# Why does an acid have a low pH value?

When study biology in class, I don't know how my teacher get to the topics on pH value. So, she says that pH stands for percentage hydrogen. Then, I thought there is something strange about it cause acid has a higher percentage of hydrogen then base. But why does an acid have a low pH value? Is there any mistake on the pH definition or my understanding?

• Just a fact: The $p$ in front of something as in $pH$, literally means, "take the negative log of." This is why we have those cool identities, $pH+pOH=14$, and using log rules we get $[H^+][OH^-]=10^{-14}$. If you have $pABCD$, it means $-\log (ABCD)$. – Equinox Jun 7 '17 at 20:30

Actually in pH the p is for potenz or power of hydrogen. The concentration of proton $(\rm H^+\equiv H_3O^+)$ in water is often $10^{-3},10^{-5},10^{-14}$ which is very tedious to write, so pH is defined as $3,5,14$ for these solutions. Or: $$\rm [H^+]=10^{-pH}\iff \log_{10}[H^+]=-pH\iff pH=-\log_{10}[H^+]$$