This achievement standard involves describing properties of aqueous systems using equilibrium principles.

Aqueous systems are limited to those in which proton transfer occurs and those involving a sparingly soluble ionic solid.

Properties of aqueous systems are related to the nature and the concentration of the species present in the solution. Description, explanation and application, or discussion of these properties may be qualitative and/or quantitative.

Qualitative evidence may include

• correlation between acid or base strength, K_a and pH
• relative equilibrium concentrations of dissolved species
• variability in solubility of a sparingly soluble salt due to the formation of a complex ion, the addition of a common ion, or the reaction of a basic anion with added acid
• features of titration curves including buffer region, equivalence point and selection of indicator (titrations of weak acids with weak bases are excluded)
• the nature of buffer solutions.

Quantitative evidence includes calculations involving

• K_a, K_w and pH limited to
  • solutions of bases, monoprotic acids and buffers
  • those in which the extent of reaction is small so that the equilibrium concentration of a dissolved weak acid can be approximated by the initial concentration, ie [HA] = c(HA)
  • pH at a particular point in a titration;
• K_s and solubility limited to
  • AB, A₂B and AB₂ type solids where neither of the ions A or B react further with water
  • calculating the concentration of one ion given the other
  • calculating the solubility in water and in solutions already containing one of the ions A or B (a common ion)
  • predicting precipitation or dissolution.