Water Softening (WJEC GCSE Chemistry)

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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

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

Examiner Tip

If you are studying for the foundation paper, you only need to be able to name the methods used to soften hard water and the pros and cons of each one.

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 

CO32- (aq)  +   Ca2+ (aq) → CaCO3 (s) 

CO32- (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 

ion-exchange-column-

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

Examiner Tip

For higher tier, as well as explaining how each process works, you must also make sure you learn the equations.

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Alexandra

Author: Alexandra

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.