14.7 Required Practicals: The Earth's Resources

A free GCSE Chemistry lesson for AQA (8462) from OBA School: aQA required practical: analysis and purification of water samples (pH, dissolved solids and distillation). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Plan and carry out tests to find the pH and mass of dissolved solids in water samples from different sources
• Purify a water sample by distillation and test the distillate
• Identify independent, dependent and control variables and write a risk assessment
• Process results, including concentration of dissolved solids in g/dm³, means and anomalies
• Evaluate the method, identify sources of error and suggest improvements

Key words

• Dissolved solids: Substances (mainly salts) dissolved in the water, left behind as a solid when the water evaporates.
• Distillate: The liquid collected after distillation.
• Constant mass: Heating and reweighing until the mass stops changing, which shows all the water has gone.
• Resolution: The smallest change a measuring instrument can show, e.g. 0.01 g on a balance.
• Anomaly: A result that does not fit the pattern of the others.

Aim and equipment

Aim: to analyse water from different sources (for example tap water, rain water, river water and sea water) by measuring pH and the mass of dissolved solids, and to purify a sample by distillation.

Collecting representative samples: use clean containers rinsed with the water being sampled; take several samples from different places (and depths or times) at each source and mix them or test each; label every container with the source, place and time; seal and test them soon after collecting.

Equipment: water samples; universal indicator paper and colour chart, or a pH meter; evaporating basin; balance (resolution 0.01 g); measuring cylinder; Bunsen burner, tripod and gauze (or water bath); distillation apparatus (round-bottomed flask, thermometer, condenser, beaker); anti-bumping granules; eye protection.

Method

A. pH

• 1 Dip a glass rod into the sample and touch it on universal indicator paper. Compare the colour with the chart and record the pH.
• 2 Or measure with a calibrated pH meter, which gives a more precise value (e.g. to 0.1).

B. Mass of dissolved solids

• 3 Weigh a clean, dry evaporating basin.
• 4 Use a measuring cylinder to add a known volume of the sample, e.g. 50.0 cm³.
• 5 Heat gently until most of the water has evaporated, then leave to dry (or use a water bath to stop spitting).
• 6 Let the basin cool and reweigh it. Reheat and reweigh until the mass is constant.
• 7 Mass of dissolved solids = final mass − mass of empty basin.

C. Distillation

• 8 Put about 100 cm³ of the salty sample in a round-bottomed flask with a few anti-bumping granules.
• 9 Turn on the cold water through the condenser (in at the bottom, out at the top).
• 10 Heat the flask. Water boils, the steam condenses in the condenser and the distillate is collected. The thermometer should read 100 °C.
• 11 Test the distillate: pH should be 7; evaporating a sample leaves no residue; it boils at 100 °C.
• 12 To find its boiling point, heat some distillate in a boiling tube with an anti-bumping granule and read a thermometer held in the vapour just above the liquid. Pure water boils at 100 °C (at normal air pressure); dissolved solids raise the boiling point.

Exam tip: Anhydrous copper(II) sulfate turning from white to blue, or cobalt chloride paper turning from blue to pink, only shows that water is present. Salty water gives exactly the same result, so these tests cannot show that water is pure. Use the boiling point (100 °C) instead.

Variables and risk assessment

• Independent; In this practical the source of the water sample
• Dependent; In this practical pH; mass (or concentration) of dissolved solids
• Control; In this practical volume of each sample; same balance and basin; heat until constant mass; same pH method

• Hot apparatus; Risk burns; Control measure let it cool before touching; use tongs
• Spitting when evaporating; Risk hot solid in eyes; Control measure wear eye protection; heat gently; stop before fully dry
• Sudden boiling in flask; Risk hot liquid sprayed; Control measure add anti-bumping granules
• Pressure build-up; Risk apparatus breaks; Control measure never heat a sealed system; keep condenser outlet open
• Microbes in river or pond water; Risk infection; Control measure do not drink samples; wash hands; cover cuts

Processing results

• Sea water; Volume (cm³) 50.0; Empty basin (g) 32.46; Basin + solid (g) 34.21; Mass of solid (g) 1.75
• Tap water; Volume (cm³) 100.0; Empty basin (g) 32.46; Basin + solid (g) 32.49; Mass of solid (g) 0.03

Worked example: concentration of dissolved solids

Sea water: mass = 34.21 − 32.46 = 1.75 g in 50.0 cm³. Volume in dm³ = 50.0 ÷ 1000 = 0.0500 dm³
concentration = mass ÷ volume = 1.75 ÷ 0.0500 = 35.0 g/dm³
Tap water: 0.03 ÷ 0.100 = 0.3 g/dm³

Answer: sea water 35.0 g/dm³; tap water 0.3 g/dm³

Precision: The tap water mass (0.03 g) is only 3 times the balance resolution (0.01 g), so the result is not precise. Using a larger volume of water gives a bigger mass and a smaller percentage uncertainty.

Sources of error and improvements

• solid spits out of the basin; Effect on result mass of solid too low; Improvement heat gently / use a water bath; cover with a watch glass
• solid not completely dry; Effect on result mass of solid too high; Improvement heat to constant mass
• basin weighed while hot; Effect on result reading unreliable (convection currents); Improvement let it cool first, ideally in a desiccator
• indicator colour is hard to judge; Effect on result pH only to the nearest whole number; Improvement use a calibrated pH meter
• mass of solid very small; Effect on result large percentage uncertainty; Improvement use a larger volume of sample

Common mistakes

• Saying distilled water is safe because it is “clean”. Distillation removes dissolved solids; you test purity by pH 7, boiling point 100 °C and no residue.
• Forgetting to convert cm³ to dm³ (÷ 1000) when calculating concentration in g/dm³.
• Giving “water” as the independent variable. It is the source of the water sample.
• Weighing the basin only once at the end. You must weigh it empty and after evaporation, and heat to constant mass.
• Including an anomalous result when calculating a mean.
• Using anhydrous copper(II) sulfate to “prove” water is pure. It only shows that water is present.

Revision questions

Try these, then check your answers below each one.

Question 1. In this practical, what is the independent variable? (Choose from: The source of the water sample, The mass of dissolved solids, The volume of each sample, The mass of the empty basin.)

Answer: The source of the water sample

Working: The source of the sample (tap, river, sea…) is changed. Mass of dissolved solids and pH are dependent variables; volume is a control variable.

Question 2. Why are anti-bumping granules added to the flask before distillation? (Choose from: To stop violent boiling, To speed up condensation, To remove salt from the water, To keep the thermometer at 100 °C.)

Answer: To stop violent boiling

Working: Anti-bumping granules give smooth boiling and stop the liquid boiling violently and spraying out.

Question 3. True or false? A distillate that boils at exactly 100 °C, has pH 7 and leaves no residue on evaporation is pure water.

Answer: True

Working: True. Pure water boils at 100 °C, is neutral (pH 7) and leaves no residue.

Related lessons

• Previous lesson: 14.5–14.6 Life Cycle Assessments and Reduce, Reuse, Recycle
• Next lesson: 15.1 Rusting
• In this chapter: 14.1 Finite and Renewable Resources
• In this chapter: 14.2 Water Safe to Drink
• In this chapter: 14.3 Treating Waste Water

Continue in the full course

This lesson is section 14.7 (ARPCH1407) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.