Course 6.3Lesson 1 of 3

The pH scale

The pH scale runs from 0 to 14. It measures how many hydrogen ions a solution has. A pH below 7 is acidic, 7 is neutral and above 7 is basic. A change of one unit in pH is a tenfold change in the concentration of hydrogen ions.

about 25 min1 simulation2 practice questions1 Python program4 questions to check yourself

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A number for acidity

Indicators tell us if a solution is an acid or a base. They do not say how strong. For this we use the pH scale. The "p" in pH stands for a German word that means power, and the "H" is the hydrogen ion. A universal indicator is a mixture of indicators that gives a different colour for each pH. A strip of pH paper, matched with a colour chart, reads the pH.

The scale

pH below 7: the solution is acidic. The lower the number, the stronger the acid.
pH = 7: the solution is neutral. Pure water has a pH of 7.
pH above 7: the solution is basic. The higher the number, the stronger the base.

On the pH scale, the numbers go from $0$ (very acidic) to $14$ (very basic).

The pH scale as a coloured bar from 0 at the left, red, to 14 at the right, purple. The left is acidic and the right is basic, with neutral at 7. Stomach acid is about 1.5, lemon juice 2.2, vinegar 3, tomato juice 4.1, milk 6.5, pure water 7, blood 7.4, baking soda solution 8.3, milk of magnesia 10.5, soap solution 12 and drain cleaner 13.5.

What does the number mean?

The pH is linked to the concentration of hydrogen ions, $[\text{H}^+]$ , in $\text{mol/L}$ .

\displaystyle \text{pH} = -\log_{10}[\text{H}^+]

You do not have to calculate this in school. But it helps to see its meaning.

$[\text{H}^+]$ (mol/L)	pH
$1$	0
$0.01$	2
$10^{-6}$	6
$10^{-7}$ (pure water)	7
$10^{-11}$	11

Each step of $1$ in pH is a change of ten times in the hydrogen ions. A solution of pH $2$ has ten times more hydrogen ions than one of pH $3$ , and a hundred times more than a solution of pH $4$ .

For a strong base, we use the hydroxide ions. At $25\ ^{\circ}\text{C}$ , $\text{pH} + \text{pOH} = 14$ , where $\text{pOH} = -\log_{10}[\text{OH}^-]$ . A $0.0001\ \text{M}$ solution of NaOH has pOH $4$ , and so pH $10$ .

You should find that lemon juice has about ten thousand times the hydrogen ions of pure water.

Strong or weak is not the same as concentrated or dilute

A strong acid gives most of its ions. A concentrated acid has a lot of acid in the water. A very dilute solution of a strong acid can have a higher pH than a concentrated solution of a weak acid.

Practise with new numbers

Try it in Python

This program finds the pH from the concentration, for acids and bases.

Remember these

pH below 7: acidic. pH 7: neutral. pH above 7: basic.
$\text{pH} = -\log_{10}[\text{H}^+]$ . One unit of pH is a factor of ten in the hydrogen ions.
Universal indicator and pH paper show the pH by their colours.