Softening Hard Water - Chemistry - Unit 3 - AQA

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).

• 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.

• 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)

• 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.