Hard and Soft Water

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Water hardness

Hard water: Water that contains dissolved calcium and magnesium ions

Temporary hard water: Contains calcium and or magnesium hydrogencarbonate. Temporary hardness can be removed by boiling.

Boiling temporary hard water leads to the formation of solid calcium carbonate:

\(Ca(HCO_3)_2(aq) \rightarrow CaCO_3(s) + H_2O(l) + CO_2(g)\)

Calcium carbonate, known as limescale, clogs up pipes in appliances and restricts flow.

Permanent hard water: Contains chlorides and/or sulfates of calcium and magnesium. Permanent hardness cannot be removed by boiling.

Soft water forms a lather easily with soap, but hard water does not.

The organic acid in the soap reacts with the calcium and magnesium ions to form insoluble calcium and magnesium salts.

These salts form a greyish scum instead of a lather.

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Softening hard water

1. Boiling

Removes temporary hardness only

Easy and cheap

Only practical for small quantities

Causes deposits on the inside of the water container

\(Ca(HCO_3)_2(aq) \rightarrow CaCO_3(s) + H_2O(l) + CO_2(g)\)

\(Mg(HCO_3)_2(aq) \rightarrow MgCO_3(s) + H_2O(l) + CO_2(g)\)

2. Adding sodium carbonate (Washing soda)

Removes temporary and permanent hardness

Cheap

Prevents calcium and magnesium ions bonding to the detergent, so less detergent needs to be used

Forms limescale - Carbonate ions from sodium carbonate react with calcium and magnesium ions to produce insoluble precipitates

\(Ca^{2+}(aq) +Na_2CO_3(aq) \rightarrow CaCO_3(s) + 2Na^+(s)\)

3. Ion exchange columns

Removes temporary and permanent hardness

Quite expensive, but can treat a large amount of water

Filled with resin containing sodium ions

Calcium/magnesium ions in hard water exchanged with sodium ions in resin

Magnesium/calcium ions are removed from water, softening it

Eventually the resin has to be 'regenerated' by passing concentrated sodium chloride through it to replace the lost sodium ions

Ion exchange resin beads:

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Advantages and Disadvantages of Hard Water

Advantages:

Tastes better

Provides a dietary source of essential minerals

People living in hard water areas across the world have lower rates of heart disease

Brewers prefer hard water for making beer

Hard water does not dissolve lead, and as a result of this it does not lead to lead poisoning if lead pipes are used in transferring the water into households

Disadvantages:

Causes build up of limescale which can clog pipes and means more energy is needed to boil kettles

More soap needed

It is not advisable to use hard water in washing white fabrics since it tends to stain white fabrics by making them appear grey

Hard water is not good for dyeing materials - cannot be used by dyeing industry