Hardness of Water

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  • Created by: bethany
  • Created on: 10-04-13 12:25

Hard Water Makes Scum and Scale

With soft water you get a nice lather with soap. but with hard water you get a nasty scum instead- unless you're using soapless detergent.

The problem is dissolved calcium ions and magnesium ions in the water reacting with the soap to make scum which is insoluble.

So to get a decent lather, you need more soap and because this isnt free, it means it costs more money.

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Hard Water in Everday Life

When heated hard water also forms furring or scale (mostly calcium carbonate) on the insides of pipes, boilers, or kettles. 

Badly scaled up pipes and boilers reduce the efficiency of heating systems and may need to be replaced- all of which COSTS MONEY. Scale can even eventually block pipes.

Scale is also a bit of a thermal insulator. This means that a kettle with scale on the heating element takes longer to boil than a clean kettle, so it becomes less efficient.

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Cause of Hard Water

Hardness is caused by Ca2+ and Mg2+ ions.

Most hard water is hard because it contains a lot of calcium ions and magnesium ions.

Rain falling on some types of rocks (e.g. limestone, chalk and gypsum) can dissolve compunds like magnesium sulphate (whihc is soluble) and calcium sulphate (which also soluble, but only slightly)

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What is Hard Water Good For?

1) Ca2+ ions are good for healthy teeth and bones.


2) Studies found that people living in hard water areas are less likely to devleop heart disease. This could be to do with the minerals in hard water.


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Making Hard Water Soft- Temporary Hardness

There are 2 kinds of hardness- temporary and permenant.

Temporary hardness is caused by the hydrogencarbonate ion.

  • Temporary hardness is removed by boiling.
  • When heated the calcium hydrogencarbonate decomposes to form calcium carbonate
  • Calcium Carbonate is insoluble.
  • This solid is the 'limescale' on your kettle.

Calcium hydrogencarbonate -----> Calcium Carbonate + Water + Carbon Dioxide

This method wont work for permenant hardness though. Heating a sulphate ion does nothing.

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Making Hard Water Soft- Permenant Hardness

Permenant Hardness can be removed in two ways:

Both types of hardness can be softened by adding washing soda (sodium carbonate).

The carbonate ions react with the Ca2+ and Mg2+ ions to make a insoluble precipitate of calcium carbonate and magnesium carbonate.

The Ca2+ ad Mg2+ ions are no longer dissolved in the water therefore cannot make it hard.

Both types of hardness can also be removed by running water through 'ion exchange columns' which are sold in shops. 

The columns have lots of sodium ions or hydrogen ions and 'exchange' them for calcium or magnesium ions in the water that run through them.

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