A bag of cement lying in the open gets wet due to rain during the night. The next day the sun shines brightly. Do you think the changes, which have occurred in the cement, could be reversed?

Change in shape/nature of an object can be of two types. They are reversible changes and irreversible changes.

a) The changes in which the object/matter can be brought back to its original state is known as reversible changes.

    e.g., melting of wax, stretching of a rubber band etc.

b) The changes in which the object/matter cannot be brought back to its original state is called irreversible changes.

    e.g., burning of wood, cooking of food etc.

The addition of water to cement is called hydration, which hardens in sunlight. During this process, the composition of the cement changes, resulting in a chemical change. As chemical change is a permanent change, it cannot be reversed. Thus, the change to the cement is irreversible.