Which of the following are chemical changes?
(i) Decaying of wood
(ii) Burning of wood
(iii) Sawing of wood
(iv) Hammering of a nail into a piece of wood

Options

A. (i) and (ii)
B. (ii) and (iii)
C. (iii) and (iv)
D. (i) and (iv)

Explanation

To determine which of the given options are chemical changes, we need to understand what constitutes a chemical change. A chemical change is characterized by the formation of new substances, where the original substances undergo a chemical reaction and cannot be easily reverted to their original form.

Let's analyze each option step by step:

1. **Decaying of wood**:
- When wood decays, it undergoes a breakdown process where microorganisms break down the wood into simpler substances. This process results in the formation of new products such as carbon dioxide, water, and organic matter.
- **Conclusion**: This is a chemical change.

2. **Burning of wood**:
- Burning wood involves combustion, where wood reacts with oxygen to produce carbon dioxide, water, and ash. This reaction results in new substances being formed, and the original wood cannot be recovered.
- **Conclusion**: This is also a chemical change.

3. **Sawing of wood**:
- Sawing wood is a physical process where the wood is cut into smaller pieces. No new substances are formed, and the chemical composition of the wood remains unchanged.
- **Conclusion**: This is not a chemical change.

4. **Hammering of a nail into a piece of wood**:
- Hammering a nail into wood is another physical change. The shape and position of the nail change, but the chemical composition of the nail and the wood remains the same.
- **Conclusion**: This is not a chemical change.

**Final Answer**: The chemical changes among the options provided are (i) Decaying of wood and (ii) Burning of wood.

### Summary:
- Chemical changes: (i) Decaying of wood, (ii) Burning of wood
- Not chemical changes: (iii) Sawing of wood, (iv) Hammering of a nail into a piece of wood