Which chemical change among the following involves absorption of heat?

Options

A. `CaO+H_(2)OrarrCa(OH)_(2)`
B. `Na(g)rarrNa^(+)(g)+e^(-)`
C. `underset("conc.")(H_(2)SO_(4))+underset("excess")(H_(2)O)rarr H_(2)SO_(4)(aq)`
D. `Al^(3+)+6H_(2)Orarr[Al(H_(2)O)_(6)]^(3+)`

Answer

To determine which chemical change involves the absorption of heat, we need to analyze the options provided in the question. The key concept here is to identify whether the process is endothermic (absorbing heat) or exothermic (releasing heat).

### Step-by-Step Solution:

1. **Understanding Endothermic and Exothermic Processes**:
- An endothermic process is one that absorbs heat from the surroundings. This means that the system takes in energy, resulting in a decrease in temperature of the surroundings.
- An exothermic process is one that releases heat to the surroundings, leading to an increase in temperature of the surroundings.

2. **Analyzing the Given Options**:
- **Option 1**: Ionization of Sodium (Na → Na⁺ + e⁻)
- This process requires energy to remove an electron from a sodium atom, indicating that it absorbs heat. Therefore, this is an endothermic process.

- **Option 2**: Dissolving Calcium Oxide (CaO) in Water
- This reaction releases heat, making it an exothermic process. It does not absorb heat.

- **Option 3**: Mixing Concentrated Sulfuric Acid (H₂SO₄) with Water
- This process also releases heat, indicating it is exothermic. Thus, it does not absorb heat.

3. **Conclusion**:
- Among the options analyzed, the ionization of sodium is the only process that involves the absorption of heat. Therefore, the correct answer is the ionization of sodium.

### Final Answer:
The chemical change that involves the absorption of heat is the **ionization of sodium**.