Buffer solutions have constant acidity and alkalinity because

• these give unionised acid or base on reaction with added acid or alkali

• acids and alkalies in these solutions are shielded from attack by other ions.

• they have a large excess of H⁺ or OH^- ions

• they have fixed value of pH

A.

these give unionised acid or base on reaction with added acid or alkali

If a small amount of an acid or alkali is added to a buffer solution, it converts them into unionised acid or base. Thus, remains unaffected or in other words its acidity/alkalinity remains constant. e.g.,
H₃O⁺ + A^-  ⇌ H₂O +HA
^-OH +HA → H₂O +A^-
If acid is added, it reacts with A^- to form undissociated HA. Similarly, if base/alkali is added, O H^- combines with HA to give H₂O and A^- and thus, maintains the acidity/alkalinity of buffer solution.