Pure water and potable water
Pure water has only water molecules (H₂O) and nothing else. Potable water means water that is safe to drink. These are not the same thing.
Potable water still has small amounts of dissolved salts. That is fine, and it even gives water its taste. But it must have no harmful germs and only low levels of salts and other substances.
Where does our water come from?
- Fresh water from rain collects in lakes, rivers and under the ground (groundwater). This is the easiest source.
- Sea water covers most of the planet but is far too salty to drink.
Only a tiny part of Earth's water is fresh and easy to reach. Using it carefully, and not wasting or polluting it, is part of sustainable development: meeting our needs today without spoiling the needs of people tomorrow.
Making fresh water safe: settle, filter, disinfect
A water works treats fresh water in a few steps:
- Choose a source that is as clean as possible.
- Screening and sedimentation: a mesh stops sticks and leaves. In big tanks the water stands still, so sand and heavy bits sink. Sometimes a chemical is added to make tiny bits clump together and sink faster.
- Filtration: water passes through beds of sand and gravel. These trap the fine solid bits that did not sink.
- Adsorption: activated carbon (charcoal with millions of tiny holes) holds colour and smelly chemicals on its surface. Adsorb means 'stick to the surface'.
- Sterilisation (disinfection): germs are killed with chlorine, ozone or ultraviolet (UV) light.
Filtering removes things you can catch. It does not remove dissolved salts, and many germs are too small to catch. That is why the last step kills germs.
Desalination: drinking water from sea water
When fresh water is short, we can remove salt from sea water. This is called desalination.
- Distillation: heat salty water until it boils. The water turns to steam and leaves the salt behind. The steam is cooled in a condenser and turns back into liquid. This water is very pure.
- Reverse osmosis (RO): salty water is pushed at high pressure through a membrane (a very thin sheet with tiny holes). Water passes through; most salt does not.
Both methods need a lot of energy, so they are costly. They are used in dry places with a lot of sea and cheap energy.
Testing your water in the lab
To check if water is pure: it boils at exactly 100 °C (at normal air pressure). Evaporate a sample on a dish: pure water leaves no solid behind. You can also test the pH (pure water is 7).
Waste water treatment
After we use water at home, in factories and on farms, it becomes waste water (sewage). It holds food bits, soap, human waste, germs and sometimes harmful chemicals. It must be cleaned before it goes back into rivers or the sea.
- Screening and grit removal: rags, plastic and grit are taken out.
- Sedimentation: in big tanks, heavy sludge sinks. The lighter liquid on top is called effluent.
- Aerobic treatment of effluent: air is bubbled in so helpful bacteria can digest the remaining waste.
- Anaerobic digestion of sludge: without air, other bacteria break down the sludge. This makes biogas (a fuel) and a solid that can be used as fertiliser.
Factory waste water may need extra steps to remove toxic chemicals. Treating sewage is harder and needs more steps than treating fresh water, but it protects rivers and our health.
Hard and soft water
Soft water lathers easily with soap. Hard water does not lather well and leaves a grey scum.
Water becomes hard when it flows over rocks like limestone and chalk and picks up dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
- Temporary hardness comes from calcium hydrogencarbonate. Boiling removes it; the white scale inside a kettle is the result.
- Permanent hardness comes from calcium or magnesium sulfate. Boiling does not remove it.
Ways to soften water: boiling (temporary only), adding washing soda (sodium carbonate), or an ion-exchange column that swaps Ca²⁺ and Mg²⁺ for sodium ions. Distillation removes both types too.
Hard water is good for teeth and bones (it has calcium), but scale blocks pipes and makes kettles and heaters waste energy.
Key formulas and definitions
- Potable water = safe to drink (low salts, no harmful germs) ≠ pure water (only H₂O)
- Treatment order: screen → sediment → filter → (carbon adsorb) → disinfect (chlorine / ozone / UV)
- Desalination: distillation (boil + condense) or reverse osmosis (membrane + pressure)
- Waste water: screen → settle (sludge + effluent) → aerobic for effluent → anaerobic for sludge
- Hard water = dissolved Ca²⁺ and Mg²⁺; temporary removed by boiling, permanent is not
- Pure water test: boils at 100 °C, leaves no solid on evaporation
Worked examples
1. A village gets water from a muddy river. Which steps make it safe to drink, and in what order?
1) Let it stand in a tank so mud settles (sedimentation). 2) Pass it through sand and gravel to remove fine bits (filtration). 3) Kill germs with chlorine, or by boiling at home (disinfection). The water is now potable, even though it still has some dissolved salts.
2. Why can't a sand filter make sea water drinkable?
Salt in sea water is dissolved: its particles are ions spread among the water molecules. They pass straight through the gaps in sand. To remove salt we need distillation (water leaves as steam, salt stays) or reverse osmosis (a membrane that lets water through but not most ions).
3. A student evaporates 50 cm³ of tap water and 50 cm³ of distilled water on two dishes. What does she see, and what does it show?
The tap water dish has a thin white solid left on it: dissolved salts. The distilled water dish is clean. This shows tap water is potable but not pure, while distilled water is pure.
Common mistakes
- Thinking clear water is safe water. Germs and dissolved salts are invisible; clear water can still make you ill.
- Saying filtration removes dissolved salt. Filters only catch solid bits; dissolved salts pass through.
- Mixing up 'potable' and 'pure'. Potable means safe to drink; it still has small amounts of dissolved substances.
- Saying boiling removes all hardness. Boiling removes only temporary hardness; permanent hardness stays.