4.23 describe reversible reactants such as the dehydration of hydrated copper(II) sulfate and the effect of heat on ammonium chloride

Dehydration of hydrated copper(II) sulfate
Copper(II) sulfate is a white solid. When you added water to copper(II) sulfate it forms blue crystals, forming hydrated copper(II) sulfate. If heat this hydrated copper(II) sulfate it turns white as the water evaporates, forming (dehydrated) copper(II) sulfate. If you add water, it will turn blue again, if you heat it again it will turn white... etc

Ammonium chloride
Ammonium chloride is a white solid, when it is heated it breaks down into ammonia gas and hydrogen chloride gas. However, if you let these products (ammonia gas and hydrogen chloride) cool down, they will react with each other, forming ammonium chloride.