Types of Hardness
Two types of hardness:
- Temporary hardness is caused by the hydrogencarbonate ion, HCo3- in Ca(HCO3)2.
- Permenant hardness is caused by dissolved calcium sulfate (amongst other things).
Removing Temporary Hardness
- Temporary hardness is removed by boiling.
- When heated the calcium hydrogencarbonate decomposes to form calcium carbonate which is insoluble. This solid is limescale on your kettle.
calcium hydrogen carbonate ----> calcium carbonate + water + carbon dioxide
Ca(HCO3)2 (aq) ----> CaCO3 (s) + H2O (l) + CO2 (g)
- This won't work for permenant hardness, heating sulfate ion doesn't do anything.
Adding Washing Soda
- Both types of hardness can be softened by adding washing soda to the water, (sodium carbonate, Na2CO3).
- The added carbonate ions react with the magnesium and calcium ions to make an insoluble precipitate of calcium carbonate and magnesium carbonate. The Mg2+ and Ca2+ ions are no longer dissolved in the water to make it hard.
Ca2+ (aq) + Co3 2- (aq) ----> CaCO3 (s)
Ion Exchange Columns
- Both types of hardness can also be removed by running water through 'ion exchange columns' which are sold in shops.
- The columns have lots of sodium ions (or hyrdogen ions) and 'exchange' them for magnesium or calcium ions in the water that runs through them.
Na2Resin (s) + Ca2+ (aq) ----> CaResin (s) + 2Na+ (aq)
- Resin is a huge insoluble resin molecule.