GCSE · Chemistry · AQA · Spec 8462

Potable water

Turn on a tap and you get water that's safe to drink. But it isn't pure. And sterilising never took the bacteria out. It killed them.

Potable water · Follow the route

What water have you got?

Start at the top. Pick the supply you've got, then follow the route it decides, one step at a time, until you reach water that's safe to drink.

There isn't one single recipe. The method used to make potable water depends on the supplies of water available and on local conditions. The kind of source largely decides what the water carries, and how much of it.

5 paths through this tree, and every one of them ends at potable water.

On We need water that is safe to drink. 2 branches to choose from.

Exam line: Name the supply first, then the route. Plenty of fresh water: choose a source, filter beds, sterilise. Fresh water limited: desalinate salty water by distillation or reverse osmosis.

Watch out: Chlorine, ozone and UV light are three options for ONE step, sterilising. They are not three separate steps done one after another.

Potable vs pure

Every path ended at potable water. So what is it, exactly?

Water of the right quality is essential for life, which is why all that treatment is worth doing. Now picture a glass of potable water, poured from a tap.

Which of these is closest to what you think about that glass of water right now?

It's pure water. If it's safe to drink, then it's pure.Treatment took out everything except the water, so it's just H₂O now.It's safe to drink, but it still has substances dissolved in it, so it isn't pure.It's potable, so there can't be a single microbe or dissolved salt in it.

How sure are you?

I'm sureI think soI'm guessing

It's pure water. If it's safe to drink, then it's pure.

This is a common misconception

This is the big one, and it comes from everyday language, where 'pure' means clean and safe. In chemistry, pure water contains only H₂O molecules. Potable water still has substances dissolved in it. So it's safe to drink, but it isn't pure.

Treatment took out everything except the water, so it's just H₂O now.

This is a common misconception

Treatment doesn't strip water down to nothing but H₂O. Potable water still contains dissolved substances. What makes it safe is that the levels of dissolved salts and microbes are low enough. Only pure water is nothing but H₂O molecules.

It's safe to drink, but it still has substances dissolved in it, so it isn't pure.

This matches the evidence

Exactly right. Potable means safe to drink: the water has sufficiently low levels of dissolved salts and microbes. It still contains dissolved substances, so it isn't pure water, which would contain only H₂O molecules.

It's potable, so there can't be a single microbe or dissolved salt in it.

This is a common misconception

Look at the actual test: drinking water should have sufficiently LOW levels of dissolved salts and microbes. The bar is 'low enough to be safe', not 'none at all'.

Potable means safe to drink. Pure means only H₂O molecules. Tap water is the first, not the second.

Exam line: In chemistry, pure water means only H₂O molecules. In everyday talk 'pure' just means clean, so keep the everyday meaning out of your answers.

Why each step?

Reason it through

Fresh water already has low levels of dissolved substances. So why is it filtered and then sterilised?

Link 1 of 4

First link · your turn

Rain has fed a lake with fresh water. Can it go straight from the lake to your tap?

Locked — reveal the link above first

Two sources, two very different jobs

Fresh water or ground watervsSalty water or sea water

The UK has sea all around it. So why doesn't it just desalinate sea water? Read down the rows and the answer builds up.

Focus

What the source carries

Fresh water or ground water

Only low levels of dissolved substances and of microbes

Salty water or sea water

High levels of dissolved substances

The insight

The source sets the job. Sea water is loaded with dissolved substances, so it needs a completely different route.

How it is made potable

Fresh water or ground water

Choose a source, pass it through filter beds, sterilise it

Salty water or sea water

Desalination, by distillation or by a membrane process such as reverse osmosis

Energy needed

Fresh water or ground water

Little

Salty water or sea water

Large amounts

Cost

Fresh water or ground water

Far less than distilling seawater

Salty water or sea water

Distilling seawater costs far more than treating fresh water

When it is used

Fresh water or ground water

Where fresh water is plentiful, as in the UK

Salty water or sea water

When supplies of fresh water are limited

Required practical 8

Two samples on the bench. What will you find?

In Required practical 8 you analyse water samples from different sources. You test their pH and their dissolved solids, and you purify water by distillation. Picture two samples in front of you: one from a river fed by rain, and one from the sea.

Before you test them for dissolved solids, which prediction would you back?

The sea water sample will hold far more dissolved solids than the river water sample.The river water sample will hold no dissolved solids at all, because rain gives pure water.Both samples will hold about the same, because where water comes from doesn't change what's dissolved in it.The river water sample is low in dissolved solids, so it's safe to drink without any treatment.

How sure are you?

I'm sureI think soI'm guessing

The sea water sample will hold far more dissolved solids than the river water sample.

This matches the evidence

Spot on. The source decides what the water carries. Sea water holds high levels of dissolved substances, while fresh water from rain holds only low levels. That's exactly the kind of difference the analysis is there to show.

The river water sample will hold no dissolved solids at all, because rain gives pure water.

This is a common misconception

Close, but low isn't zero. Rain gives fresh water with LOW levels of dissolved substances, not none. And pure water, in chemistry, is only H₂O molecules, which river water isn't.

Both samples will hold about the same, because where water comes from doesn't change what's dissolved in it.

