When vapour condenses into liquid

Options

A. it absorbs heat
B. it liberates heat
C. its temperature increases
D. its temperature decreases

Answer

To solve the question regarding the process of water vapor condensing into liquid, we can break it down into clear steps:

### Step-by-Step Solution:

1. **Understanding the Phases of Water**:
- Water exists in three phases: solid (ice), liquid (water), and gas (water vapor). In this question, we are focusing on the transition from the gaseous phase (water vapor) to the liquid phase (water).

2. **Identifying the Process**:
- The process of water vapor changing into liquid is known as condensation. This typically occurs when the vapor cools down to its dew point or when it comes into contact with a cooler surface.

3. **Heat Exchange During Condensation**:
- During condensation, water vapor releases heat. This is because the molecules in the gas phase have higher kinetic energy compared to those in the liquid phase. When the vapor condenses, the kinetic energy decreases, and the excess energy is released as heat.

4. **Temperature Behavior**:
- It is important to note that during the phase change from vapor to liquid, the temperature of the substance does not change. This is a characteristic of phase changes; while heat is being released, the temperature remains constant until the entire vapor has condensed into liquid.

5. **Conclusion**:
- Therefore, when water vapor condenses into liquid, heat is liberated, and the temperature remains constant during this phase change.

### Final Answer:
- **Heat is liberated (option B)**.