5) Strong and weak acids

• Strong acids are completely ionised (split up into ions) in water
• Hydrochloric acid is a strong acid: HCI(g) + aq➡ H+(aq) + Cl-(aq)
• Ethanoic acid is a weak acid:

  CH,COOH(I) + aq -> CH ̧COO ̄(aq) + H+(aq)
•  The pH of a solution is a measure of the concentration of H+ ions.

  A pH decrease of one unit indicates that the concentration of hydrogen ions has increased by a factor of 10.
• For a given concentration of acid, a strong acid will have a higher concentration of hydrogen ions and, therefore, a lower pH.
• The terms 'dilute' and 'concentrated' are also applied to acids sometimes.
• An acid that has a concentration of 2mol/dm3 is more concentrated than an acid that has a concentration of 0.5mol/dm3.
• Strong acids such as hydrochloric acid, nitric acid and sulfuric acid are completely ionised in water.
• Weak acids such as ethanoic acid, citric acid and carbonic acid are only partially ionised in water.