AQA GCSE Required Practical 8: Water Analysis

Scientific Principles & Theory

Scientific Background: Pure water is defined as containing only water molecules, with a boiling point of exactly 100 °C and pH of 7. Potable water is water that is safe for humans to drink, containing low levels of dissolved salts and microbes. Pure water is not the same as potable water.

Purification is achieved in the laboratory using simple distillation. The water is heated, evaporates to form steam (leaving behind dissolved solids), and is cooled to condense back into liquid water. Chemical tests (flame tests and silver nitrate tests) are used to confirm that dissolved ions have been removed.

Experimental Variables

Independent Variable

The source of the water sample being analysed (e.g., seawater, spring water, tap water).

Dependent Variable

The pH of the water and the mass of dissolved solids remaining after evaporation.

Control Variables

The volume of water evaporated (10.0 cm³), the calibration of the pH meter, and the cooling temperature.

Lab Risk Assessment

Hazard	Associated Risk	Control Measure
Hot apparatus and boiling water	Severe thermal burns and scalding	Use tongs to handle the hot evaporating basin; wear safety goggles; use boiling chips to prevent liquid spitting during boiling.

Apparatus & Procedure

Required Apparatus

Water samples from different sources (tap water, spring water, seawater)
Universal indicator paper or calibrated pH probe
Evaporating basin
Digital balance (0.01 g resolution)
Measuring cylinders (10 cm³ and 25 cm³)
Bunsen burner, tripod, gauze, and heatproof mat
Conical flask (100 cm³) with delivery tube and bung
Test tube in a beaker of ice-cold water (simple condenser)
Boiling chips
Qualitative testing reagents (concentrated HCl, silver nitrate, nitric acid)

Step-by-Step Procedure

Part 1 (Analysis): Test the pH of each water sample using universal indicator paper or a calibrated pH probe. Record the results.
Weigh a dry, empty evaporating basin on a digital balance and record its mass.
Measure exactly 10.0 cm³ of the first water sample and pour it into the basin.
Heat the basin gently over a Bunsen burner until all the water has evaporated. Do not heat to absolute dryness to prevent spitting.
Allow the basin to cool completely, then reweigh it. Calculate the mass of dissolved solids.
Repeat the analysis for all other water samples.
Part 2 (Purification): Pour 20 cm³ of seawater into a conical flask containing a few boiling chips. Set up a delivery tube from the flask to a test tube immersed in a beaker of ice-cold water.
Heat the flask gently with a Bunsen burner. Water evaporates, travels through the tube, and condenses as pure water in the cold test tube.
Test the distillate (purified water) for pH (should be 7) and evaporate a sample to verify that no dissolved solids remain.
Test the seawater and distillate for sodium ions (flame test) and chloride ions (nitric acid + silver nitrate test) to verify purification.

Fig 1. Laboratory experimental setup for Required Practical 8.

Sample Data & Calculations

This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:

Water sample source	pH recorded	Mass of empty basin (g)	Mass of basin + residue (g)	Dissolved solids in 10 cm³ (g)
Tap Water	7.2	52.41	52.42	0.01
Spring Water	7.5	52.41	52.45	0.04
Seawater	8.1	52.41	52.76	0.35
Distilled Water (Distillate)	7.0	52.41	52.41	0.00

Calculate mass of dissolved solids in 10.0 cm³: Mass = Mass of basin with residue - Mass of empty basin.
For Seawater: Mass = 52.76 g - 52.41 g = 0.35 g.
Convert volume: 10.0 cm³ = 0.010 dm³.
Calculate concentration of dissolved solids (in g/dm³): Concentration = Mass / Volume (dm³) = 0.35 g / 0.010 dm³ = 35.0 g/dm³.

Conclusion & Evaluation

Chemical Explanation: Evaluating the experimental outcomes against known values ensures validity. Understanding the source of systematic and random deviations allows for better experimental designs in future trials.

Experimental Error Analysis

Error Type & Source	Effect on Final Result	Mitigation Strategy
Systematic Error Incomplete cooling of the evaporating basin before weighing	Convection currents around the hot basin alter balance readings, leading to inaccurate residue masses.	Allow the evaporating basin to cool completely to room temperature before placing it on the balance scale.
Random Error Splitting / spitting of solid residue near dry point	Some solid is ejected out of the basin, underestimating the mass of dissolved solids.	Heat very gently, especially as the water level drops; use a watch glass or decrease heat once crystallization starts.

Exam Practice

Practise this practical

Write your answer and the AI examiner checks it against the mark scheme, showing exactly which marks you earned and what to add.

[6 marks]

A student is given a sample of seawater. Plan an investigation to determine the concentration of dissolved solids in the seawater and describe how you would purify it to obtain pure water.

View Model Answer & Mark Scheme

Marking guidance: award 1 mark for each correct, clearly explained step in the model answer below, up to 6 marks.

Model Answer (6/6 Marks):

Analysis of dissolved solids: Weigh a clean, dry evaporating basin on a digital balance and record its mass.
Evaporation: Measure exactly 10.0 cm³ of seawater using a measuring cylinder, pour it into the basin, and heat it gently over a Bunsen burner.
Weighing: Heat until all water has evaporated, then allow the basin to cool completely to room temperature. Weigh the basin and calculate the residue mass by subtraction. Calculate concentration using Concentration = Mass / 0.010 dm³.
Purification setup: Pour another portion of seawater into a conical flask. Insert a bung with a delivery tube connected to a test tube placed in a beaker of ice-cold water. Add a few boiling chips to the flask.
Distillation: Heat the flask gently to boil the water. Water vapour rises, passes through the tube, and condenses back to liquid in the ice-cooled test tube. Heat must be removed before the flask goes completely dry.
Verification: Test the collected distillate: it should have a pH of 7 and produce 0.00 g of residue upon evaporation, indicating it is pure water. Flame tests must show no sodium (no yellow flame) and silver nitrate must show no chloride (no white precipitate).

Examiner Tip:

Always mention verification tests (both chemical checks and evaporating check) to show how you confirm that the collected distillate is indeed pure water.

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Required Practical 8: Analysis and Purification of Water