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: $$\rm [H^+]=10^{-pH}\iff \log_{10}[H^+]=-pH\iff pH=-\log_{10}[H^+]$$

Note that pH scale and proton concentration works in reverse direction, one increases so the other decreases.

The pH Scale at normal room temperature is from 0 to 14 (debatable) and 7 is neutral (no excess proton or no deficiency of proton) and 0-7 is acidic (excess proton) wheras 7-14 is basic (deficiency of proton). This is defined because purest form of water at this temperature has pH 7 and pH more than this (and hence concentration of proton less than this) is caused by bases which take up proton and and pH less than this (and hence concentration of proton more than this) is caused by acids which release proton.

• I wouldn't say it's "debatable" whether the pH scale in water goes from 0 to 14 at room temperature; confining aqueous pH to those limits is objectively incorrect. There are many examples of aqueous solutions with pH lower than 0 or higher than 14. They're just not something the average person will come across because they of course present significant safety hazards. I make a point of bringing attention to it because it's a common misunderstanding. See, for example, this Chem.SE question. – Nicolau Saker Neto Apr 25 '15 at 13:30
• @NicolauSakerNeto i knew that someone will say that, and maybe this is nothing else thana debate. – RE60K Apr 25 '15 at 13:31
• Also, the p in pH is not potenz. There hasn't been an accurate definition of it, it is very debatable. – Asker123 Apr 25 '15 at 19:25
• @Asker123 you would like to see this – RE60K Apr 26 '15 at 0:49
• Mais, non! C'est ne pas potenz, mais le puissance Hydrogen. sciencehq.com/chemistry/ph-scale-puissance-of-hydrogen.html – DrMoishe Pippik Oct 7 '18 at 22:24

I am very old and maybe something changed : pH ( nothing to do with % ) used to be " negative the log ( base 10) of the hydrogen ion concentration in g-moles per liter of water" . Because it is a negative, the smaller the pH the greater the number on hydrogen ions. So pH of 1 means a lot of H ions and pH 13 means much fewer H ions.