The water we drink is not pure water because it contains dissolved substances.It should be safe to drink water that has been treated.This means that the water does not contain anything that could cause us harm. Some of the dissolved substances are beneficial to our health but some cause hard water.
Candidates should use their skills, knowledge and understanding to:
■ evaluate the use of commercial water softeners
■ consider and evaluate the environmental, social and economic aspects of water quality and hardness
■ consider the advantages and disadvantages of adding chlorine and fluoride to drinking water.
Candidates may be asked to evaluate different methods of softening water, or of providing drinking water of sufficient quality
Purifying water - Reasons for water of the correct quality Water of the correct quality is essential for life. For humans, drinking water should have sufficiently low levels of dissolved salts and microbes. Producing water of the correct quality ■ choosing an appropriate source ■ passing the water through filter beds to remove any solids ■ sterilising with chlorine. Addition of chlorine and fluoride ions to drinking Hard and soft water - Reaction with soap Soft water readily forms lather with soap. Hard water reacts with soap to form scum and so more soap is needed to form lather: the presence of calcium ions and magnesium ions in the water makes it hard. Soapless detergents do not form scum with hard water. Origin of scum (hard water + soap) Hard water contains dissolved calcium or