Memes

Water is the most common substance that can be either an acid or a base (on earth), but lots of other compounds are also amphoteric.

In fact, on other planets where ammonia fills the same role as water, ammonia would be the most common amphoteric substance, so most solutions would be in a liquid ammonia solvent. This means neutral pH on those planets would substantially higher!

K_w is the auto dissociation constant for water, and at room temp, K_w is about 10^(-14). Taking the negative log of the square root of K_w gives the pH of pure water of about 7. The auto dissociation constant of ammonia, however, is about 10^(-30), so the pH of pure liquid ammonia is about 15! Basically, as soon as we start using solvents other than water, pH gets really funky

Edit: and before anyone jumps in to say "ack-shully, pH is based on the concentration of hydronium ions in solution, so you can't use pH for systems based on solvents other than water," pH can also be considered to be based on the protonated form of whatever the solvent is. So in an ammonia-based solution, you would find the pH by taking the negative log of ammonium instead of hydronium. Instead of defining pH as

pH = -log [H_3 O^(+)]

A more universal definition would be

pH = -log [H_2 A^(+)]

Where the auto dissociation reaction of any amphoteric solvent can be written as

HA + HA -> H_2 A^(+) + A^(-)

This is more detail than most people care about, but there's always lurking pedants on the Internet, so I thought I would leave a more detailed explanation