In an experiment to determine the percentage of water in a solid hydrate by heating, what is the best indication that all the water has been removed?

Options

A. The solid melts
B. The solid changes colour.
C. Water vapour no longer appears.
D. Successive weighings give the same mass.

Answer

To determine the percentage of water in a solid hydrate by heating, the best indication that all the water has been removed is to observe that successive weighings of the solid yield the same mass. Here’s a step-by-step solution to the question:

### Step-by-Step Solution:

1. **Understanding the Experiment**:
- When a hydrate is heated, the water of crystallization is released as vapor. The goal is to determine how much water was originally present in the hydrate.

2. **Heating the Hydrate**:
- Place the solid hydrate in a crucible and heat it gently. The water will start to evaporate.

3. **Observing Changes**:
- As the hydrate is heated, you may observe steam or vapor rising from the crucible. This indicates that water is being released.

4. **Allowing the Sample to Cool**:
- After heating for a sufficient amount of time, remove the crucible and allow it to cool to room temperature.

5. **Weighing the Sample**:
- Weigh the crucible with the remaining solid after cooling. Record this mass.

6. **Repeating the Weighing**:
- Heat the crucible again for a few minutes, then allow it to cool and weigh it once more. Repeat this process several times.

7. **Determining Consistency in Mass**:
- The best indication that all the water has been removed is when successive weighings of the solid yield the same mass. This means that no more water is being lost from the hydrate.

8. **Final Calculation**:
- Once you have consistent mass readings, you can calculate the percentage of water in the original hydrate using the formula:
\[ \text{Percentage of Water} = \left( \frac{\text{Mass of Water Lost}}{\text{Mass of Hydrate}} \right) \times 100 \]

### Conclusion:
The best indication that all the water has been removed from the hydrate is when successive weighings give the same mass.