Treating Drinking Water
This topic covers the chemical additives used in public water treatment to ensure it is safe for consumption and provides public health benefits. You need to know the specific roles of chlorine and fluoride ions in this process.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Chlorine is added to drinking water as a disinfectant to kill harmful microorganisms, such as bacteria and viruses.
- The process of adding chlorine is called sterilisation, which prevents the spread of waterborne diseases like cholera and typhoid.
- Fluoride ions (F⁻) are added to drinking water in a process known as fluoridation.
- The primary purpose of adding fluoride ions is to improve dental health by preventing tooth decay.
- Fluoride ions work by strengthening tooth enamel, making it more resistant to acid attacks from bacteria in the mouth.
Definitions
- Potable water
- Water that is safe to drink and free from harmful microorganisms and pollutants.
- Disinfection
- The process of killing pathogenic microorganisms in water to make it safe for consumption. Chlorine is a common chemical disinfectant.
- Fluoridation
- The controlled addition of fluoride compounds to a public water supply for the purpose of reducing tooth decay.
Worked example
A water treatment plant adds chlorine and a soluble fluoride salt to its water supply. Which of the following correctly identifies the primary reason for each addition?
- 1
First, recall the function of chlorine in water treatment.
Chlorine is a powerful oxidising agent used to sterilise water.
- 2
Sterilisation means killing harmful microorganisms like bacteria and viruses to make the water safe to drink.
- 3
Next, recall the function of fluoride ions.
These are added specifically for public health, not water purification.
- 4
The benefit of fluoride ions is to strengthen tooth enamel and make teeth more resistant to decay.
- 5
Combine these two distinct functions to find the correct answer:
Chlorine is for disinfection/sterilisation, and fluoride is for dental health.
Answer: Chlorine is added to kill harmful microorganisms, and fluoride is added to help prevent tooth decay.
Common mistakes
- ×Confusing the roles of chlorine and fluoride. A common mistake is to think fluoride disinfects water or that chlorine helps with dental health.
- ×Assuming fluorine gas (F2) is used. It is the fluoride ion (F⁻), typically from a salt like sodium fluoride (NaF), that is added. Fluorine gas is highly toxic and reactive.
- ×Mistaking chlorination or fluoridation for filtration. These are chemical treatment steps that occur alongside physical processes like filtration, but they have different purposes.
No-calculator tips
- ✓Use a mnemonic to link the chemical to its purpose, for example: 'Chlorine Cleans' (kills germs) and 'Fluoride Fights' (cavities).
- ✓This is a knowledge-based topic, not a calculation one. Focus on memorising the distinct purpose of each chemical additive.