Potable water is safe to drink, not chemically pure
Definition
Potable water
Water that is safe to drink, although it may still contain dissolved substances that cause no harm.
- Water is described as potable when it is safe to drink, which is not the same as being chemically pure.
- Potable water still contains dissolved substances, and at the levels present they do no harm.
- Fresh water from rivers, lakes and the ground carries insoluble particles, dissolved substances and harmful microorganisms.
- Treatment has to deal with the particles and the microorganisms before the water is safe.
- Sea water needs different treatment, because its problem is the dissolved salt rather than the particles.
Key Idea
- Potable is not the same as pure, because harmless dissolved substances are allowed to remain.
- The treatment matches the problem, so the source of the water decides the method.
Fresh water is treated in three stages
Definition
Sedimentation
A stage of water treatment in which suspended insoluble particles are left to settle out under gravity.
Definition
Chlorination
A stage of water treatment in which chlorine is added to kill harmful microorganisms.
- In sedimentation the water is left to stand so that suspended insoluble particles sink under gravity.
- The settled sludge is drawn off, leaving far fewer solids for the next stage to handle.
- Filtration then passes the water through beds of sand and gravel, which trap the insoluble particles that remain.
- Dissolved substances pass straight through the filter beds along with the water.
- In chlorination, chlorine is added to kill the harmful microorganisms that filtration leaves behind.
- The three stages run in that order because each one prepares the water for the next.
Common Mistake
- Filtration does not remove dissolved salts, because dissolved particles pass through with the water.
- Chlorine kills microorganisms rather than filtering them out, which is why it comes last.
Sea water is made potable by distillation
Definition
Desalination
The removal of dissolved salts from sea water, usually by distillation, to make it potable.
- Sea water holds so much dissolved salt that filtration and chlorination cannot make it drinkable.
- In distillation the sea water is heated until the water turns to vapour.
- The dissolved salts do not vaporise, so they stay behind in the original container.
- The vapour is cooled in a condenser and collected as liquid water carrying no dissolved salt.
- Removing the salt from sea water this way is called desalination.
- Boiling that much water takes a great deal of energy, so desalination is expensive and is used mainly where fresh water is scarce.
Example
- Where it is used: countries with very little fresh water rely on desalination for their drinking water.
- Why it is a last resort: heating sea water costs far more energy than treating fresh water does.
Water for chemical analysis must contain no dissolved salts
- Chemical analysis tests a sample to find out which substances it contains.
- The water used in analysis must have no dissolved salts in it at all.
- Dissolved salts separate into ions, and those ions can react with the chemicals used in the test.
- A test would then respond to something that came from the water rather than from the sample.
- Tap water contains chloride ions, so it would give a white precipitate with acidified silver nitrate even when the sample contained none.
- Distilled water is used instead, because distillation leaves every dissolved salt behind.
Self review
- What does potable mean, and why is potable water not chemically pure?
- What does sedimentation remove from the water?
- Why is chlorine added after filtration rather than before it?
- How does distillation make sea water potable?
- Why must water used for chemical analysis contain no dissolved salts?
