Which of the following change cannot be reversed?

When powdered cement is mixed with water, it hardens. This is because of formation of new structures, called crystals, which after a fixed time get attached to each other, providing strength and hardening the cement. Therefore, as hardening of cement includes a change in its physical state and it’s chemical properties, it cannot be reversed back.

● A frozen ice- cream melts when kept in a warm room, a door can be closed after it is opened and a melted chocolate can be frozen back when kept in a refrigerator.

● As all these changes can be undone, or the items used can be brought back to their original state, they are said to be reversed.