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 analyzed (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 |
Data Processing & Analysis
- 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
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
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.