How to separate water into hydrogen and oxygen

We’ve all been told that water is made up of hydrogen and oxygen. But how do we really know that? Can this wet substance that quenches our thirst and cools our bodies on hot summer days really be made up of two gases?

We tried to separate water into oxygen and hydrogen using electrolysis. We managed it after a series of experiments that left us with even more questions than we had before we started. Which isn’t necessarily a bad thing – curiosity is a great learning state!  (See the mysterious case of the missing oxygen, below.)

You can benefit from our mistakes and perform electrolysis the quick way.  Here’s how to split water into hydrogen and oxygen using electrolysis. Afterwards I’ll tell you about what we did first, which produced a different gas entirely.

How to separate water into hydrogen and oxygen

What you need
  • glass or plastic tub
  • 2 elastic bands
  • 2 test tubes (with lids if possible)
  • bicarb of soda (1 tbsp)
  • graphite pencil leads
  • water
  • battery (we used 6V, a bit like this one)
  • 2 pairs of crocodile clips
  • waterproof tape
Electrolysis of water for kids

What you do:

Connect one end of each crocodile clip to a piece of graphite, and the other to the battery. Secure the graphite ends to the bottom of the tub with the graphite sticking up, and place an inverted test tube over each piece of graphite (held in place by the elastic bands). Dissolve the bicarb of soda in the water and fill the tub. Finally, remove each test tube, fill it with the water, and carefully replace it over the graphite. Any gases collected during the electrolysis will replace the water in the tubes, so make sure there are no air bubbles.

What happens

Bubbles of gas quickly start to form at each electrode. More gas collects at the negative electrode (cathode) than at the positive (anode).

How to test your gases

When you’ve collected plenty of gas at each electrode, carefully put the lids on your test tubes (while they’re still underwater).

To test for hydrogen

We hypothesised that the gas at our (negative) cathode was (positively charged) hydrogen. Hydrogen is explosive. It won’t wreck your house in these quantities, but it will make a cool popping noise in the presence of a lighted splinter of wood. You can hear it in the video below.

To test for oxygen

We test for oxygen with a glowing splint. If enough oxygen is present, the splint rekindles. The gas we collected at our anode gave a brief glow which confirmed it to be oxygen, but after the excitement of the popping hydrogen, we were a bit disappointed. We produced much more oxygen later using a different method – see below for a video of our relighting splint.

How does electrolysis work?

Water is a covalent molecule (H20) held together by shared electrons in covalent bonds.

During electrolysis, the molecules are reduced at the cathode to to hydrogen gas, and oxidised at the anode to oxygen gas.

Pure water doesn’t conduct electricity, so we need to add an electrolyte, like bicarbonate of soda. (You wouldn’t believe the number of websites that tell you to use salt. We tried it, and collected a completely different gas. More on that later.)

Twice as much hydrogen as oxygen is produced, reflecting the molecular composition of water.

Electrolysis of Water


I am scientist

I am scientist is a result of small effort to make huge impact on children. To stimulate the curiosity of young scientist towards science. Each idea will start with a experiment followed by reasoning the result , scientific learning, vocabulary (recap the big words used), useful reference website and books, along with free downloadable worksheets.

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