Q. Define Conjugate acid-base pair and buffer?

Buffers are substances that resist changes in pH. To "balance pH" means to buffer.

Weak acids and bases make good buffers. A pair of substances that differ by only one H⁺ ion are called a conjugate acid-base pair and make good buffers. For example, H₂Se and HSe^- are a conjugate acid-base pair, as they differ by only one H⁺ ion.

The conjugate acid, H₂Se, can produce an H⁺, and the conjugate base, HSe^-, can accept an H⁺:

H₂Se -> H⁺ + HSe^-
HSe^-  +  H⁺-> H₂Se

These reactions help to buffer a solution, and thus resist changes in the pH level.