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activity: Plan your own hard water investigation

Click here for more information on hard water.

You can use the method below to test samples of water to see how hard they are. Plan an investigation using this test to compare the hardness of different samples of water. Read the description of the hardness test first, then make your plan. Get your teacher to check your plan before you start the practical activity. Bear in mind the following questions:

  • how will you make sure that your tests are fair?
  • how will you make the investigation safe?
  • what information will you record about each sample you test?
  • what precautions will you need to take to make sure your results are as accurate as possible?

Click here for a help sheet on science investigations.

Use the basic test to compare the hardness of the following samples of water:

  • distilled water
  • local tap water
  • local tap water that has been boiled
  • sea water
  • mineral water (you could try several different brands)
  • distilled water with a little sodium chloride dissolved in it
  • distilled water with a little calcium chloride dissolved in it

The Basic Hardness Test

  1. Put 2 cm3 of water sample in a test tube.
  2. Use a dropping pipette to add a drop of soap solution to the water.
  3. Shake the tube and see if you get a lather that lasts more than a few seconds.
  4. If you don't, add two or three more drops of soap solution and shake again, until you get a lasting lather.
  5. Record the total number of drops of soap solution added, and look to see if any white precipitate (scum) has formed.

What kind of hardness?

If your local tap water is hard, you could try this activity to find out what kind of hardness is present. You could also test samples of mineral water. These could be used as a replacement for hard water if your local water is soft.

  1. Take another sample of water in a small beaker and gently boil it for about ten minutes.
  2. Allow the water to cool until you can safely handle the beaker.
  3. Repeat the basic hardness test with a sample of boiled water.