0.8 Reactions in aqueous solutions  (Page 3/10)

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.

Acid rain

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

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. We will look at each of these gases in turn.

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.

1. Carbon dioxide Carbon dioxide reacts with water in the atmosphere to form carbonic acid ( ${\mathrm{H}}_2{\mathrm{CO}}_3$ ).
   $${\mathrm{CO}}_2+\mathrm{H}{}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{CO}}_3$$
   The carbonic acid dissociates to form hydrogen and hydrogen carbonate ions. It is the presence of hydrogen ions that lowers the pH of the solution making the rain acidic.
   ${\mathrm{H}}_2{\mathrm{CO}}_3\to {\mathrm{H}}^{+}+{\mathrm{HCO}}_3^{-}$
2. Nitric oxide Nitric oxide ( $\mathrm{NO}$ ) also contributes to the natural acidity of rainwater and is formed during lightning storms when nitrogen and oxygen react. In air, $\mathrm{NO}$ is oxidised to form nitrogen dioxide ( ${\mathrm{NO}}_2$ ). It is the nitrogen dioxide which then reacts with water in the atmosphere to form nitric acid ( ${\mathrm{HNO}}_3$ ).
   $3\mathrm{NO}{}_2\left(\mathrm{g}\right)+\mathrm{H}{}_2\mathrm{O}\left(\mathrm{l}\right)\to 2\mathrm{HNO}{}_3\left(\mathrm{aq}\right)+\mathrm{NO}\left(\mathrm{g}\right)$
   The nitric acid dissociates in water to produce hydrogen ions and nitrate ions. This again lowers the pH of the solution making it acidic.
   $\mathrm{HNO}{}_3\to {\mathrm{H}}^{+}+{\mathrm{NO}}_3^{-}$
3. Sulphur dioxide Sulphur dioxide in the atmosphere first reacts with oxygen to form sulphur trioxide, before reacting with water to form sulphuric acid .
   $2\mathrm{SO}{}_2+\mathrm{O}{}_2\to 2\mathrm{SO}{}_3$
   $\mathrm{SO}{}_3+\mathrm{H}{}_2\mathrm{O}\to \mathrm{H}{}_2{\mathrm{SO}}_4$
   Sulphuric acid dissociates in a similar way to the previous reactions.
   $\mathrm{H}{}_2{\mathrm{SO}}_4\to \mathrm{H}{\mathrm{SO}}_4^{-}+{\mathrm{H}}^{+}$

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 .