Synthesize chlorine via electolysis of sodium chloride solution

15 minutes
Experiment's video preview
Chemical formula



  • Put protective eyewear on.

  • Conduct the experiment on the tray.

  • Make sure Petri dish is clean and dry.

  • Conduct the experiment only in a well-ventilated room.

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

I don’t have a clean Petri dish with a blue circle. What to do?

If you have already used the Petri dish with a blue circle in the nitrogen dioxide NO2 synthesis experiment, simply rinse it and dry with a paper towel. The other Petri dish would not work for this experiment because the height of the cap placed in the center would not allow you to seal the Petri dish properly.

What’s the template for?

Pick a template appropriate for this experiment and place a Petri dish onto it. Tags by the circles will tell you which chemical has been used at the beginning of the experiment, so that after the experiment you could easily compare the color of compounds in a vial and in a drop.

How to attach the electrodes?

Use square pieces of white double-sided adhesive tape provided in the set. Remove protective film from one side and attach the pieces to the Petri dish as shown in the instruction. Then, remove the upper protective film from the adhesive squares and stick the crocodile clips to them.

I cannot fix the graphite leads in the central drop. When I cover the Petri dish with the lid, they “float.”

Remove the lid and bend the wires to the shape of the Petri dish profile. Now, force the electrodes (graphite leads) back into the drop, and carefully replace the lid. The point is to make leads at least barely touch the drop.

The central drop has smeared. What should I do?

Use an absorbent from the set to wipe the drop out. Make sure the surface is absolutely dry and repeat the step 4.

Nothing is happening in the central drop.

First of all, double-check that you correctly installed the batteries in the holder. If everything is right, and the polarity is correct, but it still doesn’t work, then try replacing the batteries. Make sure the leads are immersed into the drop or at least touch it. It is important that the crocodile clips do not touch the solution (i.e. the drop). If the crocodile clips became wet, wipe them dry, and repeat the experiment.

I can smell gas when conducting the experiment. Is it dangerous?

Indeed, a negligible amount of chlorine is produced in this experiment. The amount of gas released is enough to make you feel the smell but too small to cause any harm to health. Nevertheless, avoid bringing the open Petri dish close to your face to sniff it! In order to smell any chemical, the proper technique is to waft: move your hand in a sweeping motion over the container with the chemical. This would be enough to bring the scent toward you.

My thymol blue solution is yellow. Is it normal?

It’s okay. A solution of thymol blue may turn yellow as it absorbs carbon dioxide CO2 from air in the vial. You may use this thymol blue solution for your next experiments. However, color of the liquid will change from yellow to red, instead of usual blue to yellow.

Yet, if you would like to obtain a blue solution, pour out all the thymol blue into a plastic cup and add there 1 drop of sodium hydroxide NaOH using a pipette from the previous experimental sets. The solution will turn blue, and you’ll be able to use it in the next experiments.

Step-by-step instructions

  1. Take 2 graphite leads and fix them on the edges with the red and black crocodile clips, as shown.

  2. Take a Petri dish with a blue circle on it. Glue pieces of double-sided adhesive tape in the center at a 2 cm distance from each other. Connect the electrodes to the pieces of tape, so that the graphite leads were 5 mm away from each other.

  3. Add 2 drops of each reagent, as shown: 0.1M potassium iodide KI solution, 0.5% iodine I2 solution, and 0.0002M thymol blue solution.

  4. Add 5 drops of 1M sodium chloride NaCl solution and 5 drops of 1M copper sulfate CuSO4 solution between the leads.

  5. Close the Petri dish with a cover marked with a blue circle. Make sure the leads are still immersed in the drop.

  6. Insert two AAA batteries into the holder. Connect the red crocodile clip to the red wire and the black crocodile clip – to the black one. Wait 510 minutes.

  7. In about 10 minutes, look at the drops of the solutions applied: iodine will lose its color, thymol blue will become pale-yellow, and potassium iodide will turn yellow.

Graphical step-by-step instruction


Dispose of the experiment residues along with regular household trash.