Concept:Reversible changes are usually physical changes where no new substance is created. The original material can be recovered easily.
Explanation:- Preparing a coating of Plaster of Paris (POP) involves a chemical reaction that forms a hard solid. This process cannot be reversed.
- Preparing molten wax: Wax melts on heating and solidifies on cooling. This is a physical, reversible change.
- Preparing paneer from milk involves curdling, which changes milk proteins permanently. This is an irreversible chemical change.
- Preparing biogas from cow dung involves decomposition by microorganisms, producing new gases. This is an irreversible chemical change.
Only molten wax can be changed back to its original solid form.
Answer:B. Preparing molten wax
