Question 1 (1 marks)
Knowledge
Which process can separate water from many dissolved salts?
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Simple distillation (1).
Required Practical 13: Water Purification | AQA GCSE Science | The Science Department
About this practical and its aim
Aim
Analyse water samples and use simple distillation to obtain purified water.
Introduction
In these experiments, you will have samples of water which you will test the pH and compare with drinking water. You will then evaporate some of the samples of water to see if there are any solid crystals left behind showing that there were dissolved solids. Finally, you get to carefully purify a sample through distillation.
The science behind the practical
Dissolved substances remain when water is evaporated; distillation boils and condenses water, leaving many dissolved solids behind.
Key things to remember
Test pH; compare residue after evaporation; set up safe simple distillation and collect condensate.
Equipment and apparatus
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Water samples
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pH paper or meter
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evaporating basin
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heating apparatus
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simple distillation flask
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condenser
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receiver
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thermometer
Method: How to carry out the experiment
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Method 1: Analysis of water samples
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Using universal indicator, test and record the pH of all of the available water samples.
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Using a measuring cylinder, accurately measure out 25 cm3 of the first sample.
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Weigh an evaporating basin and record the mass
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Add the water sample then heat with a Bunsen. It is important to stop heating while there is still a small amount of water left.
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Leave the evaporating basin to cool and the rest of the water to evaporate.
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Reweigh the basin, record the new mass and calculate the mass of dissolved solids in the sample.
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Method 2: Purifying a sample of water using distillation
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Carefully setup the distillation equipment as shown by your teacher
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Before heating the impure water, check that all of the glassware is stable.
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Heat the water until it is boiling and collect at least 1 cm of water in the collection tube.
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Stop heating the sample and allow it to cool.
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Gently clamp the boiling tube with your pure water at a slight angle and add a thermometer
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Heat the sample until it boils, if it is pure water, its boiling point should be exactly 100°C
Variables and fair testing
| Type of variable | What is measured or controlled? |
|---|---|
| Independent variable | Water sample or purification method |
| Dependent variable | pH, residue and collected distillate properties |
| Control variables | Starting sample volume; heating conditions; apparatus cleanliness |
Safety and managing risks
Follow your teacher's instructions and your school's risk assessment. This page is not a substitute for a formal risk assessment.
| Hazard | Risk | Precaution |
|---|---|---|
Hot distillation apparatus |
Burns and pressure hazards |
Never seal heated apparatus; use safe condenser setup and allow glass to cool. |
Water sample contamination |
Exposure to unknown sample constituents |
Use teacher-approved water samples; do not taste samples or condensate. |
Results, calculations and analysis
Recording, presenting and interpreting results
Record pH, appearance, residue and distilled-water observations in a results table.
Calculations
Possible extension: mass of residue per sample volume, with consistent units.
Conclusions, accuracy and evaluation
Drawing a conclusion
Distilled water usually contains fewer dissolved solids than the starting sample; not all impurities are removed by every treatment.
Evaluating the investigation
Avoid contamination; compare equal sample volumes; do not distil to dryness.
More to learn
What distillation removes
Simple distillation separates water from many dissolved solids. It does not guarantee removal of every volatile impurity.
Common exam mistake
The condenser must have a continuous cooling-water flow where a water-cooled condenser is used.
GCSE examination practice
Answer each question in full before opening the model answer. Check your work against the marking points and look for scientific key words.
Question 2 (2 marks)
Method
What is the purpose of the condenser?
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To cool the vapour (1) and turn it into liquid distillate (1).
Question 3 (2 marks)
Analysis
Why does evaporation leave a visible residue for some water samples?
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Those samples contain dissolved non-volatile solids (1) that remain after water evaporates (1).
Question 4 (2 marks)
Evaluation
Suggest one improvement to make the investigation more reliable and explain why it helps.
Show model answer / marking points
Repeat measurements and calculate a mean (1), reducing the influence of random variation/anomalies (1). Alternative valid improvement specific to the experiment with explanation can gain both marks.
This page was updated on: 19th June 2026