When an acid and a base are mixed, the excess ions: This can be answered with more than one answer ...

increase

decrease

neutralize each other

combine to form water

Combine to form water.

Explanation:

When equal amount of acid and base are mixed then there is excess of hydronium ions or hydroxide ions. Both are combined to from the water.

Reaction between acid and base:

When a base such as sodium hydroxide react with acid HCl it gives the salt and water.

The salt produced is sodium chloride which is formed by the combination of cation of base and anion of acid.

NaOH + HCl → NaCl + H₂O

The cation of base is Na⁺ while anion of acid is Cl⁻ both combine to form sodium chloride.

The anion of base OH⁻ and cation of acid H⁺ combine to form water.