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Hard Water

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What is hard water?

Hard water contains dissolved calcium and magnesium compounds such as:

magnesium hydrogen carbonate [Mg(HCO3)2]

calcium hydrogen carbonate [Ca(HCO3)2]

magnesium sulfate [MgSO4]

calcium sulfate [CaSO4]Hard water can be temporary or permanent.

Permanent hard water contains magnesium sulfate and calcium sulfate.

Temporary hard water contains magnesium hydrogen carbonate and calcium hydrogen carbonate.

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Hard and soft water in the UK

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Hard water and soap

One of the chemicals in soap is sodium stearate.

The dissolved calcium and magnesium compounds in hard water react with sodium stearate to form a solid called calcium stearate (or ‘scum’).

Soap ‘scum’ is formed by the reaction:

The soap will only form a lather when all of the dissolved calcium hydrogen carbonate in the water has reacted.

sodiumstearate

2C17H35COONa (s) 2NaHCO3 (aq)(C17H35COO)2Ca (s)

calciumhydrogencarbonate

Ca(HCO3)2 (aq)

+ calciumstearate

sodiumhydrogencarbonate

+

+ +

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Planning to test water hardness

How could you use soap to investigate the hardness of samples of water from different sources?

What would be the independent variable?

What would be the dependent variable?

What variables would you need to control?

What method would you use?

Water hardness can affect how suitable a water supply is for different uses, like washing and use in heating systems.

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Testing hard water with soap