C3bL2 Hard and Soft Water

Key Words

Dissolve - a solid or liquid becomes dispersed as single atoms or molecules in a solvent.

Hard water - water with calcium and magnesium ions dissolved in it.

Limescale - calcium carbonate deposited on pipes, heating elements, etc.

Soft water - water without calcium or magnesium ions dissolved.

Solvent - a substance that dissolves another substance.

Washing soda - sodium carbonate.

Grade E

Although soft water is not pure, it does not have many minerals dissolved in it.  It certainly does not have calcium or magnesium salts dissolved.  Soft water areas are found mostly in the Pennines of Northern England, and the hilly areas of Wales and Scotland.  Soap lathers well in soft water, and the textile industry was based in the soft water areas.

Hard water is water in which calcium and magnesium salts are dissolved.  Water is an excellent solvent, dissolving carbon dioxide to form a weak acid that reacts with calcium and magnesium in rocks like limestone.

The water in Huntingdonshire is particularly hard.  There are advantages:

• Water tastes nicer;
• Hard water reduces the risk of heart disease.
• Bones and teeth are stronger.

There are also disadvantages:

• Limescale forms on heating elements, making the elements less efficient, or even overheat.
• It is much harder to form a lather
• People with skin conditions like eczema can find that their condition is made much worse with hard water.

Hard water can be made soft by washing soda, or passing water through an ion exchange resin.  Both methods remove calcium and magnesium ions.

Grade C

If we find that soap lathers well under the tap, we can say that the water is soft.  In hard water areas, the soap does not lather well, and layers of scum are formed round baths and wash-basins.

calcium sulphate + sodium stearate ® calcium stearate + sodium sulphate

Sodium stearate is a soap (not a very pleasant one) and calcium stearate is the scum.  The lather forms only when all the sodium and calcium ions are removed.

Sodium carbonate in washing soda reacts with calcium sulphate to remove the calcium ions as a calcium carbonate precipitate.

sodium carbonate + calcium sulphate ® sodium sulphate + calcium carbonate

Na₂CO_3(aq) + CaSO_4(aq)  ® Na₂SO_4(aq) + CaCO_3(s)

Water softeners use ion-exchange resins:

• The resin beads have sodium ions.

• Calcium and magnesium ions swap places with the sodium ions.

• The water leaving has low levels of calcium and magnesium.

• However the water has high levels of sodium and is unpleasant to drink.

• The column needs recharging from time to time, and is washed through with sodium chloride solution, replacing the calcium ions on the resin with sodium ions.

Grade A

Hard water can be:

• Permanent
• Temporary.

Permanently hard water has calcium chloride and sulphate (and/or magnesium salts).  The salts become even more soluble when the water is heated, so are not removed by boiling.

Temporary hardness is caused by calcium (or magnesium) hydrogen carbonates.

Carbon dioxide reacts with water to form carbonic acid (a very weak acid):

water + carbon dioxide ® carbonic acid

H₂O_(l) + CO_2(g) ® H₂CO_3(aq)

The carbonic acid reacts with calcium carbonate in limestone to form calcium hydrogen-carbonate:

carbonic acid + calcium carbonate  ® calcium hydrogen-carbonate

H₂CO_3(aq) + CaCO_3(s) ® Ca(HCO₃)_2(aq)

When heated, the reaction reverses, and the carbonic acid decomposes to form carbon dioxide and water.  The calcium carbonate precipitates out.