AQA Chemistry 5.10.1.2–5.10.1.3
Potable water and water treatment
Safe to drink is not the same as pure. Analyse three water samples, work out how much is dissolved in them, then follow fresh water, sea water and sewage to the tap or the river.
Triple Science · Foundation tier
Start here
White crust in a kettle.
In many homes, a white crust builds up inside the kettle after weeks of boiling tap water. Nobody put anything in except water from the tap.
If you had to guess, is tap water just water, or water with things dissolved in it?
Potable, not pure
Safe to drink still has things dissolved in it.
Potable water is water that is safe to drink. It is not pure in the chemical sense, because it contains dissolved substances. Pure water is water only. In the UK, rain provides fresh water, with low levels of dissolved substances, that collects in the ground and in lakes and rivers. Tap each step to see why it is done.
Salty water: desalination
Where fresh water is scarce, sea water can be made potable by distillation or by reverse osmosis, which uses membranes. Dissolved salt passes straight through a filter, so filter beds can’t remove it. Both methods need large amounts of energy.
Required practical · Combined RP13 · Chemistry RP8
Analyse and purify three water samples.
Analysis and purification of water samples from different sources, including pH, dissolved solids and distillation. Pick a sample and run each step.
Measure the pH
Universal indicator, or a pH probe.
Dissolved solids
Weigh an empty basin. Evaporate a measured 50 cm³ to dryness. Reweigh.
Purify by distillation
Heat; the water boils off, is condensed and collected.
Equation · concentration of dissolved solids
Tap the quantity you want to find. Volume must be in dm3: divide cm3 by 1000.
Equation
Concentration of dissolved solids
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
concentration = mass ÷ volume
- m
- mass of solid
- g
- c
- concentration
- g/dm³
- V
- volume
- dm³
Waste water · sewage treatment
Two streams, two kinds of bacteria.
Sewage and agricultural waste water must have organic matter and harmful microbes removed before it is put back into the environment. Industrial waste water may also need harmful chemicals removed.
Ground water is usually the easiest to make potable: filter and sterilise. Waste water needs many more stages. Salt water needs desalination, which uses large amounts of energy.
Think again · filter out the salt?
“You can filter the salt out of sea water.”
Your call
ChooseSea water is poured through a filter bed. What comes out?
Sort
Ground, waste or salt water?
Place each treatment with the water it belongs to.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Give reasons
- Each step, then what it removes or kills.
- Calculate
- Mass by subtraction, then mass ÷ volume in dm3.
- Comment
- Compare how easy each source is, and why.
Key fact
Potable water is safe to drink but not pure: it contains dissolved substances. Most UK potable water comes from fresh water passed through filter beds and sterilised.
Key note · AQA 5.10.1.2–5.10.1.3
Potable water and water treatment
- Potable water is safe to drink. It is not pure: it contains dissolved substances.
- UK potable water: choose a source of fresh water, pass it through filter beds, then sterilise with chlorine, ozone or UV.
- Salty water: distillation or reverse osmosis. Both need large amounts of energy.
- Required practical: pH; dissolved solids by evaporating a measured volume and weighing the residue; purification by distillation.
- Concentration = mass ÷ volume, with volume in dm³ (cm³ ÷ 1000).
- Sewage: screening and grit removal; sedimentation; anaerobic digestion of sludge; aerobic biological treatment of effluent.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Not sure why volume goes in dm³?
Sample masses and pH values are typical round figures for practice, not measurements.