Many reactions, such as burning fuel or a precipitation reaction, are irreversible - they go to completion and cannot be reversed easily. Reversible reactions are different. In a reversible reaction, the products can react or break down to produce the original reactants again.
Ammonium chloride is a white solid. It breaks down when heated, forming ammonia and hydrogen chloride. When these two gases are cool enough, they react together to form ammonium chloride again. This reversible reaction can be modelled as:
ammonium chloride ⇌ ammonia + hydrogen chloride
NH4Cl(s) ⇌ NH3(g) + HCl(g)
The symbol ⇌ has two half arrowheads, one pointing in each direction. It is used in equations that model reversible reactions:
Write the balanced chemical equation for the forward reaction in the breakdown of ammonium chloride.
NH4Cl(s) → NH3(g) + HCl(g)
Blue copper sulfate is described as hydrated. The copper ions in its crystal lattice structure are surrounded by water molecules. This water is driven off when blue hydrated copper sulfate is heated, leaving white anhydrous copper sulfate. This reaction is reversible:
hydrated copper sulfate ⇌ anhydrous copper sulfate + water
CuSO4.5H2O(s) ⇌ CuSO4(s) + 5H2O(l)