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
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
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.
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
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
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