Science & Technology in Action

1st Edition

Protecting Our Natural Water Supplies


This lesson examines the topical issue of water hardness. The chemistry of hard and soft water is described. The calculation of water hardness is explained and methods of removing hardness are described.

Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

Water is an essential resource
Water is critical to the survival and health of the human race and other living organisms. There are many types of natural water found on the earth’s surface. These include rain water, river water and sea water. In fact approximately 80% of the surface of the earth is covered with water. Nevertheless, millions of people around the world don’t have access to enough water. Even in Ireland, where we normally have plentiful supplies, it is crucial to monitor the sources and quality of our water. In this lesson we look at a fundamental quality issue - water hardness.

What is hard water?
Water is an excellent solvent. All water supplies contain a wide range of dissolved substances and as a result it is difficult to get pure water. The dissolved substances may be present in solid, liquid or gas form. Water described as hard is high in dissolved minerals, particularly calcium (Ca2+) and magnesium (Mg2+). Hardness in water is a common water quality problem.

How is hardness defined?
Hard water will not easily form a lather with soap, mainly due to the presence of Ca2+ or Mg2+ ions. It contains more than 150 p.p.m. CaCO3. Soft water readily forms a lather with soap and contains less than 50 p.p.m. CaCO3. Note that, for convenience, hardness is always quoted in parts per million (p.p.m) of CaCO3 (1 p.p.m = 1 mg/l).

How can hard water be identified?
Hard water can easily be identified. It leaves chalky deposits on the element of the kettle, soap scum on shower tiles, baths and basins. It can result in a build-up of film on hair and body, producing dull hair and dry skin. Also, the build-up of limescale in boilers and heat exchange equipment can result in increased water heating costs and shorter life of machines.

Quiz questions

  1. Approximately 60% of land surface is covered by water false
  2. Water on its own is not a good solvent false
  3. Hard water is damaging to your health false
  4. Limescale is an indicator of the presence of water hardness true
  5. Soap is chemically known as calcium stearate false
  6. Boiling removes all hardness false
  7. Distillation is an ideal process for large scale removal of hardness false
  8. Soft water is better for people with skin irritations true
  9. Calcium hydrogencarbonate is used when calculating water hardness false
  10. Deionised water is the purest form of water false
  11. Ion exchange resins contain iron ions false