Anaerobic bacteria digest animal waste and produce biogas (change A). The biogas is then burnt as fuel (change B). The following statements pertain to these changes. Choose the correct one :

Options

A. A is a chemical change whereas B is a physical change.
B. B is a chemical change whereas A is a physical change.
C. Both A and B are physical changes.
D. Both A and B are chemical changes.

Answer

To solve the question regarding the changes occurring when anaerobic bacteria digest animal waste to produce biogas and the subsequent burning of that biogas, we will analyze both changes step by step.

### Step-by-Step Solution:

1. **Identify Change A**:
- Anaerobic bacteria digest animal waste to produce biogas.
- This process involves the breakdown of complex organic materials (animal waste) into simpler substances (biogas).
- **Type of Change**: This is a **chemical change** because new products (biogas) are formed, and the composition of the original material (animal waste) changes.

2. **Identify Change B**:
- The biogas produced is then burned as fuel.
- When biogas is burned, it reacts with oxygen to produce carbon dioxide and other byproducts, along with heat and energy.
- **Type of Change**: This is also a **chemical change** because burning biogas results in the formation of new products (carbon dioxide and heat), and the composition of the original biogas changes.

3. **Conclusion**:
- Both changes (A and B) involve the formation of new products and a change in composition.
- Therefore, the correct statement is that both A and B are chemical changes.

### Final Answer:
Both A and B are chemical changes.

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