pH scale

The pH scale is used to measure the acidity of a solution. Acidity is defined as the relative presence of hydrogen ions (protons, H+) in solution. That is, high acidity occurs when many hydrogen ions are present.

To a small degree pure water ionises, H2O(l) ↔ H+(aq) + OH-(aq). In a pure, neutral sample of water the H+ and OH- concentrations are 0.0000001M. That's 10-7M and that 7 is the same 7 that shows up as the neutral point on pH scales.

pH is defined, technically, as the negative logarithm of the hydrogen ion concentration. Without worrying about the maths, this essentially says that a solution in which the concentration of hydrogen ions is 0.1M is pH 1. 0.1 is 10-1. That –1 is the log of the hydrogen ion concentration, so the negative log is +1.

pH reading

H+ concentration

rating

0

1M

VERY acidic

1

0.1M (10-1M)

very acidic

3

0.001M (10-3M)

acidic

7

10-7M

neutral

10

10-10M

basic

14

10-14M

VERY basic

HCl is a strong acid. When mixed in water it ionises completely, providing lots of protons, and a low pH value. Note that a really concentrated acid, like 2M HCl, is off the scale. The pH scale is generally restricted to living organisms and their environments. 2M HCl would kill anything it came into close and prolonged contact with, so we don't worry about it in this context.