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fact sheet: Hard Water

Hard water isn't water that's hard as rock. Instead, it's water that contains dissolved substances called minerals. These minerals contain the elements calcium or magnesium.

Do I have hard water?

You'll know if you live in an area where the water is hard by looking in your kettle. Unplug your kettle and take a look inside when it's cold. If you can see lots of solid white material, often called a fur, and your cup of tea sometimes has scum floating on it, then you've got hard water.

How does water become hard?

A compound called calcium hydrogencarbonate is the main cause of hard water. It forms when rain falls on limestone and chalk rocks. These rocks are made of calcium carbonate, which is insoluble in water. Because rain water contains carbon dioxide, dissolved from the air, this makes it acidic. The rain water reacts with the rocks to form calcium hydrogencarbonate which is soluble. This is the white solid that ends up in our kettles.

An equation for the reaction is:

water + carbon dioxide + calcium carbonate = calcium hydrogencarbonate

H2O (l) + CO2 (g) + CaCO3 (s) = Ca(HCO3)2 (aq)

Is it dangerous?

Although hard water can be a problem in our kettles and to our water pipes, it is not dangerous to us. In fact, soft water is more likely to contain harmful dissolved metals. In areas where soft water is common, the water companies take precautions to control the amount of dissolved metals in our water.

Click here to investigate hard water for yourself.

Year 10 pupils finding out about water hardness

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