A buffer solution is a system that minimises pH changes when small amounts of an acid or a base are added.

When acid is added to an acidic buffer solution:

1. The concentration of H⁺ ions increases.
2. The excess H⁺ ions react with the conjugate base (A^-) to form the weak acid (HA).
3. The equilibrium position shifts to the left, consuming most of the added H⁺ ions and minimising the change in pH.

When alkali is added to an acidic buffer solution:

1. The concentration of OH^- ions increases.
2. The added OH^- ions react with H⁺ ions from the buffer, forming water: H⁺_(aq) + OH^-_(aq) ➔ H₂O_(l)
3. To replenish the H⁺ ions, the weak acid (HA) dissociates further, shifting the equilibrium position to the right and minimising the change in pH. 

The buffer solution loses its ability to resist pH changes, and further additions of acid or alkali will lead to significant pH changes.

Mix a solution of a weak acid with a solution of one of its salts, for example, mix together ethanoic acid and sodium ethanoate.

Mix a solution of a weak base with a solution of one of its salts, for example, mix together ammonia and ammonium chloride.

The resulting solution contains the weak base and its conjugate acid, for example, ammonia and ammonium ions.