Short Answer

Temperature remains constant during a phase change because the heat energy supplied is used to break or form intermolecular forces, not to increase the kinetic energy of the particles. The energy goes into changing the phase rather than raising temperature.

For example, when ice melts at 0°C, heat is absorbed to overcome hydrogen bonds in water molecules, but the temperature stays the same until all ice has turned into liquid. Similarly, water boiling at 100°C absorbs heat without a rise in temperature until all water becomes steam.

Detailed Explanation

Temperature and Phase Changes

During a phase change, a substance transforms from one state to another, such as solid to liquid, liquid to gas, or solid to gas. Although heat is absorbed or released, the temperature remains constant because the energy is used to overcome or strengthen intermolecular forces instead of increasing particle motion.

Temperature measures the average kinetic energy of particles. During a phase change, energy goes into potential energy changes, associated with particle arrangement and separation, keeping kinetic energy, and thus temperature, constant.

1. Role of Intermolecular Forces

Phase changes involve breaking or forming intermolecular bonds:

• Melting (Solid → Liquid): Heat breaks some forces holding particles in a rigid structure.
• Freezing (Liquid → Solid): Energy is released as forces form a fixed structure.
• Vaporization (Liquid → Gas): Energy breaks attractions, allowing molecules to escape as gas.
• Condensation (Gas → Liquid): Energy is released as molecules come closer together.
• Sublimation (Solid → Gas): Energy breaks all attractions to convert solid directly into gas.
• Deposition (Gas → Solid): Energy is released as molecules form solid lattice.

During all these processes, energy is involved in potential energy changes, not kinetic energy. Therefore, the temperature, which depends on kinetic energy, remains constant.

2. Latent Heat and Constant Temperature

The latent heat is the heat required for a phase change:

• Latent Heat of Fusion: Energy absorbed or released during melting/freezing.
• Latent Heat of Vaporization: Energy absorbed or released during boiling/condensation.

When heat is supplied during melting or boiling:

1. Energy goes into overcoming forces between molecules.
2. Temperature does not rise until the phase change is complete.
3. After the phase change, additional heat increases kinetic energy, raising temperature.

For example:

• Melting 1 kg of ice at 0°C requires 334,000 J. Temperature remains 0°C until all ice melts.
• Boiling 1 kg of water at 100°C requires 2,260,000 J, but temperature remains 100°C until all water becomes steam.

This explains why temperature remains constant despite energy absorption or release.

3. Examples in Daily Life

• Ice melting in drinks: Heat from the drink melts ice without raising temperature of ice-water mixture until fully melted.
• Boiling water: Water absorbs heat at 100°C, turning into steam, with no temperature change until all water vaporizes.
• Frost formation: Water vapor deposits as ice, releasing heat without temperature rise in ice.

These examples demonstrate the principle in practical situations.

4. Importance of Constant Temperature During Phase Change

• Predictable Energy Requirement: Engineers can calculate energy needed for melting or boiling.
• Climate and Environmental Effects: Melting glaciers and evaporation absorb energy without changing air temperature immediately.
• Industrial Processes: Freeze-drying, distillation, and heating/cooling systems rely on constant temperature during phase changes.

Understanding this concept is crucial in thermodynamics and practical applications.

5. Relationship Between Kinetic and Potential Energy

• Kinetic Energy: Determines particle motion and temperature.
• Potential Energy: Increases or decreases during phase change as particles overcome or form forces.
• During a phase change, energy is transferred to potential energy, keeping kinetic energy constant, so temperature does not change.

This balance explains the stability of temperature despite large energy input or release.

Conclusion

Temperature remains constant during a phase change because the energy supplied is used to overcome or form intermolecular forces rather than increasing particle motion. Latent heat ensures that phase transitions like melting, boiling, sublimation, freezing, condensation, and deposition occur at a constant temperature. This concept is essential for understanding energy transfer, industrial applications, and natural processes involving phase changes.