C3.2 WATER

a) Soft water readily forms lather with soap. Hard water reacts with soap to form scum and so more soap is needed to form lather. Soapless detergents do not form scum.

b) Hard water contains dissolved compounds, usually of calcium or magnesium. The compounds are dissolved when water comes into contact with rocks. 

c) There are 2 types of hard water. Permanent hard water remains hard when it is boiled. Temporary hard water is softened by boiling. 

d) Temporary hard water contains hydrocarbonate ions (HCO3-) that decompose on heating to produce carbonate ions which react with calcium and magnesium ions to form precipitates. 

e) Using hard water can increase costs because more soap is needed. When temporary hard water is heated it can produce scale that reduces the efficiency of heating systems and kettles. 

f) Hard water has some benefits because calcium compounds are good for the development and maintenance of bones and teeth and also help to reduce heart disease.

g) Hard water can be made soft by removing the dissolved calcium and magnesium ions. This can be done by: adding sodium carbonate, which reacts with the calcium and magnesium ions to form a precipitate of calcium carbonate and magnesium carbonate; using commercial water softeners such as ion exchange columns containing hydrogen ions or sodium ions, which replace the calcium and magnesium ions when hard water passes through the column.

a) Water of the correct quality is essential for life. For humans, drinking water should have sufficiently low levels of dissolved salts and microbes.

b) Water filters containing carbon, silver and ion exchange resins can remove some dissolved substances from tap water to improve the taste and quality. 

c) Chlorine may be added to drinking water to reduce microbes, and fluoride may be added to improve dental health.

d) Pure water can be produced by distillation.