AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Potable water: Sterilising agents used for potable water include chlorine, ozone or ultraviolet light.
Potable water must be free of disease-causing microorganisms, so treatment works use a sterilising agent.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Potable water must be free of disease-causing microorganisms, so treatment works use a sterilising agent. Chlorine gas or a chlorine compound is added to water and kills bacteria by disrupting their cell processes; it is widely used in the UK because it is effective, relatively cheap and leaves a residual that protects water as it travels through pipes. Ozone, O₃, is a strong oxidising agent that destroys microorganisms and leaves no chemical residual, but it must be generated on site and does not protect the water afterwards. Ultraviolet light damages the genetic material of microorganisms so they cannot reproduce; it adds no chemicals but only works where the water is clear enough for the light to penetrate. All three agents kill or inactivate microorganisms; none removes dissolved substances.
What earns the marks
- Chlorine, ozone and ultraviolet light are all sterilising agents used to make potable water safe.
- Chlorine kills microorganisms and leaves a residual that continues to disinfect the water in the distribution system.
- Ozone is a strong oxidising agent that kills microorganisms and leaves no persistent chemical residual.
- Ultraviolet light inactivates microorganisms by damaging their genetic material.
- Sterilising agents kill microorganisms but do not remove dissolved substances from the water.
- The choice of agent depends on factors such as cost, effectiveness, residual protection and the clarity of the water.
Common mistakes and how to put them right
Writing that chlorine, ozone or ultraviolet light remove dissolved salts.
Put it right: They only kill microorganisms, so dissolved substances remain.
Confusing ozone with oxygen.
Put it right: Ozone is O₃, a stronger oxidising agent than O₂.
Assuming ultraviolet light works in cloudy water.
Put it right: Suspended particles can shield microorganisms, so the water must be clear for UV treatment to be effective.
Exam tips
- Name all three sterilising agents and give one advantage of each, such as chlorine leaving a residual or UV adding no chemicals.
- State clearly that sterilising agents kill microorganisms rather than remove dissolved substances.
- Link the choice of sterilising agent to a practical factor such as cost, residual protection or water clarity.
Try the lesson
The start of the Potable water: Sterilising agents used for potable water include chlorine, ozone or ultraviolet light. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of these is used to sterilise potable water?
Listen
Potable water must be free of disease-causing microorganisms, so treatment works use a sterilising agent. Chlorine gas or a chlorine compound is added to water and kills bacteria by disrupting their cell processes; it is widely used in the UK because it is effective, relatively cheap and leaves a residual that protects water as it travels through pipes. Ozone, O₃, is a strong oxidising agent that destroys microorganisms and leaves no chemical residual, but it must be generated on site and does not protect the water afterwards. Ultraviolet light damages the genetic material of microorganisms so they cannot reproduce; it adds no chemicals but only works where the water is clear enough for the light to penetrate. All three agents kill or inactivate microorganisms; none removes dissolved substances. A strong answer earns marks for these points. Chlorine, ozone and ultraviolet light are all sterilising agents used to make potable water safe. Chlorine kills microorganisms and leaves a residual that continues to disinfect the water in the distribution system. Ozone is a strong oxidising agent that kills microorganisms and leaves no persistent chemical residual. Ultraviolet light inactivates microorganisms by damaging their genetic material. Exam tip: Name all three sterilising agents and give one advantage of each, such as chlorine leaving a residual or UV adding no chemicals.
See it
Key words
Say what each word means before you turn the card.
Sterilising agents
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Chlorine
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Ozone
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Ultraviolet light
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See every definition
- Sterilising agents
- Chlorine, ozone or ultraviolet light are used for potable water to kill or inactivate microorganisms.
- Chlorine
- Added as a gas or compound, it kills bacteria by disrupting their cell processes and leaves a residual that protects water in pipes.
- Ozone
- A strong oxidising agent that destroys microorganisms and leaves no chemical residual, but must be generated on site and does not protect the water afterwards.
- Ultraviolet light
- Damages the genetic material of microorganisms so they cannot reproduce, adds no chemicals, and only works where the water is clear enough.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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