Obtain an oxyhydrogen mixture via electrolysis!

40 minutes



  • Put on protective gloves and eyewear.
  • Remove protective gloves before lighting the candle.
  • Conduct the experiment on the safety underlay.
  • Keep a bowl of water nearby while working with fire.
  • Keep flammable materials and hair away from flame.
  • Observe safety precautions when working with batteries.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

The solution is leaking from the pipette! What should I do?

If this happens, first disconnect the battery holder from the electrolyzer. Carefully pour the remaining NaOH into the beaker. Then, separate the cable plug from the pipette and wipe it off with a paper towel. You need to fix the leak. Wrap the rubber O-ring from the set around the cable plug and insert it into the pipette again. Once you’re sure the leak has stopped, continue with your experiment. In addition, you can try using the spare electrolyzer from the set.

I connected the batteries, but nothing is happening.

Make sure you connected all the clips and wires correctly: the red crocodile clip connects to the inner wire end, and the black clip to the outer end; the red wire connects to the red clip on the battery holder, and the black wire to the black clip. If everything is correct, try changing the batteries.

The cable plug turned green. Why?

The cable plug is partially made of copper. The process of electrolysis involves some redox reactions, and copper can turn green when oxidized.

The mixture is bubbling out of the pipette. Is everything okay?

Yes, this should not affect the experiment. Just wait the allotted time and move on to the next step!

I did everything correctly but there was no ‘pop.’

A: Don't worry! First, repeat the previous steps starting from Step 2. Try running the electrolysis a bit longer to let more gas accumulate. Try to move the bottle to the candle quickly, without inverting or turning it. Bring the bottle of oxyhydrogen closer to the candle. If there is still no pop, then bring the bottle even closer (yes, the bottleneck may start to melt a little) or try holding the bottle at a different angle.

How many times can I fill the bottle with gas and try to blow the candle out?

Quite a few, actually! Check it out!

Step-by-step instructions

To make the water electrolysis process effective, prepare an alkali solution. The reaction between calcium hydroxide Ca(OH)2 and sodium carbonate Na2CO3 yields sodium hydroxide NaOH—exactly the alkali we need.


Aside from the main product, NaOH, the reaction produces some CaCO3. To filter it off, fold a filter paper as shown.


The calcium carbonate solids (ordinary chalk) remain in the filter, while the sodium hydroxide solution passes through, into the flask.


Prepare an apparatus for water electrolysis. This is called an electrolyzer.


Fill the electrolyzer halfway with sodium hydroxide NaOH solution.


Set up a vial to collect the oxyhydrogen gas. Start the process of water electrolysis.


Now, try to extinguish a candle using the reaction between hydrogen and oxygen.


To repeat the experiment, reconnect the electrolyzer to the batteries and repeat steps 6 and 7.

Expected result

When electrolyzed, water decomposes into two gases: oxygen O2 and hydrogen H2. The end result is twice as much hydrogen as oxygen. Such a mixture of gases is called oxyhydrogen. When a bottle full of oxyhydrogen is placed near a burning candle, the gas ignites immediately and blows out the candle.


Please refer to local regulations when disposing of chemicals. Dispose of other solid waste with household garbage. Pour leftover solutions down the sink. Wash with an excess of water.

Scientific description

Water molecules H2 are rather strong, and “disassembling” them requires a lot of energy. The electricity  from the batteries is applied to break the strong bonds in the water molecules. This process forms molecules of oxygen O2  and hydrogen H2 , which then escape into the air.

A mixture of H2  and O2  in a 2 to 1 ratio is called oxyhydrogen and is quite flammable—easily ignited by a candle flame. The reaction between the gases takes place with a characteristic loud sound and emission of energy . Incidentally, this is the energy that was spent on “disassembling” the water molecules earlier, just in a different form. Water H2 is another product of this reaction.

We don’t usually use this method to obtain water; instead, we capture and use the emitted energy. For instance, this reaction is used to create the driving force in rocket engines. However, if uncontrolled, it can be quite dangerous. In 1937, an airship called the Hindenburg exploded because the 200 thousand cubic meters of hydrogen filling the blimp reacted with the oxygen in the air. The reaction completely destroyed the airship and formed over 150 tons of water.

Chemically, the process of water formation looks rather simple:

2H2 + O2 → H2O

However, it is not as simple as it seems. This is an oxidation-reduction reaction, with oxygen as an oxidizer (taking electrons from hydrogen) and hydrogen as a reducer (giving its electrons to oxygen):

O20 + 4e → 2O2-

H20 - 2e → 2H+

This reaction proceeds vigorously, especially when oxygen is mixed with hydrogen in a 1:2 ratio as it is in our experiment. This is because the resulting product, water vapor, contains one oxygen atom and two hydrogen atoms – its ratio is also 1:2.

How does electrolysis occur?

To understand this, let’s first consider what happens inside the initial solution in the electrolyzer. Water  itself, when liquid, can split into ions:

H2O → H+ + OH-

Ordinary water contains very, very few of these ions. But sodium hydroxide can split in water as well:

NaOH → Na+ + OH-

So now our water contains a lot of hydroxide ions OH- . An electrolyzer (the device used in water electrolysis) has a positively-charged anode , which attracts negatively charged ions like OH-, and a negatively-charged cathode , which attracts positively charged ions like H+ .

And so these OH- and H+ begin to move toward the respective parts of our electrolyzer. Then, H+ ions take electrons from the cathode and turn into hydrogen H2 , while hydroxide ions OH- give their electrons to the anode and turn into oxygen O2 .

In our experiment, the electrolyzer is made of an RCA plug: its metallic ring is a cathode, and its pin is an anode. However, one can change the poles by connecting the wires of the plug and the battery holder the other way round – this will not affect the experiment.

What is an RCA plug?

The RCA plug was once widely used in audio and video systems. It could connect, for example, a video player with a TV set. It is still used in some visual equipment, but is not so widespread these days. It consists of two metallic parts: an outer ring and a pin, separated by a plastic insulating ring. Separate wires are connected to each of the metallic components: the shorter wires are connected to the metallic ring, while the longer wires connect to the pin.