# Why is low pH acidic?

Because, by definition, a low $p H$ indicates a high concentration of $\text{hydronium ions}$.
By definition, $p H = - {\log}_{10} \left(\left[{H}_{3} {O}^{+}\right]\right)$. Given the logarithmic function, ${H}_{3} {O}^{+}$ $=$ ${10}^{- p H}$. And thus the more negative $p H$, the higher the concentration of ${H}_{3} {O}^{+}$. If you have difficult with this, you should review the logarithmic function.