According to the Brönsted-Lowry theory, an acid is a proton ($H^+$) donor. When a Brönsted acid donates a proton, the remaining species with one less proton is called its conjugate base .

• For $H_2O$: $H_2O ightarrow H^+ + OH^-$. Therefore, the conjugate base of the acid $H_2O$ is the hydroxyl ion, $OH^-$ .
• For $HF$: $HF ightarrow H^+ + F^-$. Therefore, the conjugate base of the acid $HF$ is the fluoride ion, $F^-$ . Thus, the conjugate bases of $H_2O$ and $HF$ are $OH^-$ and $F^-$, respectively.