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

If you add water to copper sulphate you can make hydrated copper sulphate.
If you remover the water from hydrated copper sulphate you can make copper sulphate.

When heated, ammonium chloride splits into to hydrogen chloride and ammonia.
Hydrogen chloride and ammonia can be reacted to make ammonium chloride.