Which of the following is a chemical change ?

Options

A. A. Souring of milk
B. B. Stretching of rubber
C. C. Preparation of starch paste
D. D. Melting of wax.

Answer

To determine which of the given processes is a chemical change, we need to understand the differences between chemical changes and physical changes.

### Step-by-Step Solution:

1. **Understanding Chemical Changes**:
- A chemical change involves a transformation that alters the chemical composition of a substance. This can include changes in taste, smell, or the formation of new substances. Chemical changes are typically irreversible.

2. **Understanding Physical Changes**:
- A physical change involves a change in physical properties (like state, shape, or size) without altering the chemical composition of the substance. Physical changes are usually reversible.

3. **Analyzing Each Option**:
- **Option A: Storing of Milk**:
- When acid (like lemon juice) is added to milk, it results in curdling, which changes the taste and creates new substances (curds and whey). This is a chemical change.
- **Option B: Stretching of Rubber**:
- Stretching rubber changes its shape but does not alter its chemical composition. This is a physical change.
- **Option C: Preparing of Starch Paste**:
- Mixing starch with water changes its state from powder to paste but does not change its chemical composition. This is also a physical change.
- **Option D: Melting of Wax**:
- Melting wax changes its state from solid to liquid without altering its chemical composition. This is a physical change.

4. **Conclusion**:
- Among the options provided, only the **storing of milk** (Option A) is a chemical change because it involves a change in the chemical properties and the formation of new substances.

### Final Answer:
The correct answer is **Storing of Milk** (Option A).

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