Water Softening (WJEC GCSE Chemistry)

• 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

Method	Advantages	Disadvantages
Boiling	Relatively easy and cheap method	Wastes energy  Only useful for small volume Does not remove permanent hardness
Washing soda	Inexpensive  Softens water that has temporary and permanent hardness	Leads to the formation of limescale which can block narrow pipes
Ion-exchange columns	Is a continuous process  Uses concentrated sodium chloride which is cheap and widely available Softens water that has temporary and permanent hardness	Ion-exchange resins are expensive

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

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

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