When water is boiling, it

Analysis of the Question

The question asks about the physical state of water during the process of boiling. You need to understand what happens to the molecules of water when heat is continuously applied at its boiling point.

Explanation of the Concept

Boiling is a phase transition process where a liquid turns into a gas (vapor) at a constant temperature known as the boiling point.

1. Constant Temperature: Once water reaches its boiling point (100°C at standard atmospheric pressure), adding more heat does not increase the temperature of the water. Instead, the energy is used to break the intermolecular forces holding the water molecules together in the liquid state.
2. Phase Change: As the molecules gain enough energy, they escape the surface of the liquid and enter the gaseous phase, which we call steam (or water vapor).
3. Mass Considerations: While the mass of the water in the container decreases as it turns into steam, the fundamental physical event occurring is the change of state from liquid to gas.

Step-by-Step Solution

• Step 1: Evaluate Option A. Boiling occurs at a constant temperature (100°C for water). Therefore, the water does not get hotter while it is boiling.
• Step 2: Evaluate Option B. Boiling involves the escape of molecules into the air; it does not add mass to the water.
• Step 3: Evaluate Option C. While the mass of the liquid water decreases, this is a consequence of the process, not the definition of the process itself. The primary physical phenomenon is the change of state.
• Step 4: Evaluate Option D. This accurately describes the phase transition that defines boiling: the conversion of liquid water into gaseous steam.

Conclusion

When water boils, it undergoes a phase change from liquid to gas. Therefore, the most accurate description of the process is that it changes to steam.

Exam Tip

Remember the "Latent Heat" rule: During any phase change (melting, boiling, freezing, or condensing), the temperature of the substance remains constant. If you see a question asking about temperature change during boiling, always remember it stays the same!