Water Treatment and Potable WaterAQA GCSE Chemistry: Flashcards

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Define potable water.

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Define potable water.: Potable water is water that is safe to drink. It does not mean the water is pure, but rather that it is free from harmful microorganisms and toxic substances at levels that would cause harm.
Name the three main stages in treating fresh water to make it potable in the UK.: Sedimentation (settling) - allows suspended particles to settle out Filtration - water passes through sand and gravel beds to remove remaining particles Sterilisation - kills microorganisms using chlorine, ozone, or ultraviolet light
What is sedimentation in water treatment?: Sedimentation is the process where water is left to settle in large tanks, allowing suspended particles and sediment to sink to the bottom where they can be removed.
Describe filtration in the treatment of potable water.: Water is passed through layers of sand and gravel beds. These layers trap small particles that were not removed during sedimentation, further purifying the water.
Name three methods used to sterilise water during potable water treatment.: Chlorine gas or chlorine compounds Ozone gas Ultraviolet (UV) light
What is desalination?: Desalination is the process of removing dissolved salts from sea water or brackish water to produce fresh water suitable for drinking or other uses.
Describe the process of desalination by distillation.: Sea water is heated until it evaporates. The water vapour is then condensed to form pure water, whilst the dissolved salts are left behind as a residue.
Describe the process of desalination by reverse osmosis.: High pressure is applied to sea water to force it through a partially permeable membrane. Water molecules pass through the membrane whilst dissolved salt ions are blocked, producing fresh water.
Why is desalination more energy-intensive and expensive than UK water treatment?: Desalination requires either significant heating (distillation) or high pressure (reverse osmosis) to remove salts from sea water. These processes consume large amounts of energy, making them costly. UK water treatment only requires settling, filtration, and sterilisation, which use less energy.
Explain why countries with limited freshwater supplies use desalination differently from the UK.: Countries with limited freshwater (like Middle Eastern nations) must desalinate sea water or brackish water despite the high cost and energy use, as it is their only viable source of potable water. The UK has abundant freshwater from rainfall and reservoirs, so it only treats natural fresh water sources.
State two advantages of adding fluoride to water supplies.: Reduces tooth decay and cavities by strengthening tooth enamel Improves dental health, especially in children
State two disadvantages of adding fluoride to water supplies.: Excessive fluoride can cause dental fluorosis (brown staining of teeth) Some people believe it is unethical to add chemicals to water without individual choice In high concentrations, fluoride can be toxic
State one advantage of adding chlorine to water supplies.: Chlorine kills harmful microorganisms (bacteria and viruses), preventing waterborne diseases and making water safe to drink.
State two disadvantages of adding chlorine to water supplies.: Chlorine can react with organic compounds in water to form potentially harmful disinfection by-products (such as trihalomethanes) Some people object to adding chemicals to water supplies Chlorine can affect the taste and odour of water
Why must water be sterilised even after sedimentation and filtration?: Sedimentation and filtration remove particles and sediment but do not kill microorganisms such as bacteria and viruses. Sterilisation is essential to eliminate these harmful microorganisms and make the water safe to drink.