Introduction and concepts

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

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 ( $\mathrm{Na}{}_3\mathrm{PO}{}_4$ )

Applications

1. Water hardness

   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.

   Interesting fact

   The simplest way to check whether water is hard or soft is to use the lather/froth test. If the water is very soft, soap will lather more easily when it is rubbed against the skin. With hard water this won't happen. Toothpaste will also not froth well in hard water.

   A water softener works on the principle of ion exchange . Hard water passes through a media bed, usually made of resin beads that are supersaturated with sodium. As the water passes through the beads, the hardness minerals (e.g. calcium and magnesium) attach themselves to the beads. The sodium that was originally on the beads is released into the water. When the resin becomes saturated with calcium and magnesium, it must be recharged. A salt solution is passed through the resin. The sodium replaces the calcium and magnesium and these ions are released into the waste water and discharged.

2. Acid rain

   The acidity of rainwater comes from the natural presence of three substances ( $\mathrm{CO}{}_2$ , $\mathrm{NO}$ , and $\mathrm{SO}{}_2$ ) in the lowest layer of the atmosphere. These gases are able to dissolve in water and therefore make rain more acidic than it would otherwise be. Of these gases, carbon dioxide ( $\mathrm{CO}{}_2$ ) has the highest concentration and therefore contributes the most to the natural acidity of rainwater.

   Acid rain

   Acid rain refers to the deposition of acidic components in rain, snow and dew. Acid rain occurs when sulphur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain, snow, mist, dry dust, hail, or sleet. This increases the acidity of the soil and affects the chemical balance of lakes and streams.

   Although these reactions do take place naturally, human activities can greatly increase the concentration of these gases in the atmosphere, so that rain becomes far more acidic than it would otherwise be. The burning of fossil fuels in industries, vehicles etc is one of the biggest culprits. If the acidity of the rain drops to below 5, it is referred to as acid rain .

   Acid rain can have a very damaging effect on the environment. In rivers, dams and lakes, increased acidity can mean that some species of animals and plants will not survive. Acid rain can also degrade soil minerals, producing metal ions that are washed into water systems. Some of these ions may be toxic e.g. ${\mathrm{Al}}^{3+}$ . From an economic perspective, altered soil pH can drastically affect agricultural productivity.

   Acid rain can also affect buildings and monuments, many of which are made from marble and limestone. A chemical reaction takes place between ${\mathrm{CaCO}}_3$ (limestone) and sulphuric acid to produce aqueous ions which can be easily washed away. The same reaction can occur in the lithosphere where limestone rocks are present e.g. limestone caves can be eroded by acidic rainwater.

   $${\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{CaCO}}_3\to {\mathrm{CaSO}}_4·\mathrm{H}{}_2\mathrm{O}+{\mathrm{CO}}_2$$