GCSE Chemistry — Pure Substances, Potable Water & Treatment

Taught through the animal room water supply

Approx. 55 minutes · KS4 · Also usable as two halves

Before you start

Every animal at Pets on the Green drinks, swims in, or is misted with water that came out of a tap in Surrey. Almost none of them can have it straight.

This lesson is about why, and it covers a large slice of the GCSE Chemistry specification on purity, potable water and separation techniques.

There is no timer. Stop wherever you like.

What you'll be able to do by the end

• Give the chemical definition of a pure substance, and explain why it is not the everyday one.
• Explain why potable water is not pure water.
• Describe how water is treated to make it potable, and why that treatment makes it unsafe for some animals.
• Explain hard water in terms of dissolved ions, and account for limescale.
• Choose the right separation technique for a given water problem, and justify it.

Part 1 — Two meanings of "pure"

A bottle of spring water says pure on the label. In chemistry, that label is wrong.

In chemistry, a pure substance is a single element or a single compound, with nothing else in it.

Spring water is water plus dissolved calcium, magnesium, sodium, chloride, sulfate and more. It is a mixture. The label means "nothing nasty added," which is an everyday meaning, not a chemical one.

How you can tell, without a lab

A pure substance melts and boils at fixed, specific temperatures. Pure water boils at exactly 100 °C at atmospheric pressure and freezes at exactly 0 °C.

A mixture melts and boils across a range, and the presence of dissolved solute shifts those points — boiling point up, freezing point down.

Check yourself

The terrapin pool is topped up with tap water. A sample is heated and begins to boil at 100.4 °C, with the temperature continuing to rise slowly as it boils.

Is this sample pure water? Justify your answer.

Part 2 — Potable water is not pure water

Potable water is water that is safe to drink. That is a different test from chemical purity.

Potable water is allowed to contain dissolved substances. It must simply have low enough levels of dissolved salts and no harmful microbes. Tap water is a mixture, and deliberately so.

How UK tap water is made potable

1. Choose the source. Fresh water from reservoirs, rivers or aquifers — low in dissolved salts to begin with.
2. Sedimentation. Large solids settle out.
3. Filtration. Passed through filter beds of sand and gravel to remove finer insoluble solids.
4. Sterilisation. Microbes killed — by chlorine, ozone, or ultraviolet light.

Note what step 4 does. It puts something in.

Part 3 — Where this becomes an animal welfare problem

Chlorine is added to tap water precisely because it is toxic to microorganisms. That property does not switch off when the water reaches an aquarium.

Fish and aquatic amphibians. Chlorine damages gill tissue. In an enclosed tank there is no dilution and nowhere to escape to. Water for the tropical tank and the terrapins is dechlorinated before it goes anywhere near an animal.

Amphibians in general. White's tree frogs and the African bullfrog absorb water and dissolved substances directly through permeable skin. They do not have the barrier a mammal has. What is in the water goes into the animal.

Two ways to remove it. Chlorine is a dissolved gas and will leave standing water over about 24 hours. A chemical dechlorinator — usually sodium thiosulfate — does it in minutes by reducing the chlorine to chloride ions. Where a supplier uses chloramine rather than chlorine, standing will not work, because chloramine is far more stable and does not readily leave solution.

Check yourself

A keeper adds sodium thiosulfate to a bucket of tap water before filling a frog enclosure.

Explain what the sodium thiosulfate does, and why the frog is at greater risk from untreated water than a mammal would be.

Part 4 — Hard water and limescale

Surrey sits on chalk. Rainwater, which is slightly acidic from dissolved carbon dioxide, passes through it and dissolves calcium and magnesium compounds. The result is hard water — water containing dissolved calcium and magnesium ions.

When hard water is heated or evaporates, those ions come out of solution as solid calcium carbonate: limescale.

In an animal room this shows up constantly:

• White crusting on the misting nozzles that keep the chameleons hydrated — a blocked nozzle means an animal is not drinking.
• Scale on heaters and thermostats, which reduces heat transfer and drives up running costs.
• White deposits on glass where water has evaporated. The water left; the dissolved solids did not.

That last one is a demonstration of a chemical idea you will be examined on. When a solution evaporates, the solvent leaves and the solute remains. The mark on the glass is the dissolved solids, made visible.

Check yourself

Explain, in terms of dissolved ions, why a misting system in Surrey blocks more often than the same system in a soft-water area.

Part 5 — Getting to actually pure water

Sometimes potable is not enough. Reptile misting systems and some aquarium setups need water with the dissolved solids removed, or the scale problem never stops.

Distillation. Boil the water, condense the vapour, collect the distillate. The dissolved solids have much higher boiling points and stay behind. Produces very pure water. Enormously energy-expensive, which is why it is used at small scale and not for a public supply.

Reverse osmosis. Force water through a partially permeable membrane under pressure. The membrane passes water molecules and holds back dissolved ions. Cheaper to run than distillation, and it is the method used to desalinate seawater.

Why not use these for tap water? Cost and energy. Treating an entire city's supply this way would be indefensible when filtration and sterilisation produce water that is already safe to drink.

Choosing the right method

Part 6 — Exam-style questions

1. A student says bottled spring water is pure. Explain why a chemist would disagree. (2 marks)

2. Describe the stages used to treat fresh water to make it potable, and state the purpose of each. (4 marks)

3. Tap water is safe for humans but must be treated before use in a tropical fish tank. Explain why. (3 marks)

4. A sample of water from an animal enclosure is suspected to contain dissolved solids. Describe a practical method to determine whether it does, and how you would use the result. (4 marks)

5. Explain why distillation is used to purify water in a laboratory but not to supply water to a city. (3 marks)

If you want to go further

• Look up your own water company's supply report and find whether they use chlorine or chloramine. The husbandry advice changes depending on the answer.
• Related lessons: Separating Mixtures · Rates of Reaction · Energy Changes.
• The care guides for the terrapins, the White's tree frogs and the panther chameleons each show this chemistry as a daily task rather than a topic.

Notes for the adult

Curriculum: GCSE Chemistry — pure substances, formulations, potable water, and separation techniques including filtration, evaporation, distillation and reverse osmosis. Overlaps with the required practical on the analysis and purification of water samples.

The misconception this lesson targets: that "pure" and "safe" mean the same thing, and that filtration removes dissolved substances. Both are addressed directly rather than left to correct themselves.

Running it in halves: Parts 1–3 stand alone as a session on purity and potable water. Parts 4–6 cover hardness and separation and assume the first half has been done.

A note on the chloramine point. It is beyond the specification and is included because it is true and because it is the kind of detail that makes a student trust the material. It is flagged as extension credit in question 3 rather than required.

Access: read-aloud, dyslexia-friendly font, enlarged text, high contrast and calm mode on every page, changeable mid-lesson. Answer sections collapse by default and can be set to stay open.

No timing, no scoring. Nothing here is marked, logged or reported.

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