Purifying Water (Edexcel GCSE Combined Science): Revision Note
Exam code: 1SC0
Purifying Water
Making Water Potable
In the UK, drinking water is sourced from rivers, lakes, and underground aquifers (rock layers that contain groundwater)
This water is usually stored in reservoirs before being treated
Fresh water from these sources often contains:
Large objects (e.g. leaves and twigs)
Insoluble particles (e.g. grit and silt)
Soluble substances (e.g. salts, pesticides, fertilisers)
Microorganisms (e.g. harmful bacteria)
To make this water safe to drink, it undergoes several treatment steps:
Screening and Sedimentation
Water first passes through a sieve to remove large debris
Then, it is left still in large tanks so that sedimentation can occur: larger insoluble particles sink to the bottom
Chemicals like iron sulfate or aluminium sulfate may be added to help smaller particles clump together and settle
Filtration
Water is filtered through beds of sand and gravel to remove remaining small solid particles.
Chlorination
Chlorine is added to kill harmful bacteria and other microorganisms that can’t be removed by filtration.
This step helps prevent diseases such as cholera and typhoid, which can be caused by drinking untreated water.
Sedimentation, filtration and chlorination
Examiner Tips and Tricks
For GCSE Edexcel the order for making water potable is:
Sedimentation: Large particles sink to the bottom.
Filtration: Water passes through sand and gravel to remove smaller particles.
Chlorination: Chlorine is added to remove harmful bacteria and microorganisms.
Disinfection comes last so that microorganisms are removed after all solid particles are cleared.
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
Examiner Tips and Tricks
Waste and ground water can be made potable by sedimentation, filtration and chlorination. Sea water can be made potable by using simple distillation. Water used in analysis must not contain any dissolved salts which would interfere with the sensitivity of the tests.
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