Sulfur dioxide

Synthesize sulfur dioxide from sodium disulfite

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

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.

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 steps 2 - 5. If the central drop has merged with the other drop(s), thorough wash the Petri dish, dry it, and repeat the experiment.

After waiting for the time specified, I still don’t see the drops change their color. Why?

Make sure the Petri dish is tightly closed with the lid, and that the gas is not leaking through any openings. Also, try adding a couple more drops of each compound and wait for 5 - 7 minutes more.

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 a Petri dish and 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.

  2. Add 10−15 drops of 0.1M sodium pyrosulfite Na2S5O2 solution onto to center of the Petri dish.

  3. Now, add into a central drop 5 drops of 3M sodium hydrosulfate NaHSO4 solution.

  4. Close the Petri dish with a cover and wait 5−10 minutes. In the central drop, bubbles of sulfur dioxide SO2 will appear.

  5. In about 10 minutes, iodine drop will lose its color, thymol blue drop will become yellowish-brown, white potassium iodide drop will not change at all.

Graphical step-by-step instruction


Dispose of the experiment residues along with regular household trash.