Acids and bases

The pH scale

How acidic or alkaline a substance is (the pH of the substance) can be measured using the pH scale, a continuous range that stretches from below 0 to above 14. Most common pH values occur between 0 and 14.

Acids have a pH of less than 7.

Alkalis have a pH more than 7.

Water and neutral solutions have a pH of exactly 7.

A coloured pH scale, ranging from dark red at pH 0, orange at pH 3, green at pH 7, blue at pH 10 to dark purple at pH 14.

The pH is a measure of the concentration of hydrogen ions in a solution.

Forming acids and alkalis

Alkalis are soluble bases. An alkaline solution can be formed when a metal oxide is dissolved in water. An acidic solution can be formed when a non-metal oxide is dissolved in water.

For example, magnesium oxide dissolves to form alkaline solutions.

\[Magnesium\,oxide + water \to magnesium\,hydroxide\]

\[MgO + H_2^{}O \to Mg(OH)_2^{}\]

Sulfur dioxide and nitrogen dioxide however, will dissolve to form acidic solutions.

\[Sulfur\,dioxide + water \to sulfurous\,acid\]

\[SO_2^{} + H_2^{}O \to H_2^{}SO_3^{}\]

\[Nitrogen\,dioxide + water \to nitric\,acid\]

\[NO_2^{} + H_2^{}O \to HNO_3^{}\]

Soluble metal oxides produce alkalis when dissolved in water.

Soluble non-metal oxides produce acids when dissolved in water.


What will carbon dioxide produce when it is dissolved in water?

Carbon dioxide is a non-metal oxide. It produces carbonic acid when is dissolves in water.

\[carbon\,dioxide + water \to carbonic\,acid\]

\[CO_2^{} + H_2^{}O \to H_2^{}CO_3^{}\]

Watch this video to see the procedure for an experiment investigating the effect of carbon dioxide on pH.

How to investigate the effect of carbon dioxide on the pH of water.