Carboxylic acids are weak because they only partially ionise

1. A weak acid ionises only partially in water, so only a small proportion of its molecules form ions.
2. Carboxylic acids are weak acids, because most of their molecules stay un-ionised in solution.
3. Ethanoic acid is a typical carboxylic acid, and its partial ionisation is shown here.
   1. ${CH}_{3}COOH(aq)\rightleftharpoons H^{+}(aq)+{CH}_3{COO}^{-}(aq)$

Partial ionisation gives fewer hydrogen ions and a higher pH

1. The pH of a solution depends on its concentration of $H^{+}(aq)$ ions.
2. A higher $H^{+}(aq)$ concentration gives a lower pH.
3. A weak carboxylic acid makes fewer $H^{+}(aq)$ ions than a strong acid of the same concentration, because it only partially ionises.
4. So a carboxylic acid has a higher pH than a strong acid of the same concentration, though still below 7.

Strength describes ionisation, not concentration

1. Acid strength is the proportion of acid molecules that ionise in water.
2. Concentration is how much acid is dissolved in a given volume of solution.
3. A concentrated carboxylic acid is still weak, because only a proportion of its molecules ionise.
4. A dilute strong acid is still strong, because the molecules present ionise completely.
5. A weak carboxylic acid is not neutral, because partial ionisation still makes some $H^{+}(aq)$ ions.

Putting the explanation together

1. Start by saying carboxylic acids partially ionise in water.
2. Explain that partial ionisation makes fewer $H^{+}(aq)$ ions than complete ionisation at the same concentration.
3. Link the lower hydrogen ion concentration to the higher pH.
4. Do not say a weak acid does not ionise, because some molecules do form ions.