Hard water –.......... by Timothy Connolly 12E/2000

Hard water is defined as water that does not lather readily with soap. There is a scum produced, and hardly any lather.

Advantages of hard water

·         Calcium compounds in hard water strengthen bones and teeth

·         Calcium is beneficial to people who tend to develop heart disease

·         The leather industry prefers hard water

·         The brewing industry likes it for the taste it gives to the beer

Disadvantages of hard water

·         Wastes soap

·         Forms scale in pipes and radiators, and fur in kettles

·         Spoils special finishes on fabrics

Hardness in water is caused by dissolved calcium and magnesium salts which do not get removed at the waterworks, mainly:

·          Calcium Hydrogen carbonate                              Ca (HCO₃)₂

·          Calcium Sulphate                                                   CaSO₄

·          Magnesium Hydrogen carbonate                       Mg (HCO₃)₂

·          Magnesium Sulphate                                             Mg SO₄

·          Calcium Chloride                                                     CaCl₂

·          Magnesium Chloride                                                 MgCl₂

How do these chemicals get into the water?

1.       How Calcium Hydrogen carbonate gets into the water

While calcium carbonate does not dissolve in water, it will dissolve in water that has carbon dioxide in it. When rainwater falls carbon dioxide dissolves into it from the air.

Water   + Carbon dioxide   + Calcium carbonate ® Calcium Hydrogen carbonate       

H₂O        +             CO₂                 +                   CaCO_{3                         →}                             Ca(HCO₃)_{2 (aq)}

This particular chemical causes what is referred to as temporary hardness, because it can be removed quite easily. Heating reverses the above reaction.

Calcium Hydrogen carbonate  ® Water   + Carbon dioxide   + Calcium carbonate

             Ca (HCO₃)_{2 (aq)}          ®   H₂O    +          CO₂           +          CaCO₃

When this happens in a kettle, the calcium carbonate deposits are known as fur, and when it happens in a radiator, the insoluble calcium carbonate deposits are known as scale. This process can also form stalagmites and stalactites in a limestone cave.

Permanent Hardness

The main salts that cause permanent hardness are the sulphates and carbonates of calcium and magnesium. These chemicals get into the water when it passes over/through rocks and soil that contain them. These chemicals cannot be removed by boiling in water.

The removal of permanent hardness

·          Washing Soda is sodium carbonate-10 water. This is able to soften temporary and permanent hardness. A precipitation reaction occurs, and the calcium and magnesium ions form insoluble carbonates.

Calcium Chloride + Sodium Carbonate-10 water ®  Calcium carbonate_¯  +  Sodium Chloride

·          Exchange resins take ions of one kind out of aqueous solution and replace them with ions of a different kind. Permutits are an example of a manufactured ion exchange resin. They replace dissolved calcium and magnesium ions in water with sodium ions.

Calcium ions          +    Sodium Permutit   ® Sodium ions    +   Calcium Permutit   

Magnesium ions   +      Sodium Permutit   ® Sodium ions    +   Magnesium Permutit

Chemistry Department: Loreto College, Coleraine.