0.4 The hydrosphere  (Page 5/5)

In Reactions in aqueous solutions we saw how compounds can dissolve in water. Most of these compounds (e.g. ${\mathrm{Na}}^{+}$ , ${\mathrm{Cl}}^{-}$ , ${\mathrm{Ca}}^{2+}$ , ${\mathrm{Mg}}^{2+}$ , etc.) are safe for humans to consume in the small amounts that are naturally present in water. It is only when the amounts of these ions rise above the safe levels that the water is considered to be polluted.

You may have noticed sometimes that when you pour a glass a water straight from the tap, it has a sharp smell. This smell is the same smell that you notice around swimming pools and is due to chlorine in the water. Chlorine is the most common compound added to water to make it safe for humans to use. Chlorine helps to remove bacteria and other biological contaminants in the water. Other methods to purify water include filtration (passing the water through a very fine mesh) and flocculation (a process of adding chemicals to the water to help remove small particles).

pH of water is also important. Water that is to basic (pH greater than 7) or to acidic (pH less than 7) may present problems when humans consume the water. If you have ever noticed after swimming that your eyes are red or your skin is itchy, then the pH of the swimming pool was probably to basic or to acidic. This shows you just how sensitive we are to the smallest changes in our environment. The pH of water depends on what ions are dissolved in the water. Adding chlorine to water often lowers the pH. You will learn more about pH in grade 11.

Experiment: water purity

Aim:

Apparatus:

Method:

1. Look at each water sample and note if the water is clear or cloudy.
2. Examine each water sample under a microscope and note what you see.
3. Test the pH of each of the water samples.
4. Pour some of the water from each sample through filter paper.
5. Refer to Testing for common anions in solutions for the details of common anion tests. Test for chloride, sulphate, carbonate, bromide and iodide in each of the water samples.

Results:

Discussion:

Conclusion:

Project: water purification

Prepare a presentation on how water is purified. This can take the form of a poster, or a presentation or a project. Things that you should look at are:

• Water for drinking (potable water)
• Distilled water and its uses
• Deionised water and its uses
• What methods are used to prepare water for various uses
• What regulations govern drinking water
• Why water needs to be purified
• How safe are the purification methods

Summary

• The hydrosphere includes all the water that is on Earth. Sources of water include freshwater (e.g. rivers, lakes), saltwater (e.g. oceans), groundwater (e.g. boreholes) and water vapour. Ice (e.g. glaciers) is also part of the hydrosphere.
• The hydrosphere interacts with other global systems , including the atmosphere, lithosphere and biosphere.
• The hydrosphere has a number of important functions . Water is a part of all living cells, it provides a habitat for many living organisms, it helps to regulate climate and it is used by humans for domestic, industrial and other use.
• Despite the importance of the hydrosphere, a number of factors threaten it. These include overuse of water, and pollution .
• Water is not pure, but has many substances dissolved in it.

End of chapter exercises

1. What is the hydrosphere? How does it interact with other global systems?
   
2. Why is the hydrosphere important?