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23.6 How pure is our water?

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23.6 How pure is our water? (ESAHV)

When you drink a glass of water you are not just drinking water, but many other substances that are dissolved into the water. Some of these come from the process of making the water safe for humans to drink, while others come from the environment. Even if you took water from a mountain stream (which is often considered pure and bottled for people to consume), the water would still have impurities in it. Water pollution increases the amount of impurities in the water and sometimes makes the water unsafe for drinking. In this section we will look at a few of the substances that make water impure and how we can make pure water. We will also look at the pH of water.

In Chapter 18 we saw how compounds can dissolve in water. Most of these compounds (e.g. \(\text{Na}^{+}\), \(\text{Cl}^{-}\), \(\text{Ca}^{2+}\), \(\text{Mg}^{2+}\), 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 too basic (pH greater than \(\text{7}\)) or too acidic (pH less than \(\text{7}\)) 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 too basic or too 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.

Water purity


To test the purity and pH of water samples


pH test strips (you can find these at pet shops, they are used to test pH of fish tanks), microscope (or magnifying glass), filter paper, funnel, silver nitrate, concentrated nitric acid, barium chloride, acid, chlorine water (a solution of chlorine in water), carbon tetrachloride, some test-tubes or beakers, water samples from different sources (e.g. a river, a dam, the sea, tap water, etc.).



  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 Chapter 18 for the details of common anion tests. Test for chloride, sulphate, carbonate, bromide and iodide in each of the water samples.


Write down what you saw when you just looked at the water samples. Write down what you saw when you looked at the water samples under a microscope. Where there any dissolved particles? Or other things in the water? Was there a difference in what you saw with just looking and with looking with a microscope? Write down the pH of each water sample. Look at the filter paper from each sample. Is there sand or other particles on it? Which anions did you find in each sample?


Write a report on what you observed. Draw some conclusions on the purity of the water and how you can tell if water is pure or not.


You should have seen that water is not pure, but rather has many substances dissolved in it.

Water purification

Prepare a presentation on how water is purified. This can take the form of a poster, or a presentation or a written report. 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