Spectator ions are ions that appear in a solution but do not take part in the chemical reaction. This ions remain unchanged before and after the chemical equation.
From the equation:
2H⁺ + SO₄²⁻ + Ca²⁺ + 2I⁻ ⇒ CaSO₄ + 2H⁺ + 2I⁻
We can see that both H⁺ and I⁻ ions remain unchanged as the reaction occurs, therefore they are spectator ions. Removing the spectator ions gives a net ionic equation:
