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Topic: Electrochemistry
Level: Ordinary & Higher Level
Exam Focus: Gas Tests and Ratio
1. Experiment Snapshot
Feature
Details
Apparatus
Hofmann voltameter or electrolysis apparatus, DC power supply, electrodes, test tubes, wires, switch.
Chemicals/Reagents
Water with a small amount of dilute sulfuric acid or sodium sulfate to improve conductivity.
Main observation
Gas bubbles form at both electrodes; hydrogen volume is about twice oxygen volume.
Main measurement
Volumes of hydrogen and oxygen gas collected.
2. Aim
To decompose water by electrolysis and identify the gases produced at each electrode.
3. Quick Theory
Pure water conducts electricity poorly, so an electrolyte is added. During electrolysis, water decomposes into hydrogen at the cathode and oxygen at the anode.
2H₂O(l) → 2H₂(g) + O₂(g)
Examiner Tip: Hydrogen forms at the cathode and oxygen forms at the anode in a 2:1 volume ratio.
4. Apparatus, Chemicals and Safety
Category
Details
Apparatus
Hofmann voltameter or electrolysis apparatus, DC power supply, electrodes, test tubes, wires, switch.
Chemicals
Water with a small amount of dilute sulfuric acid or sodium sulfate to improve conductivity.
Safety: Wear goggles. Use low-voltage DC supply. Hydrogen is flammable and oxygen supports combustion.
5. Labelled Experiment Diagram
Electrolysis of Water — Premium Practical Setup
6. Procedure
Fill the electrolysis apparatus with acidified water.
Connect electrodes to a DC power supply.
Place collection tubes over each electrode if needed.
Switch on the current.
Observe gas bubbles at both electrodes.
Collect the gases produced.
Compare the volumes of gas collected.
Test hydrogen with a lighted splint.
Test oxygen with a glowing splint.
Examiner Tip: Use direct current, not alternating current, and label cathode/anode correctly.
7. Observations and Results
Stage / Test
Observation
Conclusion
Cathode (-)
Gas collected; squeaky pop
Hydrogen
Anode (+)
Gas relights glowing splint
Oxygen
Volume ratio
Hydrogen about twice oxygen
H₂:O₂ = 2:1
Conclusion: Water decomposes into hydrogen and oxygen by electrolysis, with hydrogen produced in twice the volume of oxygen.
8. Calculations / Key Chemical Reasoning
The balanced equation shows two moles of hydrogen gas for every one mole of oxygen gas.
2H₂O → 2H₂ + O₂ ; H₂:O₂ = 2:1
Worked Example: If 20 cm³ oxygen is collected, about 40 cm³ hydrogen should be collected.
9. Sources of Error and Improvements
Source of Error
Effect
Improvement
Gas leaks
Volumes too low
Check apparatus is airtight
Electrodes mixed up
Wrong gas identified
Label cathode and anode
Poor conductivity
Slow reaction
Add suitable electrolyte
Gas dissolves slightly
Small volume error
Collect carefully
Using AC supply
Poor separation
Use DC supply
10. Student Common Mistakes
Calling the gas at cathode oxygen.
Forgetting hydrogen has twice the volume.
Testing hydrogen with glowing splint.
Using pure water with no electrolyte.
Forgetting water is decomposed, not evaporated.
11. Examiner Tips
Tip: Hydrogen gives squeaky pop.
Tip: Oxygen relights glowing splint.
Tip: Cathode is negative in electrolysis.
Tip: Use acidified water to conduct.
12. Examiner Traps
Trap: Hydrogen is not produced at the anode.
Trap: The electrolyte helps conductivity but should not be the main product.
Trap: The 2:1 ratio comes from the balanced equation.
13. 🎮 Interactive Simulator
Water Electrolysis Gas Ratio
Choose conditions and observe gas volumes.
Observation Window
H₂ / O₂ tubes
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 decompose water by electricity and identify gases [2].
Q2. Electrolysis apparatus, electrodes, DC supply, collection tubes [2].
Q3. Hydrogen at cathode and oxygen at anode [2].
Q4. Use low voltage and keep hydrogen away from flames [1].
Q5. Gas leak; make apparatus airtight [2].
Q6. Water splits into H₂ and O₂ in a 2:1 ratio [3].