10.1.2Air and the Environment
Water Treatment — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 3 of 7 Unit quiz
Part 1 of 4What is in untreated water
Insoluble impurities
- Insoluble impurities These are solid pieces that do not dissolve, such as soil, sand, mud and fragments of plants and other organic matter.
- They make the water cloudy and can be removed by letting them settle or by filtering.
Soluble impurities
- Soluble impurities These dissolve in the water and include calcium compounds, metal compounds and inorganic pollutants.
- They cannot be removed by simple settling or filtration.
Microorganisms
- Microorganisms Bacteria and other microbes are tiny and can cause disease if they are swallowed in drinking water.
Part 2 of 4Stages of water treatment
1
Sedimentation: water is pumped into large tanks and left to stand for several hours, so that mud, sand and other heavy particles sink to the bottom under gravity and form a layer of sediment.
2
Filtration: the water is passed through beds of sand and gravel, which trap smaller solid particles that did not settle out.
3
Filtration through carbon (charcoal) (IGCSE CIE courses): the water passes through carbon, which removes substances that cause unpleasant tastes and smells.
4
Chlorination: a carefully controlled amount of chlorine is added, which kills bacteria and other microorganisms because these are too small to be caught by the filters.
5
The treated water is then stored and piped to homes, industry and farms.
Part 3 of 4Why each stage is needed
| Stage | What it removes | Reason |
|---|---|---|
| Sedimentation | Large, dense solid particles | Gravity pulls them down so they do not clog the filters |
| Sand and gravel filtration | Smaller solid particles | Gives clear water by trapping solids in the gaps between grains |
| Carbon filtration (IGCSE CIE) | Substances causing taste and odour | Makes the water pleasant to drink |
| Chlorination | Bacteria and microbes | Prevents diseases carried in water |
Part 4 of 4Why chlorination matters
1Untreated water
- Untreated water It may contain disease-causing bacteria.
2Drinking it
- Drinking it Bacteria enter the body and multiply.
3Illness
- Illness Diseases such as cholera and typhoid can result, and spread quickly where water is unsafe.
4Chlorine added
- Chlorine added Microorganisms are killed, so the water is safe to drink all the way to the tap.
Key terms
| Term | Meaning |
|---|---|
| Sedimentation | Settling of heavy solid particles to the bottom of a tank under gravity. |
| Filtration | Separating insoluble solids from a liquid by passing it through a filter. |
| Chlorination | Adding chlorine to water to kill bacteria and other microorganisms. |
| Insoluble | Not able to dissolve in a solvent such as water. |
| Soluble | Able to dissolve in a solvent such as water. |
| Sediment | The solid layer that collects at the bottom of a tank or container. |
Avoid these mistakes
1
Saying filtration removes bacteria - microbes are too small for sand and gravel, which is why chlorination is needed.
2
Describing the stages without explaining their purpose; exam questions usually ask why each step is done.
3
Confusing sedimentation (settling by gravity) with filtration (trapping in a filter bed).
4
Writing that treated water is 'pure' - it is safe to drink but still contains dissolved substances.
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