NCERT Question 12 - Chapter 6 Class 7 - Physical and Chemical Changes

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.

1. Process – A is a chemical change.
2. Process – B is a chemical change.
3. Both processes A and B are chemical changes.
4. None of these processes is a chemical change.  

Answer:

• Digestion of animal waste results in the production of biogas ( a new substance).
• Biogas being burnt as fuel is accompanied by the evolution of heat and energy .
• Since neither of them can be reversed , both are chemical changes .

So, the correct answer is (iii) .