* Formation of New Substances: When sodium hydroxide (NaOH) dissolves in water, it undergoes a reaction to form sodium ions (Na+) and hydroxide ions (OH-). These ions are different chemical species than the original NaOH molecule.

* Irreversible Process: While the process can be reversed by evaporating the water, the original NaOH cannot be recovered in its entirety. This is because some of the hydroxide ions react with water to form a small amount of heat and produce some hydrogen ions (H+).

* Energy Change: Dissolving NaOH in water releases heat, indicating a chemical change. This heat is generated due to the formation of new bonds between water molecules and the ions produced.

In summary: The dissolution of sodium hydroxide in water involves the breaking of existing bonds in NaOH and the formation of new bonds with water molecules, resulting in a change in chemical composition. This makes it a chemical change, not a physical change.