Hard water is water with a lot of minerals dissolved in it. While it is perfectly safe to drink, it can cause problems. When heated, it leaves behind the minerals as scale. Scale can be seen most noticeably in kettles.
Also, it is more difficult to form a lather with soap in hard water, which leads to more soap being needed. This is because the minerals in the water react with the salts in the soap to form an insoluble scum.
Hard water can be treated to make it soft by removing the minerals. To explore whether your tap water is hard or soft you can try the following experiment.
Take three jars with lids. To the first add 25 millilitres of distilled water - distilled water is soft. To the second add 25ml of tap water, and to the third add 25ml of distilled water and a teaspoon of Epsom salts.
Make sure the salts dissolve in the water. Label the jars.
Take a dropper and add one drop of liquid soap to each jar. Replace the lids and shake for 60 seconds. Note how much lather is created in each jar.
Repeat the process, continuing to add drops of soap and observing what happens. You should find that the soft water (the distilled water) forms foam quite quickly, and the hard water (the water with the Epsom salts added) does not form foam very easily.
Now compare your results with the tap water. Do you think you can determine whether the tap water is hard or soft from your results?
Further extensions of this activity could include evaporating the different waters and observing the residue left behind.
Alternatively, you could test different mineral waters to see if they are hard or soft. Gather a selection and conduct a taste test. Which tastes best - or can't you tell the difference?
Sophie Duncan is project manager for science at the BBC www.bbc.co.uksn