The pH scale tells you how acidic or basic a solution is. It measures the H⁺ ions: the more H⁺,
the lower the pH.
Everyday solutions run from about 0 to 14. Pure water sits in the middle, at pH 7, which is
neutral. Below 7 a solution is acidic, and above 7 it is basic,
also called alkaline. The next section shows what makes a solution basic.
The pH scale, with the pH of everyday things. Lemon juice is about 2, vinegar about 2.5, pure water 7, baking soda in water about 8.3 and bar soap about 10.
Each step is ten times
The scale goes in steps of ten. A drink at pH 3 has ten times as much H⁺ as one at pH 4, and a hundred times as
much as one at pH 5. A small change in pH is a big change in H⁺.
H⁺ in everyday drinks, compared with pure water
pH
Example
H⁺ compared with pure water
7
pure water
1 ×
5
black coffee
100 ×
3
grapefruit juice
10,000 ×
2
lemon juice
100,000 ×
Even pure water has a few H⁺ ions. About 2 water molecules in every billion have split into an H⁺ and an OH⁻, a
hydroxide ion. Water has exactly as many OH⁻ ions as H⁺ ions, and that balance is what makes it
neutral.
Chemists measure pH with a pH meter, or with an indicator: a dye that changes colour with pH. In
a later lesson you will see red cabbage juice do this.