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Split water into hydrogen and oxygen with a battery. Which gas do you get more of?
Electrolysis lab · Last updated: 16 October 2026
You pass electricity from a battery through water and watch bubbles rise at two metal rods. One gas is hydrogen and the other is oxygen, and they find out which one collects more and why. Electrolysis is used to make hydrogen fuel, to plate metals and to purify copper.
What you'll discover
- An electric current can split water into hydrogen gas and oxygen gas.
- Hydrogen collects at the negative electrode and oxygen at the positive electrode.
- You get twice as much hydrogen as oxygen by volume, because each water molecule has two hydrogen atoms and one oxygen atom.
The science in plain words
Water is a compound: each molecule is made of two hydrogen atoms joined to one oxygen atom, which is why its formula is H2O. Water is very stable and does not fall apart on its own. But electrical energy from a battery can break it down. This is called electrolysis, which means splitting using electricity.
Two electrodes, usually metal or graphite, are placed in the water and connected to a battery. Pure water conducts electricity very poorly, so a few drops of dilute acid are usually added to let current flow. At the negative electrode, called the cathode, hydrogen gas bubbles off. At the positive electrode, called the anode, oxygen gas bubbles off. If each gas is collected in an upside-down tube, the hydrogen tube fills twice as fast as the oxygen tube.
The reason lies in the formula. The balanced equation is 2H2O giving 2H2 plus O2: two molecules of water make two molecules of hydrogen and one molecule of oxygen. At the same temperature and pressure, equal numbers of gas molecules take up equal volumes, so the hydrogen takes up twice the volume. You can test the gases safely in a school lab: hydrogen burns with a squeaky pop, and oxygen relights a glowing splint. Electrolysis using electricity from solar or wind power may become an important way to make clean hydrogen fuel.
Where it fits
- India, NCERT: Class 10 science, Chapter 1: Chemical Reactions and Equations
- United States: Grades 6 to 8
- Next Generation Science Standards: MS-PS1-2, MS-PS1-5
Common questions
Why do you get twice as much hydrogen as oxygen in electrolysis of water?
Each water molecule contains two hydrogen atoms and one oxygen atom. Splitting water gives two molecules of hydrogen for every one molecule of oxygen. Equal numbers of gas molecules take up equal volumes at the same conditions, so the hydrogen fills twice the volume.
Why is acid added to water for electrolysis?
Pure water contains very few charged particles, so it conducts electricity poorly and the reaction is extremely slow. Adding a few drops of dilute acid supplies more ions to carry the current, so the water splits much faster. The acid itself is not used up.
How can you test for hydrogen and oxygen?
Bring a lighted splint near a tube of hydrogen and it burns with a squeaky pop. Put a glowing splint into a tube of oxygen and it bursts back into flame. These simple tests, done under supervision in a school lab, confirm which gas is which.
Related labs
- Redox Reactions: Two metals and two solutions make a battery. Where do the electrons come from?
- Simple Circuits: Close the switch. What makes the bulbs brighter: more volts, fewer bulbs, or a different wiring?
- Chemical Reactions: Molecules must crash hard enough to react. What makes that happen more often?
See all chemistry labs or every Taz science lab.
Try it yourself
The Electrolysis lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.