Required Practical: Water Purification

This lesson covers the AQA GCSE Chemistry Required Practical on water purification. You need to know how to carry out an analysis of water samples and produce a sample of pure water by distillation. Understanding the apparatus, method, expected results and sources of error is essential for the exam, as questions on required practicals appear frequently.

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Aim of the Practical

The aim of this required practical is to:

1. Analyse a sample of water to determine what it contains
2. Purify a sample of water using distillation to produce a sample that is close to chemically pure

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Apparatus and Materials

You need to know the apparatus used in this practical:

Apparatus / Material	Purpose
Water sample (e.g. seawater, tap water, or river water)	The sample to be tested and purified
Bunsen burner, tripod, gauze and heatproof mat	To heat the water sample to boiling
Round-bottomed flask or conical flask	To hold the water sample during heating
Delivery tube	To carry the steam from the flask to the condenser
Liebig condenser	To cool the steam back into liquid water
Rubber tubing connected to cold water supply	To circulate cold water around the condenser
Collecting beaker or conical flask	To collect the purified (distilled) water
Thermometer	To monitor the temperature during distillation
Anti-bumping granules (boiling chips)	To ensure smooth, even boiling and prevent sudden violent boiling
pH paper or pH meter	To test the pH of the water before and after distillation
Silver nitrate solution (AgNO3)	To test for chloride ions in the water
Evaporating basin	To evaporate a sample and observe dissolved solids

graph LR
    A["Water sample<br>in flask"] -->|Heat with<br>Bunsen burner| B[Steam produced]
    B -->|Passes through<br>delivery tube| C["Liebig Condenser<br>Cold water flows<br>around outside"]
    C -->|Steam condenses<br>back to liquid| D["Pure water<br>collected in beaker"]
    style A fill:#bbdefb,stroke:#1565c0
    style B fill:#fff9c4,stroke:#f57f17
    style C fill:#e1bee7,stroke:#6a1b9a
    style D fill:#c8e6c9,stroke:#2e7d32

Exam Tip: You must be able to draw and label the distillation apparatus from memory. Practise sketching the setup: flask with water, delivery tube leading to Liebig condenser (with cold water flowing in at the bottom and out at the top), and a collecting vessel at the end.

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Method: Testing the Water Sample

Before distillation, you should test the original water sample to identify what is dissolved in it.

Step-by-step Testing Method

1. Measure the pH of the water sample using pH paper or a pH meter. Record the value.
2. Test for chloride ions: Add a few drops of dilute nitric acid followed by a few drops of silver nitrate solution to a small sample of the water. A white precipitate of silver chloride indicates the presence of chloride ions.
3. Test for dissolved solids: Place a small volume of water in an evaporating basin and heat gently until all the water has evaporated. Any solid residue left behind shows dissolved solids were present. Weigh the residue to find the mass of dissolved solids.

Test	Reagent Used	Positive Result	What It Shows
pH test	pH paper / pH meter	Value recorded	Whether water is acidic, neutral or alkaline
Chloride ion test	Dilute HNO3 then AgNO3 solution	White precipitate	Chloride ions (Cl-) are present
Dissolved solids	Evaporating basin and heat	White/coloured residue	Dissolved salts were present

Exam Tip: Always add dilute nitric acid before the silver nitrate solution when testing for chloride ions. The nitric acid removes any carbonate ions that might otherwise give a false positive (a white precipitate of silver carbonate).

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Method: Distillation

Step-by-step Distillation Method

1. Pour the water sample into a round-bottomed flask (fill no more than one-third full).
2. Add a few anti-bumping granules to the flask to ensure smooth boiling.
3. Set up the Liebig condenser at a slight downward angle, connected to the flask via a delivery tube.
4. Connect rubber tubing to the condenser so that cold water flows in at the bottom and out at the top (counter-current flow for maximum cooling efficiency).
5. Place a collecting beaker at the open end of the condenser.
6. Place a thermometer in the flask (the bulb should be level with the side arm / delivery tube opening).
7. Heat the water sample using a Bunsen burner until it boils.
8. Observe the temperature — it should remain steady at approximately 100 degrees C while pure water is distilling over.
9. Continue heating until a sufficient volume of distilled water has been collected.
10. Turn off the Bunsen burner and allow the apparatus to cool before dismantling.

Safety Precautions

Hazard	Risk	Precaution
Boiling water and steam	Burns and scalds	Wear safety goggles; keep hands away from steam; do not point the apparatus at anyone
Bunsen burner	Burns	Use a heatproof mat; tie back long hair; keep flammable materials away
Glassware	Breakage and cuts	Check for cracks before use; handle hot glassware with tongs or allow to cool
Silver nitrate solution	Stains skin and is irritant	Wear gloves; wash hands after use

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Expected Results

Testing the Original Sample

Test	Seawater	Tap Water	Distilled Water (control)
pH	Approximately 8 (slightly alkaline)	6.5 - 8.5 (varies by region)	7 (neutral)
Chloride test (AgNO3)	Heavy white precipitate	Slight white precipitate	No precipitate
Dissolved solids	Large amount of white residue (mainly NaCl)	Small amount of residue	No residue

Testing the Distilled Product

After distillation, the collected water should:

• Have a pH close to 7 (neutral)
• Show no precipitate with silver nitrate solution (chloride ions removed)
• Leave no residue when evaporated (dissolved solids removed)
• Boil at very close to 100 degrees C (confirming near-chemical purity)

Exam Tip: If asked about the purity of the distilled water, state that it should boil at exactly 100 degrees C. If the boiling point is not exactly 100 degrees C, this suggests trace impurities are still present. A sharp, fixed boiling point is evidence of a pure substance.

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Analysing the Results

Observation	Explanation
Temperature stays steady at 100 degrees C during distillation	Only pure water is evaporating and condensing; dissolved salts remain in the flask
White precipitate with AgNO3 in the original sample but not in the distillate	Chloride ions were present in the original water but were not carried over during distillation
Residue in evaporating basin from original sample	Dissolved solids (salts) were present in the original water
No residue from distillate	Distillation successfully separated pure water from dissolved solids

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Sources of Error