Which of the following salt on dissolution in water absorbs heat

Options

A. `NH_(4)Cl`
B. `CaO`
C. `Na_(2)SO_(4)`
D. `Na_(2)CO_(3)`

Explanation

To determine which salt absorbs heat upon dissolution in water, we need to analyze the nature of the salts provided. The key concept here is that salts can either release heat (exothermic process) or absorb heat (endothermic process) when they dissolve in water, depending on their ionization and hydrolysis properties.

### Step-by-Step Solution:

1. **Identify the Salts**: We have four salts to consider. Let's denote them as:
- A: NH4Cl (Ammonium Chloride)
- B: Na2CO3 (Sodium Carbonate)
- C: Na2SO4 (Sodium Sulfate)
- D: Ca(OH)2 (Calcium Hydroxide)

2. **Understand Hydrolysis**: Hydrolysis occurs when a salt reacts with water to form an acid or a base. This process can either absorb or release heat. Salts formed from weak acids or weak bases tend to undergo hydrolysis, which can lead to heat absorption.

3. **Analyze Each Salt**:
- **NH4Cl**: This salt is formed from a weak base (NH4OH) and a strong acid (HCl). When dissolved, it undergoes hydrolysis:
\[ NH4^+ + H2O \rightleftharpoons NH4OH + H^+ \]
This process absorbs heat, making it endothermic.

- **Na2CO3**: This salt is formed from a strong base (NaOH) and a weak acid (H2CO3). It also undergoes hydrolysis:
\[ CO3^{2-} + H2O \rightleftharpoons HCO3^- + OH^- \]
This process can absorb heat, but generally, it is not as significant as NH4Cl.

- **Na2SO4**: This salt is formed from a strong base (NaOH) and a strong acid (H2SO4). It does not undergo significant hydrolysis and thus does not absorb heat.

- **Ca(OH)2**: This salt is a strong base and does not undergo hydrolysis significantly. It will not absorb heat.

4. **Conclusion**: Among the salts analyzed, **NH4Cl** is the salt that absorbs heat when dissolved in water due to its hydrolysis process.

### Final Answer:
**NH4Cl (Ammonium Chloride)** is the salt that absorbs heat upon dissolution in water.

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