1. Experiment Snapshot
| Feature | Details |
|---|
| Apparatus | Dissolved oxygen meter/probe or Winkler method apparatus, bottles with stoppers, incubator/dark cupboard, thermometer. |
| Chemicals/Reagents | Water samples, dissolved oxygen reagents if using Winkler method. |
| Main observation | Clean water usually has higher dissolved oxygen; polluted water shows lower oxygen after incubation. |
| Main measurement | Dissolved oxygen concentration before and after incubation. |
2. Aim
To investigate dissolved oxygen in water samples and understand biochemical oxygen demand as an indicator of organic pollution.
3. Quick Theory
Dissolved oxygen is oxygen gas dissolved in water. BOD is the amount of oxygen used by microorganisms to break down organic matter in a water sample.
BOD = initial dissolved oxygen − final dissolved oxygen
Examiner Tip: High BOD usually means more organic pollution and lower water quality.
4. Apparatus, Chemicals and Safety
| Category | Details |
|---|
| Apparatus | Dissolved oxygen meter/probe or Winkler method apparatus, bottles with stoppers, incubator/dark cupboard, thermometer. |
| Chemicals | Water samples, dissolved oxygen reagents if using Winkler method. |
Safety: Wear goggles if using chemical reagents. Treat natural water samples as potentially contaminated. Wash hands after handling samples.
5. Labelled Experiment Diagram
Dissolved Oxygen and BOD — Premium Practical Setup
6. Procedure
- Collect water sample carefully with minimum aeration.
- Measure initial dissolved oxygen using a DO probe or suitable method.
- Fill a second bottle completely with no air bubbles.
- Incubate the sealed sample in the dark for a fixed time, often 5 days for standard BOD.
- Measure final dissolved oxygen.
- Calculate BOD by subtracting final DO from initial DO.
- Compare BOD values for different water samples.
- Interpret high BOD as evidence of organic pollution.
Examiner Tip: Avoid trapping air bubbles because they can add oxygen and give inaccurate results.
7. Observations and Results
| Stage / Test | Observation | Conclusion |
|---|
| Clean water | High DO, low BOD | Good water quality |
| Polluted water | DO falls greatly after incubation | High organic pollution |
| Stagnant water | Low DO | May not support aquatic life |
Conclusion: BOD measures oxygen used by microorganisms; high BOD indicates more organic waste and poorer water quality.
8. Calculations / Key Chemical Reasoning
BOD is calculated from the drop in dissolved oxygen after incubation.
BOD = initial DO − final DO
Worked Example:
If initial DO = 9.0 mg/L and final DO = 3.5 mg/L, BOD = 9.0 − 3.5 = 5.5 mg/L.
9. Sources of Error and Improvements
| Source of Error | Effect | Improvement |
|---|
| Air bubbles in bottle | DO too high | Fill bottle without bubbles |
| Sample exposed to air | Oxygen changes | Stopper immediately |
| Temperature changes | Microbial activity changes | Incubate at controlled temperature |
| Light exposure | Photosynthesis adds oxygen | Incubate in dark |
| Probe not calibrated | Wrong reading | Calibrate DO probe |
10. Student Common Mistakes
- Thinking high BOD is good.
- Confusing DO and BOD.
- Leaving air bubbles in sample.
- Incubating in light.
- Forgetting units such as mg/L.
11. Examiner Tips
Tip: High DO is usually good.
Tip: High BOD is usually bad.
Tip: BOD is oxygen demand, not oxygen present.
Tip: Incubate in dark to prevent photosynthesis.
12. Examiner Traps
Trap: High BOD means oxygen is used up, not available.
Trap: Photosynthesis can increase oxygen if sample is left in light.
Trap: BOD relates to organic pollution.
13. 🎮 Interactive Simulator
Water Oxygen Quality Lab
Choose water quality condition and observe BOD.
Observation Window
DO sample
Choose a condition and run the simulation.
Lab Score: 0/0
14. Past Paper Style Questions
- State the aim of this experiment. [2 marks]
- Name the main apparatus or reagent used. [2 marks]
- Describe the key observation expected. [2 marks]
- Give one safety precaution. [1 mark]
- Give one source of error and one improvement. [2 marks]
- Explain the chemistry behind the result. [3 marks]
15. Mark Scheme Answers
Q1. To investigate dissolved oxygen/BOD as water quality indicators [2].
Q2. DO probe/bottles/incubator or Winkler reagents [2].
Q3. DO decreases in polluted sample after incubation [2].
Q4. Treat water samples as contaminated/wear goggles [1].
Q5. Air bubbles; fill bottle without bubbles [2].
Q6. Microorganisms use oxygen to break down organic matter; high BOD indicates pollution [3].
16. Quick Revision Card
| Key Point | Memory Hook |
|---|
| DO | oxygen dissolved in water |
| BOD | oxygen used by microbes |
| High DO | usually good |
| High BOD | usually polluted |
| Calculation | initial DO - final DO |
17. Mastery Checklist
- I can define DO.
- I can define BOD.
- I can calculate BOD.
- I can explain high BOD.
- I can identify errors and precautions.