Sulfur dioxide

Synthesize sulfur dioxide and learn how it makes the indicator change colors

15 minutes
Experiment's video preview



  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray and in a well-ventilated area.
  • Keep a bowl of water nearby when working with fire.
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

It’s been 3 minutes but the flower is still blue. What should I do?

First, check that the candle is still lit up. Then, make sure that flowers are positioned right above the neck of the flask. It’s important that gas is flowing right into the “buds”. Wait 3 min. more.

Step-by-step instructions

Coat the paper flower with the acid-base indicator thymol blue.


To create sulfur dioxide SO2, combine sodium metabisulfite Na2S2O5 and citric acid to cause a chemical reaction.


The reaction speeds up when heated. Sulfur dioxide dissolves in the water imbued in the flower, forming sulfurous acid. The acid-base indicator reacts to this and changes colors.


To stop the flow of gas, blow out the candle and add water to the solution.


With exposure to air, some of the sulfur dioxide vaporizes and the indicator turns orange.



Dispose of solid waste together with household garbage. Pour solution down the sink. Wash with an excess of water.

Scientific description

How is sulfur dioxide produced?

In this experiment, a solution of sodium metabisulfite Na2S2O5 is mixed with citric acid. This produces sulfur dioxide. The resulting solution is acidic which means there are many hydrogen ions H+ or protons in it. The protons react with the sodium metabisulfite. The sodium metabisulfite then decomposes into two molecules of sulfur dioxide SO2:

Na2S2O5 + H8C6O7 → Na2H6C6O7 + H2O + 2SO2

Why do we heat the reaction mixture?

Heating greatly increases the reaction rate. With no heat is applied, the rate of reaction would be very slow. If you want to know, how heat speeds up the reaction, follow the link: Fox tail gas

Why do the flowers change in color from blue to red?

Thymol blue is a pH indicator. This means that it will change color depending on the number of protons in a solution. When sulfur dioxide dissolves in the solution of thymol blue, it increases the number of protons thereby creating an acidic medium. Thymol blue reacts to this, thus, changing its color from blue to red. Therefore, the flowers change color as well.

Why does the gas flow stop when water is added and the candle is distinguished?

The gas flow stops for two reasons: the first one is because there is no further heat supply once we have blown out the candle and added cool water. The second reason is that the reaction mixture becomes too diluted when the water is added. Both of these make the gas release stop.

Why do the flowers turn orange after a while?

If we leave the flowers for a few minutes, sulfur dioxide will start to evaporate from the surface of the petals. The more sulfur dioxide evaporates the fewer number of protons remains in the solution. In turn, the fewer number of protons create a less acidic medium. This makes thymol blue change its color from blue to orange.