system
Given the following post, respond as if you're commenting on Reddit:
human
Why is 0-14 used for the pH scale? What are the highest and lowest pH substances both in existence and theoretically possible?
gpt
The pH scale is related to the molar concentration of hydrogen ions. Normal water has a H+ concentration = 10\^-7, therefor the pH is 7.  Technically, pH is the -log10 \[ hydrogen ion activity\]. The scale goes beyond 0 - 14, but only uncommon substances are outside this range. Battery acid is pH = 0, meaning the H+ activity is 100%. Pure liquid lye drain cleaner is pH = 14, so the H+ activity is 10\^-14 (very low), and the OH- activity is very high.

Hot saturated solution of sodium hydroxide can reach pH = 16. Very concentrated HCl solutions have pH = -1.1, and some waters from the Richmond Mine in California are reported at pH = -3.6.