Hard & Soft Water

  • Created by: Katy **
  • Created on: 05-01-14 14:09

Hard & Soft Water

  • Water is either soft or hard.
  • To find out which type of water hardness it is, add soap to the water & shake

Soft water = large amount of bubbles (lather)

Hard water = few bubbles on surface (scum)

  • Water hardneness is caused by dissolved magnesium ions or calcium ions.
  • There are two types of hard water -

a) temporary hard water

b) permanent hard water

1 of 6

Temporary Hard Water

  • Removed by boiling the water.
  • Caused by dissolved magnesium hyrdogen carbonate or calcium hydrogen carbonate.
  • When the water it boiled the compounds decompose (thermal decomosition reaction).
  • Calcium hydrogen carbonate → calcium carbonate + water + carbon dioxide

Ca(HCO3) → CaCO3 + H20 +CO2

white precipiate is made

  • Calcium / magnesium carbonate are both insoluble white solids = limescale
2 of 6

Permanent Hard Water (1)

  • This can not be removed by boiling.
  • Caused by dissolved calcium sulfate or magnessium sulfate.
  • It is removed by two compounds (both also remove temporary hard water).

1. Ion Exchange Resin (IER)

Hard water → IER → Soft water

IER traps the Calcium / Magnesium sulfate & replaces them with Sodium that does not cause water hardness.

The IER will eventually run out of Sodium & stop working.

To get the IER working, wash with Sodium chloride solution & this will replace the lost Sodium.

3 of 6

Permanent Hard Water (2)

2. Add Sodium carbonate solution (washing soda)

  • This causes a precipiate reaction.
  • Sodium carbonate + Calcium sulfate → Calcium carbonate + Sodium sulfate

Na2CO3 (aq) + CaSO4 (aq) → CaCO3 (s) + Na2SO4 (aq)

Calcium carbonate = white precipiate

4 of 6

Boiling & the Ion Exchange Resin

Boiling the Water:

  • only works to temporary hard water
  • forms limescale that blocks filters & pipes
  • works every time

Ion Exchange Resin:

  • resin will stop working when the Sodium runs out
  • have to wash with Sodium chloride solution
  • works for temporary & permanent hard water
  • no limescale is made
5 of 6

Advantages & Disadvantages

Advantges of Hard water:

  • good for the bones & teeth
  • reduces risks of heart disease

Disadvantages of Hard Water:

  • problems can occur in the filters & pipe with blockages
6 of 6


No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Water hardness and solubility resources »