Dehydration of hydrates of halides of calcium, barium and strontium i.e., `CaCI_(2). 6H_(2)O, BaCI_(2).2H_(2)O, SrCI_(2).2H_(2)O`, can be achieved by heating. These become wet on keeping in air. Which of the following statements is correct about these halides?

Options

A. Act as dehydrating agent.
B. Can adsorb moisture from air
C. Tendency to form hydrate decreases from calcium to barium
D. All of the above

Answer

To solve the question regarding the dehydration of hydrates of halides of calcium, barium, and strontium, we will analyze the properties of the given compounds and their behavior when exposed to heat and moisture.

### Step-by-Step Solution:

1. **Understanding the Compounds**:
- We have three hydrated halides:
- Calcium chloride hexahydrate: \( \text{CaCl}_2 \cdot 6\text{H}_2\text{O} \)
- Barium chloride dihydrate: \( \text{BaCl}_2 \cdot 2\text{H}_2\text{O} \)
- Strontium chloride dihydrate: \( \text{SrCl}_2 \cdot 2\text{H}_2\text{O} \)

2. **Dehydration Process**:
- When these hydrates are heated, they lose their water of crystallization:
- \( \text{CaCl}_2 \cdot 6\text{H}_2\text{O} \) on heating gives \( \text{CaCl}_2 \) and releases \( 6\text{H}_2\text{O} \).
- \( \text{BaCl}_2 \cdot 2\text{H}_2\text{O} \) on heating gives \( \text{BaCl}_2 \) and releases \( 2\text{H}_2\text{O} \).
- \( \text{SrCl}_2 \cdot 2\text{H}_2\text{O} \) on heating gives \( \text{SrCl}_2 \) and releases \( 2\text{H}_2\text{O} \).

3. **Behavior in Air**:
- These compounds are hygroscopic, meaning they can absorb moisture from the air.
- When left in moist air, they can rehydrate, forming their respective hydrates again:
- \( \text{CaCl}_2 \) can absorb moisture and revert to \( \text{CaCl}_2 \cdot 6\text{H}_2\text{O} \).
- \( \text{BaCl}_2 \) can absorb moisture and revert to \( \text{BaCl}_2 \cdot 2\text{H}_2\text{O} \).
- \( \text{SrCl}_2 \) can absorb moisture and revert to \( \text{SrCl}_2 \cdot 2\text{H}_2\text{O} \).

4. **Tendency to Form Hydrates**:
- The tendency to form hydrates decreases from calcium to barium. This is due to the increasing size of the cation:
- Calcium ion (\( \text{Ca}^{2+} \)) is smaller than strontium ion (\( \text{Sr}^{2+} \)), which is smaller than barium ion (\( \text{Ba}^{2+} \)).
- Smaller cations have a higher charge density, which leads to a greater degree of hydration and a greater tendency to form hydrates.

5. **Conclusion**:
- All the statements regarding the behavior of these halides are correct. They can dehydrate upon heating, they become wet in moist air, and the tendency to form hydrates decreases from calcium to barium.

### Final Answer:
The correct statement about these halides is that they can dehydrate upon heating, become wet in air, and the tendency to form hydrates decreases from calcium to barium.