8.1 Ions in aqueous solution

Understanding pH is very important. In living organisms, it is necessary to maintain a constant pH so that chemical reactions can occur under optimal conditions.

In agriculture, farmers need to know the pH of their soils so that they are able to plant the right kinds of crops. The pH of soils can vary depending on a number of factors such as rainwater, the kinds of rocks and materials from which the soil was formed and also human influences such as pollution and fertilisers. The pH of rain water can also vary and this too has an effect on agriculture, buildings, water courses, animals and plants. Rainwater is naturally acidic because carbon dioxide in the atmosphere combines with water to form carbonic acid. Unpolluted rainwater has a pH of approximately 5,6. However, human activities can alter the acidity of rain and this can cause serious problems such as acid rain.

Calculating ph

1. Calculate the pH of each of the following solutions:
   1. A 0,2 mol $·$ dm ${}^{-3}$ KOH solution
   2. A 0,5 mol $·$ dm ${}^{-3}$ HCl solution
2. What is the concentration (in mol $·$ dm ${}^{-3}$ ) of H ₃ O ⁺ ions in a NaOH solution which has a pH of 12?
   
3. The concentrations of hydronium (H ₃ O ⁺ ) and hydroxyl (OH ${}^{-}$ ) ions in a typical sample of seawater are 10 ${}^{-8}$ mol $·$ dm ${}^{-3}$ and 10 ${}^{-6}$ mol $·$ dm ${}^{-3}$ respectively.
   1. Is the seawater acidic or basic?
   2. What is the pH of the seawater?
   3. Give a possible explanation for the pH of the seawater.
   (IEB Paper 2, 2002)
   

The following simulation allows you to test the pH of various substances.

Acid rain

The acidity of rainwater comes from the natural presence of three substances (CO ₂ , NO, and 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 (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.