C3.2 Water

The water we drink is not pure because it contains dissolved substances. It should be safe to drink water that has been treated, This means the water does not contain anything that could harm us. Some of the dissolved substances are beneficial to our health but some cause hard water.

Hard Water contains dissolved calcium and magnesium compound such as:

• magnesium hydrogen carbonate [Mg(HCO3)2]
• calcium hydrogen carbonate [Ca(HCO3)2]
• magnesium sulphate (MgSO4)
• calcium sulphate (CaSO4)

Hard Water can be temporary or permanent.

Temporary Hard Water contains magnesium hydrogen carbonate and calcium hydrogen carbonate.

Permanent Hard Water contains magnesium sulphate and calcium sulphate.

• Soft Water forms a lather easily with soap.
• Hard Water needs more soap to form a lather, adding additional cost to cleaning processes
• This is because dissolved chemicals in hard water react with soap to form scum

Rainwater becomes slightly acidic as carbon dioxide from the air dissolves in it, forming carbonic acid.

Limestone contains calcium carbonate. As the slightly acidic rainwater trickles through rocks, the calcium carbonate reacts to form soluble hydrogen carbonate:

Carbonic Acid + Calcium Carbonate ---> Calcium Hydrogen Carbonate + Carbon Dioxide + Water

H2CO3          + CaCO2                  ---> Ca(HCO3)2                          + CO2                + H20

One of the chemicals in soap is sodium stearate.

The dissolved calcium and magnesium compounds in hard water react with sodium stearate to form a solid called calcium stearate (or scum).

The soap will form a lather when all of the dissolved calcium hydrogen carbonate in the water has reacted.

Using hard water can cause problems:

• More soap is needed to get a lather
• It can be difficult to clean scum from bathtubs and sinks
• Hard Water can be unsuitable for industrial processes like dying

When hard water is heated, the dissolved calcium hydrogen carbonate decomposes to form solid calcium carbonate.

These deposits of calcium carbonate are called limescale:

• Limescale can block pipes and coat the heating elements in kettles, washing machines and heaters.
• Limescale is a poor heat conductor, and reduces the efficiency of appliances.

Weak acids, such as ethanoic acid, can be used as descalers. The acids reacts with the limescale to form soluble compounds, which are then washed away

Ethanoic acid + Calcium Carbonate ---> Calcium Ethanoate + Carbon Dioxide + Water

2CH2COOH  + CaCO3                  ---> (CH3COO2)Ca        +CO2                 + H2O

Hard Water can be good for your health:

• calcium is needed for healthy bones and teeth
• magnesium is needed for effective metabolism

Some studies have also shown that people living in hard water areas are less likely to suffer from heart disease. The World Health Organisation states that there is not yet enough evidence to confirm a link between hard water and heart disease.

Hard Water
+ improved taste
+ reduced risk of heart disease
- more soap = more cost
- scum is unsightly
- can reduce efficiency of appliances because of limescale

Soft Water
+ less soap
+ no scum
+ no limescale
- less minerals

Softening temporary hard water

Temporary hard water contains calcium hydrogen carbonate. This is relatively easy to remove because it decomposes on heating to form solid calcium carbonate:

Calcium Hydrogen Carbonate ---> Calcium Carbonate + Water + Carbon Dioxide
Ca(HCO3)2                        ---> CaCO3                + H2O   + CO2

Softening hard water

Both temporary and permanent hard water can be softened by adding sodium carbonate (washing soda). The sodium carbonate reacts with the calcium compounds in the water to form calcium carbonate and soluble sodium compounds, which do not contribute to hardness: 

Calcium Hydrogen Carbonate + Sodium Carbonate --->Calcium Carbonate + Sodium Hydrogen Carbonate

Ca(HCO3)2                          + Na2CO3                ---> CaCO3                 + 2NaHCO3

Water can be softened by removing Ca2+ or Mg2+ ions using an Ion-Exchange Column. A column contains a resin packed with sodium ions (Na+). The hard water passes through the column. The sodium ions in the resin are exchanged for the Ca2+ or Mg2+ ions in the hard water.

Some columns work by swapping hydrogen ions for the aqueous calcium or magnesium ions. This is how domestic water-softening units work.

The resin can be recharged with sodium ions after they have been exchanged for calcium and magnesium ions. The resin is washed with salt (sodium chloride) solution. This puts the sodium ions back in. This is why softeners must be kept topped up with salt.

Reservoir

1. Filter to remove any large debris e.g. twigs

2. Settlement tank (sand and soil settle to the bottom)

3. Aluminium sulphate and lime are added which cause small particles of dirt to clump together and sink as sludge

4. Chlorine is added to kill any microorganisms

5. Check pH of the water