Section 1 of 8Pure water or potable water?
| Feature | Pure water | Potable water |
|---|---|---|
| Composition | Only H2O molecules | Water with small amounts of dissolved minerals and salts |
| Safe to drink? | Yes, but not what comes from taps | Yes: safe for people to drink |
| pH | 7 | Between about 6.5 and 8.5 |
| Microbes | None | Free of harmful bacteria and other microbes |
| Dissolved substances | None | Only in very low, regulated amounts |
Section 2 of 8Sources of fresh water
Rain
- Rain Rainwater is fresh because it contains very few dissolved substances.
Surface water
- Surface water Rain collects in lakes, rivers and reservoirs.
Groundwater
- Groundwater Water soaks into aquifers, which are porous rocks that store water underground.
- In warm, dry regions such as south-east England, surface water evaporates quickly, so groundwater is often the main source.
Section 3 of 8Treating fresh water
1
Choose a suitable source of fresh water, depending on local climate and geology.
2
Pass the water through a wire mesh screen to remove twigs, stones and other large debris.
3
Allow solids to settle (sedimentation), then filter through beds of sand and gravel to remove finer particles.
4
Sterilise by adding chlorine, or by using ozone or ultraviolet light, to kill bacteria and other microbes.
Section 4 of 8When fresh water is scarce: desalination
Desalination of seawater
Distillation
- Distillation Seawater is boiled, the steam is condensed and the salts are left behind.
Reverse osmosis
- Reverse osmosis Seawater is forced through a semi-permeable membrane.
- Water molecules pass through but the larger ions and molecules are held back.
Drawback
- Drawback Both methods need large amounts of energy, so desalination is expensive and unsuitable for producing very large volumes.
Where it is used
- Where it is used Hot, dry countries with little fresh water, such as Saudi Arabia.
Section 5 of 8Comparing the treatment stages
| Stage | What it removes | Why it is needed |
|---|---|---|
| Screening | Large debris such as twigs and stones | Protects pumps and later stages from damage |
| Sedimentation | Heavier suspended particles, which sink | Reduces the load on the filter beds |
| Filtration | Fine solid particles | Leaves the water clear |
| Sterilisation | Bacteria and other microbes | Prevents diseases such as cholera and typhoid |
Section 6 of 8Testing a water sample
1
Use a pH meter or universal indicator to check that the pH lies between about 6.5 and 8.5.
2
Evaporate a measured volume of water on a dish to find the mass of dissolved solids left behind.
3
Heat a second sample: pure water boils at exactly 100 °C, while salt water boils above this temperature.
4
Conclude that water which passes these checks may be potable, but it must still be free of microbes to be safe.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Potable water | Water that is safe to drink; it may contain dissolved substances. |
| Aquifer | A layer of porous rock that holds groundwater. |
| Sterilise | To kill microbes, e.g. with chlorine, ozone or ultraviolet light. |
| Desalination | Removing salt from seawater to make fresh water. |
| Reverse osmosis | Forcing water through a semi-permeable membrane to separate it from dissolved ions. |
Section 8 of 8Exam technique
1
Writing that potable water is 'pure'. In chemistry pure means a single substance; potable water contains dissolved minerals.
2
Saying that filtering removes microbes. Sterilising is needed to kill bacteria.
3
Forgetting that the choice of treatment depends on the local water supply and geology.
4
Claiming desalination is cheap. It uses large amounts of energy.