Which of the following is not a chemical change?

Options

A. Curd formation from milk
B. Combustion of coal
C. Rusting of iron
D. Conversion of water into steam

Explanation

To determine which of the following processes is not a chemical change, we need to analyze each option provided:

1. **Formation of curd from milk**:
- This process involves the action of bacteria (lactobacilli) on milk, which leads to the conversion of milk into curd. This is a chemical change because new substances (curd) are formed, and the original substance (milk) cannot be recovered from curd.

2. **Combustion of coal**:
- When coal burns in the presence of oxygen, it undergoes a chemical reaction that produces new substances (like carbon dioxide and ash). This is also a chemical change because the original coal cannot be recovered once it has been burned.

3. **Rusting of iron**:
- Rusting occurs when iron reacts with oxygen and moisture in the air to form rust (iron oxide). This is a chemical change as well, since new substances are formed, and the original iron cannot be recovered from rust.

4. **Conversion of water into steam**:
- This process involves heating water to its boiling point, causing it to change from a liquid to a gas (steam). This is a physical change because the chemical composition of water (H2O) remains the same, and the process is reversible; steam can be condensed back into water.

Based on this analysis, the process that is **not a chemical change** is:

**Answer**: Conversion of water into steam.