Water and Water Treatment

Two different questions hide inside the water section of Chemistry of the Environment, and conflating them is the classic error: a chemical test shows water is present; only a boiling-point or melting-point measurement shows it is pure. Examiners set the trap deliberately, and the colour-change tests plus the four treatment stages supply quick marks on Papers 1–4 and Paper 6 alike.

Testing for the presence of water

Two reagents, two colour changes, both directions learnable in a minute:

Test reagent	Start colour	Colour with water
Anhydrous copper(II) sulfate	White	Blue
Anhydrous cobalt(II) chloride	Blue	Pink

The copper(II) sulfate change is the hydration of the salt: CuSO4 + 5H2O → CuSO4·5H2O. The word anhydrous (without water) belongs in your answer. Hydrated copper(II) sulfate is already blue and tests nothing. Cobalt chloride is usually met as treated paper that turns from blue to pink.

A positive result means water is present in the liquid tested. Seawater, orange juice and dilute acid all turn anhydrous copper(II) sulfate blue. That is the whole point of the next section.

Testing for purity

Purity is a physical measurement, not a colour change. Pure water boils at 100 °C and melts/freezes at 0 °C at standard atmospheric pressure. A sample that boils across a range, or above 100 °C, contains dissolved impurities: dissolved solids raise the boiling point and lower the freezing point. The sharper the melting or boiling point, the purer the substance, the same purity criterion used across experimental techniques.

So the two-part exam answer: anhydrous copper(II) sulfate turning blue shows the liquid contains water; boiling at exactly 100 °C shows it is pure water. One sentence each, two marks.

Why water is treated, and the four stages

Domestic water supplies start as river, reservoir or ground water containing insoluble solids (mud, grit), dissolved substances, and microbes. The treatment sequence for 0620:

1. Sedimentation: water stands in large tanks; larger insoluble particles settle out under gravity.
2. Filtration: the water passes through gravel and sand beds, removing smaller insoluble particles.
3. Carbon beds: activated charcoal removes dissolved substances responsible for unpleasant tastes and odours.
4. Chlorination: chlorine is added to kill bacteria and other microbes, making the water safe to drink.

Keep each stage paired with its job; the mark scheme credits the pairing, not the bare list. Note what treatment does not do: it does not remove dissolved salts in general. Treated tap water is safe, but it is not pure in the chemical sense, which is why distilled water is used in laboratories.

A related two-marker: why is water needed in industry and at home? Standard credited uses include drinking and cooking, and as a solvent and coolant in industry.

Worked exam question

A student is given a colourless liquid. (a) Describe a chemical test to show the liquid contains water. Give the result. [2] (b) Describe how the student could show the liquid is pure water. [2] (c) Drinking water is treated by filtration and chlorination. State the purpose of each. [2]

Model answer: (a) Add anhydrous copper(II) sulfate (1); it turns from white to blue if water is present (1). (b) Measure the boiling point (1); pure water boils at exactly 100 °C (1). (c) Filtration removes insoluble solid particles (1); chlorination kills bacteria/microbes (1).

Mark-by-mark: (a) needs anhydrous and both colours: “copper sulfate goes blue” without the start colour is risky, and without “anhydrous” usually fails. (b) a chemical test cannot earn these marks; the scheme wants the measurement and the exact value. (c) “cleans the water” is too vague for either mark. Each stage gets its specific function.

The mistakes that cost marks

1. Using a chemical test to claim purity. The blue colour change proves presence only. Purity needs the boiling point (100 °C) or melting point (0 °C).
2. Dropping the word “anhydrous.” Hydrated copper(II) sulfate is already blue. The test only works, and only scores, with the anhydrous (white) form named.
3. Colour changes reversed. Copper(II) sulfate: white → blue. Cobalt(II) chloride: blue → pink. Swapping the pairs is a frequent Paper 1 error.
4. Treatment stages without purposes. “Sedimentation, filtration, carbon, chlorination” as a bare list leaves the function marks unclaimed. Say what each stage removes or does.

How examiners want it phrased

Student wording	Mark-scheme wording
”Copper sulfate turns blue so it’s water"	"Anhydrous copper(II) sulfate turns from white to blue, showing water is present"
"It’s pure because the test worked"	"It boils at exactly 100 °C, so it is pure water"
"Chlorine cleans the water"	"Chlorine kills bacteria/microbes in the water"
"Filtering makes it drinkable"	"Filtration removes insoluble solid particles”

The presence/purity distinction also patrols Paper 6, where planning questions ask you to design both checks. Air quality marks fall the same way (specific over vague), as the air quality and climate change page shows. If vague phrasing is what separates your mocks from your target grade, a free trial lesson will diagnose it inside an hour.

Test yourself

Answer these three before clicking. Each answer stays hidden until you do.

Q1 (2 marks). Describe the test for water using anhydrous cobalt(II) chloride. Give the result.

Show answer

• add the liquid to anhydrous cobalt(II) chloride (or cobalt chloride paper) [1] • it turns from blue to pink if water is present [1]

Q2 (2 marks). In the treatment of domestic water, state the purpose of sedimentation and the purpose of the carbon beds.

Show answer

• sedimentation: larger insoluble particles settle out (under gravity) [1] • carbon beds: remove dissolved substances responsible for unpleasant tastes and odours [1]

Q3 (2 marks). A sample of water boils between 101°C and 103°C. Explain what this shows about the sample.

Show answer

• the water is not pure / contains dissolved impurities [1] • dissolved impurities raise the boiling point and cause boiling over a range instead of at exactly 100°C [1]