Questions: Energy in Phase Changes

Question 1

What happens to the temperature of water while it is boiling at 100°C?

• A. It continues to increase above 100°C
• B. It stays constant at 100°C
• C. It decreases slightly
• D. It fluctuates up and down

Answer: (B)

During boiling, the temperature remains constant at 100°C (at standard pressure) even as heat continues to be added. All the incoming energy is used to break the attractions between liquid water molecules, converting them to gas, rather than increasing the temperature.

Question 2

Freezing is an exothermic process.

• T. True
• F. False

Answer: True

When a liquid freezes, its particles slow down and form stronger attractions with neighboring particles. This process releases energy (heat) into the surroundings. This is why the air near a freezing lake can actually feel slightly warmer — the water is releasing heat as it turns to ice.

Question 3

Explain why a heating curve has flat sections during phase changes.

Model answer: During a phase change, all the added energy goes into breaking attractions between particles rather than increasing their speed, so the temperature remains constant.

Temperature measures the average kinetic energy (speed) of particles. During a phase change like melting or boiling, the energy being added is used to overcome the forces holding particles together in the current phase, not to make them move faster. Once all the substance has changed phase, the temperature begins to rise again.