Hard Water (WJEC GCSE Chemistry: Combined Science)

• 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

Image by Sean Horsburgh on Unsplash

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' 

• 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