Water Softening (WJEC GCSE Chemistry)

Water Softening

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

• There are many advantages to this such as not wasting soap when washing

• Using soft water is also important in many industrial processes because the ions contained in hard water can interfere with the chemical processes 

• There are three methods used to remove 'hardness' from water:

  • Boiling 

  • Adding washing soda (sodium carbonate) 

  • Using an ion exchange column 

• Boiling water removes the calcium and magnesium ions from temporary hard water 

• Washing soda is used to wash clothes and can soften permanent and temporary hard water  

  • When washing soda is added to hard water, a reaction takes place that removes the calcium and magnesium ions 

• Ion-exchange columns remove calcium and magnesium ions by replacing them with sodium ions and are used in domestic water-softening units 

  • This method also removes hardness from temporary and permanent hard water 

Summary Table of Advantages & Disadvantages

How Water Softeners Work

Higher Tier

Boiling

• 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 insoluble calcium carbonate forms scale on the heating element 

Adding Washing Soda 

• Washing soda, or sodium carbonate is used for washing clothes 

• When it is added to hard water, the carbonate ions react with the magnesium and calcium ions to form insoluble carbonates 

CO₃^2- (aq)  +   Ca²⁺ (aq) → CaCO₃ (s) 

CO₃^2- (aq)  +   Mg²⁺ (aq) → MgCO₃ (s) 

Ion-Exchange Columns

• The name of this tells us exactly what it does- exchanges ions!

• The column contains a resin packed with sodium ions

• As the hard water passes through the column, the sodium ions in the resin are exchanged for the calcium and magnesium ions in the water, removing them

  • Two sodium ions are needed to replace one calcium or magnesium ion

• Eventually all of the sodium ions will have been used up, with only magnesium and calcium ions stuck to the resin

  • The column is recharged with concentrated sodium chloride which flushes out the calcium and magnesium ions and replaces them with sodium ions 

• These are used in dishwashers and is why you replace the dishwasher salt every so often 

Diagram to show how an ion-exchange column works 

The sodium ions are exchanged for the calcium and magnesium ions in the hard water