Required Practical 11: Water Analysis (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Required Practical 11: Water Analysis

Practical 11(a): Analysing and Purifying Water and making it Safe to Drink

Objective

• To determine the amount of dissolved solid in samples of water

Hypothesis

• Analysis of the pH and dissolved solids of water samples can help determine the regions the water comes from

Materials

• Water samples A, B, C and D

• Universal indicator paper

• Mass balance

• Evaporating basin

• 25 cm³ graduated cylinder

• Bunsen burner, tripod & gauze

Practical tips

Don’t overheat during step 4 as you run the risk of thermally decomposing some of the solids, leading to erroneous results

Universal indicator paper gives pH values as whole numbers. A pH meter or probe gives more precise decimal values but these are two different methods and cannot be averaged together as they have different resolution.

Method

1. Filter the water sample to remove any suspended solid particles

2. Use the universal indicator paper to determine the pH of the water sample

3. Accurately weigh an empty evaporating basin to two decimal places

4. Add 25 cm³ of water sample A into the evaporating basin

5. Heat the evaporating basin on a tripod and gauze using a Bunsen burner until the solids start to form and the majority of water has evaporated

6. Leave for the remaining water to evaporate off

7. Weigh the cooled evaporating basin again and calculate the mass of dissolved solids

Results

Record your results in a suitable table

Evaluation

• The results could be compared to the national water safety levels

• Analysis can indicate the origin of each sample, e.g. region of high acid rain, from a salt water supply etc.

Conclusion

• The amount of dissolved solids in water can be determined and are useful indicators of water quality

Required Practical 11(b): To Purify a Water Sample by Distillation

Objective

• To separate pure clean water from a sample containing water and other substances

Hypothesis

• A simple distillation apparatus can be set up separate pure water from a mixture of water and unwanted substances

Materials

• 10 cm³ of water sample A

• Bunsen burner

• Tripod and gauze

• Heatproof mat

• Clamp and clamp stand

• Conical flask with delivery tube and bung

• Boiling tube

• Ice bath

Simple distillation apparatus

Practical Tips

The delivery tube must sit above the level of the liquid in the collecting tube — if the tube is submerged, cold water can be sucked back up into the hot flask when heating stops, causing the glass to crack.

The thermometer must be positioned to measure the temperature of the vapour, not the boiling liquid. The thermometer bulb should sit at the neck of the flask where steam passes into the delivery tube. If placed in the liquid, it will give an incorrect boiling point reading.

To improve the volume of pure water collected, place an ice bath around the collecting tube or use a Liebig condenser as both increase the rate of condensation.

Method

1. Add the water sample to the conical flask and set up the distillation apparatus as shown in the diagram

2. Heat the water using the Bunsen burner until boiling occurs

3. Reduce the heat so that the water boils gently

4. The distilled water will collect in the cooled boiling tube

5. Collect approximately 2 cm depth of water, then stop heating

6. Determine the boiling point of the distilled water to confirm it is pure

Results

• Distillate of pure water collected in the boiling tube

Evaluation

• The purity of the collected water can be confirmed by:

  • Testing its pH (should be 7)

  • Measuring its boiling point (should be 100°C)

Conclusion

• Simple distillation can be used to produce pure water from a sample of impure or contaminated water