed in sublimation and released in deposition

Why Constant Temperature Is Important

1. Helps Identify Substances

Each substance has a fixed melting and boiling point.
The constant temperature during state change helps in identification.

2. Maintains Natural Balance

Melting of ice, boiling of water, and condensation in clouds occur at constant temperatures, helping regulate climate.

3. Makes Cooling and Heating Effective

Processes like sweating, boiling, and freezing work because temperature stays constant during state change.

4. Useful in Technology

Refrigerators, air conditioners, and heat pumps depend on constant temperature during vaporization and condensation.

Conclusion

Temperature remains constant during a change of state because the heat supplied or removed is used as latent heat to break or form intermolecular forces instead of increasing kinetic energy. Since particle movement does not increase, the temperature does not rise or fall. Only after the entire substance finishes changing its state does the temperature begin to change again. This principle is essential for understanding melting, boiling, freezing, condensation, and many natural and scientific processes.