What action involves adding water to restore a substance to its original state?

The action of adding water to restore a substance to its original state is accurately described as reconstitution. This term specifically refers to the process of mixing a powdered or dehydrated substance with a solvent, typically water, to create the original liquid form in which the substance is intended to be used. This is commonly seen in pharmaceuticals, where powdered medications are reconstituted before administration. For instance, certain antibiotics and vaccines are stored in a powdered form and require reconstitution to activate or to make them ready for use again.

Rehydration is closely related, often referring to the general process of adding water to a dehydrated material but not specifically to restoring a powdered substance to a liquid state. Dilution refers to reducing the concentration of a solution by adding more solvent, which may not necessarily pertain to restoring a substance to its original state. Dehydration is the removal of water, which is the opposite of adding water, thus does not fit the description of restoring a substance.