Purifying Water (Edexcel GCSE Chemistry: Combined Science)

Making Water Potable

• Ground water from aquifers is relatively clean but surface water (from rivers & lakes) and waste water need significant treatment in order to be fit for human consumption
• Untreated water contains soluble and insoluble impurities
• Insoluble impurities include soil, pieces of plants and other organic matter and soluble impurities include calcium, metallic compounds and inorganic pollutants
• Unclean water also contains microbes which can cause illness
• Potable water means water that is clean enough for human consumption

Filtration

• This process removes large insoluble particles by passing the water through layers of sand and gravel filters that trap larger particles
• Wire mesh filters are sometimes used, depending on the level of impurities in the water

Sedimentation

• Large insoluble particles sink to the bottom of a tank of water that has been left still for some time
• Iron sulfate or aluminium sulfate is sometimes added to help the fine particles clump  together

Chlorination

• This process is used to kill bacteria and microorganisms which are too small to be trapped by the filters
• Cholera and typhoid are examples of bacterial diseases which can arise by the consumption of untreated water

Water treatment involves filtration, sedimentation and chlorination

Making Sea Water Potable

• This process is done in some areas of the world where very hot and dry climates prevail and where a lack of water.
• Sea water contains mainly salts and can therefore be distilled to separate the water and the salts.
• The salt remains in the liquid while the steam is cooled and condensed to make potable water.
• The process is extremely expensive as a lot of energy is required to heat the large volumes of water to 100 ºC.
• The wastewater produced is also extremely toxic due to the very high concentration of salts and must be disposed of correctly

Simple distillation of seawater to make it potable

Water in Chemical Analysis

• Most chemical investigations involve the use of water at some stage of the process
• Normally deionised water is used, which is water that has had metallic ions such as calcium or copper removed
• Deionisation uses specifically designed ion-exchange resins that remove ions by exchange with hydrogen and hydroxide ions in water, which then recombine to form water molecules
• Deionised water is used as the ions could react with the substances under analysis and would give the experiment a false result