The pH depends on the concentration of the solution. The strength of acids or bases depends on the extent to which they dissociate in aqueous solution.

  • Strong acids and bases dissociate almost completely in solution:
    $\rm HA(aq) + H_2O(l)$ $\rm \rightarrow HO^+_3(aq) + \dfrac{A^–(aq)}{HA(aq)}$ $\rm \rightarrow H^+(aq) + A^–(aq)$
    Strong acids are good proton donors and have weak conjugate bases:
    $\rm A^–(aq)$ is a weak base as the position of above equilibrium is completely to the right.
  • Strong bases are good proton acceptors and form weak conjugate acids.
    $\rm B(aq) + H_2O(l)$ $\rm \rightarrow BH^+(aq) + OH^–(aq)$
  • $\rm BH^+(aq)$ is a weak acid as the position of above equilibrium is completely to the right.
  • Weak acids and bases form an equilibrium mixture:
    $\rm HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^–(aq)$ / $\rm HA(aq) \rightleftharpoons H^+(aq) + A^–(aq)$
    $\rm B(aq) + H_2O(l) \rightleftharpoons BH^+(aq) + OH^–(aq)$
  • Strong acids include: $\rm HCl$, $\rm H_2SO_4$, $\rm HNO_3$
  • Strong bases include: $\rm LiOH$, $\rm NaOH$, $\rm KOH$, $\rm Ba(OH)_2$
  • Weak acids include: $\rm CH_3COOH$, $\rm H_2CO_3$
  • Weak bases include: $\rm NH_3$, $\rm C_2H_5NH_2$
  • Strong and weak acids and bases can be distinguished by:
    • $\rm pH$ measurement/indicator: stronger acids have a lower $\rm pH$
    • conductivity: stronger acids have a higher conductivity as there are more ions present in aqueous solution.
    • rate of reaction with metals, metal oxides, metal hydroxides, metal hydrogencarbonates and metal carbonates. Stronger acids have higher concentration of $\rm H^+(aq)$ and so react at a faster rate.
  • Solutions of equal concentration should be used for fair comparisons of the strength of acids and bases.
  • It is important to recognise that the terms "strong" and "weak" describe the nature of acids and bases not to their concentrations. A solution of a strong acid can be dilute.