How do you do the water purification required practical?

What does the water purification practical investigate?

How to analyse water samples from different sources, and how to purify water by distillation. You measure the pH of each sample, find the mass of solid dissolved in it, then distil a salty sample and test the water you collect.

It is a required practical on both GCSE Chemistry and Combined Science: Trilogy.

Behind it is the difference between pure water and potable water. Pure water contains nothing but water. Potable water is water that is safe to drink: it has low levels of dissolved salts and microbes, but it still contains dissolved substances, so it is not pure in the chemical sense.

What equipment do you need?

For the pH you need universal indicator paper or a pH probe, and for the dissolved solids an evaporating basin and a balance. The distillation needs a conical flask with a bung and delivery tube, and a Bunsen burner does the heating for both.

• Water samples to test, such as tap water, bottled water and salty water
• Universal indicator paper and its colour chart, or a pH probe
• An evaporating basin and a balance that reads to 0.01 g
• A 10 cm³ measuring cylinder
• A Bunsen burner, tripod, gauze and heatproof mat
• A conical flask with a bung and delivery tube, plus a few anti-bumping granules
• A boiling tube standing in a beaker of ice and water
• A clamp and stand, and eye protection

What is the method?

Test the pH of every sample, then evaporate a measured volume of each to find the mass of dissolved solids. Finally, distil the salty sample and test the water you collect.

• pH: dip a clean glass rod in the sample and touch it on universal indicator paper, then match the colour to the chart. A pH probe gives a reading directly.
• Dissolved solids: weigh a clean, dry evaporating basin and record the mass to two decimal places.
• Add 10 cm³ of the water sample and heat it gently. When solid starts to appear and only a little water is left, turn the heat right down so the last of it evaporates without the solid spitting out.
• Let the basin cool, weigh it again and subtract to find the mass of solid.
• Distillation: put some salty water and a few anti-bumping granules in the conical flask, and fit the bung and delivery tube.
• Lead the delivery tube into a boiling tube standing in ice and water, with its end well above the bottom of the tube.
• Heat until the water boils, then turn the heat down so it boils gently. Collect about 1 cm depth of water, then stop heating.
• Test the water you collected: measure its boiling point, which is 100 °C for pure water, and check its pH.

What are the variables?

You change the water sample and measure its pH and the mass of dissolved solid it contains. For a fair comparison, every sample is tested in exactly the same way.

How do you work out and present the results?

Mass of dissolved solids = mass of basin and solid − mass of empty basin. To compare samples fairly, scale it up to the mass in 1 dm³ (1000 cm³) of water.

Worked example: the empty basin weighs 45.20 g. After 10 cm³ of salty water has evaporated, the basin and solid weigh 45.55 g. Mass of dissolved solids = 45.55 − 45.20 = 0.35 g in 10 cm³. In 1000 cm³ there would be 0.35 × (1000 ÷ 10) = 35 g, so the sample holds 35 g of dissolved solids per dm³.

Present the results in a table with columns for the sample, its pH and the mass of dissolved solids. To compare the sources, draw a bar chart rather than a line graph, because the sources are separate categories, not points on a number scale.

For the distilled water, the question is whether it is pure. A pure substance boils at one fixed temperature, so pure water boils at 100 °C, while a salt solution boils at a higher temperature. Pure water also has a pH of 7 and leaves no solid behind when it evaporates.

How do you make it accurate and safe?

Weigh the basin only once it has cooled, on the same balance each time. To be sure the solid is completely dry, heat it gently again, let it cool and reweigh: once the mass stops changing, all the water has gone. A pH probe gives a more precise reading than indicator paper, which you have to match to a colour chart by eye.

For the distillation, anti-bumping granules make the water boil smoothly instead of in sudden bursts. Never let the flask boil dry, and take the heat away if the liquid looks like it will boil up into the delivery tube. Keep the end of the delivery tube above the water collecting in the boiling tube: if it dips in, cold water can be sucked back into the hot flask when you stop heating.

Wear eye protection. The flask, tubes and basin stay hot for a while, so let them cool before you handle them. And do not drink the water you make: lab glassware is not clean enough for drinking water.

What do exam questions ask about it?

Questions test the method (how to measure the dissolved solids, and how to show the distilled water is pure) and the ideas behind it. The specification asks you to know the difference between pure and potable water, so expect that too.

You may also be asked how drinking water is made. In the UK, most comes from fresh water, which is passed through filter beds and then sterilised with chlorine, ozone or ultraviolet light. Where fresh water is scarce, sea water can have its salt removed (desalination) by distillation or by reverse osmosis, but both need large amounts of energy. For example:

"A student distils a sample of sea water. Describe two ways the student could show that the water collected is pure. (2 marks)"

Measure its boiling point, which should be 100 °C, and evaporate some of it to show that no solid is left behind.

Common mistakes

Check your method and your answers against these:

• Saying potable water is pure. It is safe to drink, but it still contains dissolved substances.
• Weighing the basin while it is still hot. Let it cool first, or the reading will not be reliable.
• Drawing a line graph to compare water sources. The sources are categories, so use a bar chart.
• Letting the delivery tube dip into the distilled water. Cold water can be sucked back into the hot flask when heating stops.
• Forgetting to scale up. A mass from 10 cm³ has to be multiplied by 100 to give the mass per dm³.