pH is a logarithmic scale that measures how concentrated the hydrogen
ions (H+) are in solutions. (There are more general definitions, but
they probably aren't the 'simple form' you're looking for.)
pH is defined as the -log [H+], where [H+] is the molar
concentration of H+, meaning the number of moles of H+ per liter of
solution. In case you are unfamiliar with logarithms, Log of 10^x =
log(10^x) = x, e.g. log(100)= 2, log(0.001)= -3.)
Even in pure water, some of the water molecules (H2O) fall apart
into H+ and OH-. At room temperature in pure water [H+]= [OH-] = 10^-7
M, so pH=7.
Acids have more H+ (and less OH-) so they have pH <7. For
example, 10^-3 M of HCl will give almost 10^-3 M of H+, so it will have
pH of 3. Bases have more OH- and less H+, so they have pH >7.
You might wonder why a solution can't be both acidic and basic-
have lots of H+ and OH- ions. The reason is that if there were, most of
them would just combine to form neutral water molecules. The product
[H+] [OH-] stays at 10^-14 M^2 because of this chemical reaction.
Some pH values for common substances can be found here:
(On terminology, by "strong hydrofluoric acid" this page must mean
what we would usually call "concentrated hydrofluoric acid".)
Jason (w. Mike W)
(republished on 07/13/06)