Hard and Soft Water

These revision notes will help you revise work on hard and soft water for AQA GCSE Chemistry, unit C3.

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Hard and Soft Water

The water we drink and wash with contains more than just H2O.  Dissolved in the water are ions (atoms or molecules with an overall positive or negative charge).  Calcium and magnesium ions cause water to be hard.

Hard water can be good for your health as the calcium ions help build strong teeth and bones and can reduce your chances of heart disease.

However, hard water can also cost you more money!  Using soap leaves soap scum on your bath or sink, which is unsightly.  Washing in hard water takes more soap or detergent to make a lather.  Hard water also leaves your kettle covered in limescale, which takes longer to boil water and so increases your energy bills. 

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Types of Hard Water

There are two types of hard water:

  • Permanent Hardness:

Calcium chloride, calcium sulfate, magnesium chloride and magnesium sulfate in the water causes this type of hardness.

  • Temporary Hardness:

Calcium bicarbonate and magnesium bicarbonate causes this type of hardness.

Water can contain both permanent and temporary hardness, so:

total hardness =  permanent hardness  +  temporary hardness

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Removing Permanent Hardness

Permanent hardness is caused by calcium chloride, calcium sulfate, magnesium chloride and/or magnesium sulfate dissolved in the water.

Hard water can be softened by removing the calcium and magnesium ions.  Water softeners are added to hard water to make it soft.  One way we can soften hard water is to add sodium carbonate.  The carbonate ions which area added as sodium carbonate cling on to the calcium and magnesium ions and form a solid precipitate of calcium or magnesium carbonate.  Here is an equation to show how this happens to remove calcium ions from the water.

calcium ions  +  carbonate ions  -->  calcium carbonate

Ca2+  +  CO32-  -->  CaCO3

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Removing Temporary Hardness

Temporary hardness is caused by calcium bicarbonate and/or magnesium bicarbonate ions dissolved in the water.

Temporary hardness can be removed by boiling the water.  A thermal decomposition reaction takes place and calcium carbonate and/or magnesium carbonate forms a solid precipitate.

calcium bicarbonate  -->  calcium carbonate  +  water  +  carbon dioxide

Ca(HCO3)2  -->  CaCO3  +  H2O  +  CO2

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Other Ways of Softening Hard Water

Ion exchange is the method used in jug filters at home.  Cartridges of thousands of small beads of ion exchange resin (a polymer) swaps the ions that cause hard water (calcium (Ca2+) and magnesium (Mg2+)) for other ions like sodium (Ns+).  Replacing calcium and magnesium ions in the water removes both permanent AND temporary hardness.

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