This is a common misconception

It's the other way round. The kind of source largely decides what the water carries, and how much. Sea water holds high levels of dissolved substances; fresh water holds only low levels.

The river water sample is low in dissolved solids, so it's safe to drink without any treatment.

This is a common misconception

Don't let this catch you out. Even fresh water can contain insoluble material, dissolved substances and harmful microbes, so it still needs treating before it's potable.

The source decides what's dissolved in the water: sea water has high levels, fresh water has low levels, but not zero. To purify a sample, you distil it, the same method that can desalinate salty water.

Exam line: Required practical 8 is the analysis and purification of water samples from different sources: testing their pH and dissolved solids, and purifying water by distillation.

Your turn to write

Now say it without the scaffolding

Before it is piped to homes, water from a river goes through filter beds and is then sterilised. Give a reason for each of these two treatment steps. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Filter beds trap small insoluble particles. (1 mark)
This keeps turbidity low, which helps UV disinfection work effectively. (1 mark)
Sterilising kills harmful microbes. (1 mark)
So the water is safe to drink (the microbes can no longer cause illness). (1 mark)

Model answer

The filter beds trap small insoluble particles, which keeps turbidity low and helps UV disinfection work effectively. Sterilising kills harmful microbes, so the water is safe to drink.

'Kills' earns the mark. 'Removes the bacteria' describes something that doesn't happen.

Exam line: A step on its own is half an answer. Say what it does, then why that matters.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

Potable water is water that is safe to drink.
Drinking water should have sufficiently low levels of dissolved salts and microbes.
Potable water is not pure: it contains dissolved substances. Pure water contains only H₂O molecules.
The method used depends on the water supplies available and on local conditions.
UK route: choose an appropriate source of fresh water, pass it through filter beds, then sterilise it with chlorine, ozone or ultraviolet light.
If fresh water is limited, salty water or sea water may be desalinated by distillation or by reverse osmosis.

The big picture

Potable water is water that is safe to drink: it has sufficiently low levels of dissolved salts and microbes. It is not pure, because it still contains dissolved substances; pure water contains only H₂O molecules. How potable water is made depends on the supply. Where fresh water is plentiful, as in the UK, an appropriate source is chosen, the water is passed through filter beds, and it is sterilised with chlorine, ozone or ultraviolet light. Where fresh water is limited, salty water or sea water may be desalinated by distillation or reverse osmosis, which need large amounts of energy.

Key points

1Even fresh water can contain insoluble material, dissolved substances and harmful microbes, so it still needs treating.

2Filter beds have layers that get finer and finer, trapping the small insoluble particles and keeping turbidity low so that UV disinfection can work effectively.

3Sterilising kills harmful microbes such as bacteria. It does not remove them, but it stops them causing illness.

4Desalination processes need large amounts of energy. Treating fresh water needs little, so it costs far less than distilling seawater, which is why the UK does not rely on desalination.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

Two towns need drinking water. Riverton gets plenty of rain, and a large river runs past it. Saltmarsh is on a dry coast with very little fresh water, but it has the sea. Describe how potable water could be produced for each town.

⚠ Watch out

Writing that sterilising 'removes' or 'filters out' the bacteria. It doesn't. Chlorine, ozone or UV light kill the microbes, so they can no longer cause illness. Trapping small insoluble particles is the filter beds' job.

🧠

Memory hook

Tap water is SAFE, not PURE, and its bacteria were KILLED, not REMOVED. For the route, ask one question first: fresh or salty? Fresh: pick it, filter it, sterilise it. Salty: desalinate it.

✓

Check yourself

Cover the page and answer out loud: why is tap water potable but not pure? What happens to its bacteria, and why is it filtered before UV disinfection?

Flashcards

(14)

What is potable water?

Water that is safe to drink.

Why is potable water not pure water?

It still contains dissolved substances. Pure water contains only H₂O molecules.

Drinking water must have sufficiently low levels of which two things?

Dissolved salts and microbes.

What decides the method used to produce potable water?

The supplies of water available and local conditions.

In the UK, where does fresh water come from, and where does it collect?

From rain. It collects in the ground and in lakes and rivers.

Why does even fresh water still need treating?

It can contain insoluble material (leaves, grit, silt), dissolved substances and harmful microbes.

The three steps for producing potable water in the UK, in order?

Choose an appropriate source of fresh water, pass it through filter beds, sterilise it.

Name three sterilising agents used for drinking water.

Chlorine, ozone and ultraviolet light.

How do filter beds trap the smallest particles?

The water passes through layers that get finer and finer, which trap the very small insoluble particles.

Why is water filtered before UV disinfection?

Filtering keeps turbidity low, which helps UV disinfection work effectively.

Does sterilising take the bacteria out of the water?

No. It kills them, so they can no longer cause illness.

When may desalination be needed?

When supplies of fresh water are limited, so salty water or sea water has to be used.

Name two ways of desalinating water.

Distillation, and processes that use membranes such as reverse osmosis.

Why doesn't the UK rely on desalination?

It has plenty of fresh water, which needs little energy to treat and costs far less than distilling seawater.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

What water have you got?

Every path ended at potable water. So what is it, exactly?

Two samples on the bench. What will you find?

Now say it without the scaffolding

More AQA GCSE Chemistry topics