Hard Water (WJEC GCSE Chemistry): Revision Note

Hard Water

• Depending on which part of the country you live in, you will have hard or soft water 

• Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions 

• These dissolve in the water when it has come into contact with rocks such as limestone that contain calcium compounds 

  • E.g when rainwater flows over rocks on its way to a resevoir 

• Soft water has only low concentrations of these ions

• The difference between these two types of water can be observed via their reaction with soap: 

  • Soft water forms a lather much more easily than hard water 

  • The minerals in hard water react with soap to form scum making it harder to form a lather

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Soft water forms a lather more easily than hard water 

• The relative amount of hardness in different water samples can be determined by:

  • Measuring the volume of soap solution required to produce a given lather
    OR
    By measuring the lather produced by a given volume of soap solution 

• Details of this investigation can be located in 'Determining hardness in water' 

Temporary & Permanent Hardness

• Hard water can be softened by removing the calcium and magnesium ions  

• Sometimes this can be achieved by boiling the water: 

  • Water that can have its hardness removed by boiling it is called temporary hard water 

  • This method of removing hardness leads to the formation of scale which  causes 'furring' of the heating element in a kettle making it less efficient

  • Scale also forms in washing machines, pipes and immersion heaters and can cause pipes to eventually become clogged 

• Temporary hardness is caused by dissolved calcium hydrogencarbonate, Ca(HCO₃)₂ which decomposes when the water is heated

• Some types of hard water cannot have their hardness removed by boiling

  • This type of water is called permanent hard water

  • Permanent hardness is caused by dissolved calcium sulfate, CaSO₄ which does not decompose when the water is heated 

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Water that can have its hardness removed by boiling is called temporary hard water

Higher Tier

• Higher Tier students must be able to explain how boiling removes temporary hardness

• Temporary hardness is caused by dissolved calcium hydrogencarbonate, Ca(HCO₃)₂

  • This is formed when rainwater containing carbon dioxide flows over rocks containing calcium carbonate

calcium carbonate + water + carbon dioxide → calcium hydrogencarbonate

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

• When the water is boiled, the calcium hydrogenencarbonate decomposes to form calcium carbonate, water and carbon dioxide 

calcium hydrogencarbonate → calcium carbonate + water + carbon dioxide

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

• The calcium carbonate is insoluble which leads to the 'furring' of the kettle elements