Soft water e.g. distilled water, easily forms a lather with soap.
Hard water e.g. London tap water, does not easily form a lather and forms a scum.
Teaching activity
C7.14 Soaps and soapless detergents
Detergents are chemicals with large molecules which help clean. They have one end which dissolves in oil and one end which is ionic and dissolves in water.
oil soluble end C17H35COO-Na+ water soluble ionic end
Hard water contains soluble calcium and/or magnesium salts. It contains calcium ions Ca2+(aq), which can be detected by a flame test or by testing with sodium hydroxide and/or magnesium ions Mg2+(aq).
A soap has ions which react with ions in hard water to form a precipitate (a scum).
e.g. sodium stearate C17H35COO-Na+ is a soap.
stearate ion + calcium ion ---> calcium stearate
Calcium stearate is insoluble so forms a precipitate (a scum) in hard water.
2C17H35COO-(aq) + Ca2+(aq) ---> (C17H35COO)2Ca(s)
A soapless detergent has ions which do not react with the ions in hard water.
e.g. sodium 3-dodecylbenzene sulfonate C18H29SO3-Na+ is a soapless detergent.
C18H29SO3-Na+ and (C18H29SO3)2Ca are both soluble so no scum forms in hard water.
Task C7.14 Draw labelled diagrams to show beakers with the ions and molecules that they contain for (a) soft water, hard water, soft water + soap, soft water + soapless detergent, hard water + soap, hard water + soapless detergent.
C7.15 Limestone, chalk and gypsum and hard water
Hard water forms when calcium or magnesium salts in rock dissolve in rain water as it flows through the rock.
Gypsum is a rock containing calcium sulfate. Calcium sulfate is insoluble but a little does dissolve to leave some calcium ions in the water where is has past over gypsum.
Limestone and chalk are rocks containing calcium carbonate. Calcium carbonate is very insoluble and none of it dissolves as water passes over limestone or chalk. Calcium ions from limestone and chalk do dissolve to make water hard in a reaction with carbonic acid.
water + carbon dioxide ---> carbonic acid
H2O(l) + CO2(g) ---> H2CO3(aq)
calcium carbonate + carbonic acid ---> calcium hydrogencarbonate
CaCO3(s) + H2CO3(aq) ---> Ca(HCO3)2(aq)
C7.16 does not exist!
C7.17 Industrial and domestic problems caused by scale
Hard water normally contains dissolved calcium hydrogencarbonate. This can slowly change back into insoluble calcium carbonate but the change is speeded up by heating. The solid calcium carbonate formed is called scale.
calcium hydrogencarbonate ---> calcium carbonate + carbon dioxide +water
Ca(HCO3)2(aq)) ---> CaCO3(s) + H2O(l) + CO2(g)
scale
Scale forms on a kettle's heating element insulating it and wasting energy when it boils.
Scale forms inside hot water pipes which can even block a pipe.
Task C7.16 Stalactites and stalagmites are made of calcium carbonate and form in caves in hard water areas. Explain how this might happen.
C7.18 Benefits of hardness in water
The formation of thin layers of scale on the inside of pipes can be useful.
Corrosion is reduced because the water in the pipes is not in contact with the metal.
Poisonous metal salts of lead or copper from the metal surface of pipes cannot enter drinking water if the metal surface is covered in scale.
Calcium is needed in the diet for healthy bones and teeth.
C7.19 The treatment of water to make it suitable for domestic use
Task C7.19a Match the following descriptions to the stages in the diagram above: kills bacteria, collects rain water etc, removes tiny particles of solid, removes large objects like twigs, removes small bits of solid like leaf fragments, causes tiny particles to clump together, solid particles gather together at bottom.
Task C7.19b Illustrate each stage in the diagram above.