0.8 Reactions in aqueous solutions  (Page 2/10)

${\mathrm{K}}_2{\mathrm{SO}}_4\mathrm{(s)}\to 2{\mathrm{K}}^{+}\mathrm{(aq)}+\mathrm{SO}{}_4^{2-}\mathrm{(aq)}$

Remember that molecular substances (e.g. covalent compounds) may also dissolve, but most will not form ions. One example is sugar.

${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\mathrm{(s)}\rightleftharpoons {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\mathrm{(aq)}$

There are exceptions to this and some molecular substances will form ions when they dissolve. Hydrogen chloride for example can ionise to form hydrogen and chloride ions.

$\mathrm{HCl(g)}\to {\mathrm{H}}^{+}\mathrm{(aq)}+{\mathrm{Cl}}^{-}\mathrm{(aq)}$

Ions in solution

1. For each of the following, say whether the substance is ionic or molecular.
   1. potassium nitrate ( $\mathrm{KNO}{}_3$ )
   2. ethanol ( $\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{OH}$ )
   3. sucrose (a type of sugar) ( $\mathrm{C}{}_{12}\mathrm{H}{}_{22}\mathrm{O}{}_{11}$ )
   4. sodium bromide ( $\mathrm{NaBr}$ )
2. Write a balanced equation to show how each of the following ionic compounds dissociate in water.
   1. sodium sulphate ( $\mathrm{Na}{}_2\mathrm{SO}{}_4$ )
   2. potassium bromide ( $\mathrm{KBr}$ )
   3. potassium permanganate ( $\mathrm{KMnO}{}_4$ )
   4. sodium phosphate ( $\mathrm{Na}{}_3\mathrm{PO}{}_4$ )

Ions and water hardness

This section is not examinable and is included as an example of ions in aqueous solution.

Water hardness

Water hardness is a measure of the mineral content of water. Minerals are substances such as calcite, quartz and mica that occur naturally as a result of geological processes.

Hard water is water that has a high mineral content. Water that has a low mineral content is known as soft water . If water has a high mineral content, it usually contains high levels of metal ions, mainly calcium ( $\mathrm{Ca}$ ) and magnesium ( $\mathrm{Mg}$ ). The calcium enters the water from either ${\mathrm{CaCO}}_3$ (limestone or chalk) or from mineral deposits of ${\mathrm{CaSO}}_4$ . The main source of magnesium is a sedimentary rock called dolomite, ${\mathrm{CaMg(CO}}_3\mathrm{)}{}_2$ . Hard water may also contain other metals as well as bicarbonates and sulphates.