Reversible hydration of copper(II) sulfate and cobalt(II) chloride

Some reactions can proceed in either direction. The symbol $$\rightleftharpoons$$ shows that a reaction is reversible: substances formed in one direction can react to reform the starting substances.

Hydrated copper(II) sulfate and cobalt(II) chloride contain water as part of their compounds. Heating either hydrated compound removes this water, leaving its anhydrous form. Adding water to the anhydrous compound reforms the hydrated compound.

Reversible hydration of copper(II) sulfate

Hydrated copper(II) sulfate contains five water molecules per formula unit. Its reversible change can be written as:

Heating favours the left-to-right change, producing anhydrous copper(II) sulfate and water. Adding water to anhydrous copper(II) sulfate favours the reverse change.

Reversible hydration of cobalt(II) chloride

Hydrated cobalt(II) chloride contains six water molecules per formula unit. Its reversible change can be written as:

Heating favours the left-to-right change, producing anhydrous cobalt(II) chloride and water. Adding water to the anhydrous compound favours the reverse change, reforming hydrated cobalt(II) chloride.