<human>:  What is a buffer solution?
<bot>: A buffer solution is a solution that resists changes in pH when small amounts of an acid or base are added. It contains a weak acid, such as acetic acid (CH3COOH), and its conjugate base, such as sodium acetate (CH3COONa). The ratio of the acid to its conjugate base is such that when an acid or base is added, the resulting solution resists changes in pH.