💧 Testing pH
Put a small amount of each sample in a clean test tube or spotting tile. Add universal indicator and match the colour to the pH chart, or use a pH probe and read the number. Rinse the probe with distilled water between samples.
Earth's Resources and Water » Required Practical: Water Purification
What you'll learn this session
Study time: 30 minutes
AQA spec: 8.2.8 (Required practical 8)
- How to test the pH and the dissolved solids in water samples
- How to purify a water sample by distillation
- How to heat safely and collect representative samples
- The mistakes examiners see, and how to improve the method
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Unlock This CourseThe aim of the practical
In this practical you analyse water samples from different sources, then purify one of them. You will use three skills: measuring pH, finding out how much is dissolved in the water, and distilling.
Your samples might be rain water, water from a pond, and salt water. Each one has different things in it, so each gives different results.
Key terms:
- Representative sample: a sample that is a fair picture of all the water it came from.
- Dissolved solids: solid substances, such as salts, mixed into the water so that you cannot see them.
- Distillate: the pure liquid that is collected after distillation.
Getting a good sample
A sample is only useful if it is representative. If you scoop water from the very edge of a pond, you might collect extra mud or leaves. Take water from more than one place, and label each bottle clearly with where it came from.
Part 1: Analysing the samples
Universal indicator turns colour to show each water sample's pH - green means neutral, around pH 7
⚖ Dissolved solids
Find out how much solid is dissolved in a measured volume of each sample. The method is below.
Method: finding the dissolved solids
- Wear eye protection.
- Weigh a clean, dry evaporating basin and write the mass down.
- Measure out 20 cm3 of the water sample and pour it into the basin.
- Heat the basin gently over a beaker of boiling water (a water bath) or with an electric heater. A Bunsen burner can be used: stand the basin on a tripod and gauze, use the blue flame to heat gently, and switch back to the yellow safety flame when you are not heating.
- Stop heating when the water has gone and a solid is left. Let the basin cool.
- Weigh the basin and the solid. Repeat with each sample.
The mass of the solid is the final mass minus the mass of the empty basin.
Compare your results. Pure water has a pH of 7. A sample with a pH below 7 is acidic and above 7 is alkaline. A sample with more dissolved solid has more dissolved in it.
Worked example
An empty basin has a mass of 42.10 g. After evaporating 20 cm3 of sea water, the basin and solid have a mass of 42.80 g.
Mass of dissolved solid = 42.80 - 42.10 = 0.70 g
These numbers are an example. Rain water would be expected to leave far less solid than sea water.
Keep the volume the same for every sample. This is the control variable, so the results are a fair test.
Part 2: Purifying water by distillation
Steam rises off the salty water and cools in the condenser, leaving the dissolved salt behind in the flask
Water with dissolved solids can be purified by distillation. The water boils and turns to steam. The dissolved solids do not boil off, so they stay behind. The steam is cooled and turns back into liquid water.
Method: distillation
- Wear eye protection.
- Pour the salt water sample into a flask. Add a few anti-bumping granules so it boils smoothly.
- Fit a bung and a delivery tube to the flask. Join it to a condenser.
- Connect cold water to the condenser. It goes in at the bottom and out at the top, so the jacket stays full of cold water.
- Put a clean beaker at the end of the condenser to collect the distillate. You can stand it in cold water.
- Heat the flask. The water boils, the steam passes into the condenser, cools and drips into the beaker.
- Stop heating before the flask boils dry.
Then test the distillate. Evaporate a small amount in a clean basin: if it is pure, no solid is left behind. You can also check its pH, which should be close to 7.
Safety
Heat
Hot glass looks like cold glass. Let equipment cool before you touch it. Tie back long hair and turn a Bunsen burner to the yellow flame when it is not in use.
Eyes
Wear eye protection. Hot water can spit out of the basin if it is heated too hard.
Drinking
Never taste or drink any sample or the distillate. Lab glassware is not clean enough to drink from.
Common mistakes
Heating too strongly, so the solid spits out of the basin and the mass is too low. Weighing the basin while it is still hot, which gives a wrong reading. Using different volumes for different samples. Letting the flask boil dry. Putting the condenser water in at the top, so it does not fill the jacket.
Evaluating the method
You could improve the results by repeating each measurement and finding a mean, heating until the mass stops changing, and collecting samples from more than one spot. A pH probe is more precise than universal indicator colours.
Exam-style question
A student has a sample of sea water. Describe how the student can get pure water from the sample, and how to show that the water is pure. (4 marks)
Model answer
Heat the sea water in a flask so that it boils. The steam passes into a condenser, where it cools and turns back into liquid water. Collect the liquid in a beaker. To show it is pure, evaporate a small amount of the collected water. No solid is left behind. (You could also say that pure water boils at 100 °C at normal pressure.)
Exam tip
When asked about distillation, name the two changes of state: boiling (evaporating) then condensing. Many students forget to say that the solids stay behind in the flask.