Producing potable water (Required Practical)
Potable water is water safe to drink — low levels of dissolved salts, no harmful microbes, suitable taste and pH. (Note: potable water is not chemically pure — it usually contains beneficial dissolved minerals.)
Sources of fresh water in the UK
- Surface water: rivers, lakes, reservoirs.
- Groundwater: aquifers (underground rocks).
These usually contain manageable levels of dissolved salts but may have microbes/insoluble matter.
Standard treatment process
- Filtration: passing water through gravel/sand filter beds → removes insoluble particles.
- Sterilisation: kills microbes. Methods:
- Chlorine (most common in UK).
- Ozone.
- Ultraviolet light.
Sea water — desalination
Sea water has too much dissolved salt to drink. Two main desalination methods:
Distillation
Boil sea water → collect water vapour → condense to pure water. Salts left behind. Energy-intensive (lots of heating).
Reverse osmosis
Force sea water through a partially permeable membrane under high pressure. Water molecules pass; salt ions blocked. Less energy than distillation but expensive membranes.
Both methods are used in dry, coastal countries (e.g. Saudi Arabia, parts of Australia).
Required practical: producing pure water by distillation
Aim: distil salty water to obtain pure water and analyse purity.
Method:
- Test the salt water (note its electrical conductivity — high; boiling point — slightly above 100 °C).
- Set up a distillation apparatus: round-bottom flask + thermometer + Liebig condenser + collection beaker.
- Heat the salt water; collect the distillate.
- Test the distillate (low conductivity; b.p. exactly 100 °C).
- Compare results.
The distillate is pure water; the original salt remains in the flask.
Why two stages of treatment?
- Filtration removes solids that could block pipes or shelter microbes.
- Sterilisation kills microbes that could cause disease (cholera, typhoid).
Both are essential — filtration alone doesn't kill microbes; sterilisation alone doesn't remove dirt.
⚠Common mistakes
- Saying potable water is pure water — it has dissolved minerals. Pure (chemistry sense) water comes only from distillation.
- Confusing filtration with distillation — filtration removes solids, not dissolved salts.
- Saying chlorine "removes" microbes — it kills them by oxidation; they remain in the water (now dead).
- Forgetting reverse osmosis as an alternative to distillation.
Links
Builds on C1.1 (separation techniques). Connects to C10.3 (waste water) and C10.5 (LCA).
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