Most reactions go one way: reactants turn into products and that is the end of it. In some reactions, the products can react to produce the original reactants. These are called reversible reactions. We show them with a double arrow in place of the normal single arrow: A + B ⇌ C + D.
The reaction from left to right is the forward reaction and makes the products. The reaction from right to left is the reverse reaction and turns the products back into the reactants. The direction in which a reversible reaction goes can be changed by changing the conditions, such as heating or cooling the mixture.
Hydrated copper sulfate is a blue solid. Heating it drives off the water and leaves white anhydrous copper sulfate: CuSO4.5H2O ⇌ CuSO4 + 5H2O. Adding water to the white solid turns it blue again, so the change has been reversed.
Ammonium chloride is another example. When it is heated it breaks down into ammonia and hydrogen chloride gases. When these gases cool, they react together and form solid ammonium chloride again: NH4Cl ⇌ NH3 + HCl.
