Science 7–10 · Year 8
Splitting water: two volumes of hydrogen to one of oxygen
Periodic table and atomic structure
The idea
Water is a compound of two elements in a fixed ratio; an electric current decomposes it and the 2 : 1 gas volumes match the formula H2O.
Safety card
Hazards
- hydrogen is extremely flammable; a mixed hydrogen-oxygen pocket can pop sharply
- sodium sulfate is low hazard
- low-voltage electricity near liquid
Controls
- gas tests done only by the adult on the tiny microscale volumes
- no flames near the apparatus except for the test
- eye protection
- battery disconnected before dismantling
Note
NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.
What you need
- microscale Hoffman apparatus (two shortened plastic pipettes over inert electrodes) or a bench Hoffman voltameter with platinum electrodes
- sodium sulfate solution 0.2 mol/L, about 30 mL
- 9 V battery with crocodile-clip leads (or a low-voltage DC supply with an ammeter in series)
- Blu-Tack to seal the apparatus, ruler or the pipette graduations, stop clock, wooden splints, matches (adult only)
How to do it
- Fill the apparatus with 0.2 mol/L sodium sulfate solution so that both collecting tubes are full and free of air; seal as the kit requires.
- Connect the electrodes to the battery, note which electrode is negative (cathode), and start the clock. If a supply with an ammeter is used, record the current.
- Record the gas volume above each electrode every 2 minutes for 15 minutes.
- Plot both volumes against time and compute the ratio cathode gas : anode gas at each reading.
- Adult test: hydrogen from the cathode tube gives a pop with a lit splint; oxygen from the anode tube relights a glowing splint.
What you should see
Bubbles rise at both electrodes as soon as the circuit is closed. The cathode collects about twice the volume of the anode at every reading, so the ratio stays near 2 : 1; early readings can run a little above 2 because oxygen is more soluble in water than hydrogen and some dissolves before bubbles collect. A larger current makes both gases collect faster, but the ratio stays 2 : 1. The learner knows it worked when the plotted lines are straight, the cathode line is about twice as steep, and the splint tests match the expected gases.
What changes
- What you change
- time (and current, if a supply with an ammeter is used)
- What you measure
- volume of gas above each electrode
- What you keep the same
- electrolyte concentration
- electrode material and spacing
- temperature
- battery or supply voltage
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- The bubbles are air that was dissolved in the water.
- Water is a mixture of hydrogen and oxygen gases.
- Equal volumes of the two gases should form because there are two elements.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC4-PRT-01SC4-CHG-01
- Australian Curriculum v9AC9S8U06AC9S8U07
Sources
The pages the author read to write this activity.