Sulfur dioxide

Synthesize sulfur dioxide and watch as it makes the indicator change colors!

15 minutes
Sulfur dioxide



  • 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

What color should the buds be when I apply the thymol blue?

When the thymol blue is applied, the buds should turn bright blue. Be sure to apply enough indicator to cover the petals completely.

The flowers aren’t turning red. What should I do?

First, check that the candle is still lit. Next, make sure that the flowers are positioned right above the neck of the flask. It’s important that gas flow right into the buds. Wait a little bit more.

Why do we heat the reaction mixture?

Heating greatly increases the reaction rate. Without heat, the reaction will proceed very slowly.

Step-by-step instructions

Sodium metabisulfate Na2S2O5 reacts with acids, such as citric acid, to produce sulfur dioxide SO2 gas.


Thymol blue is an acid-base indicator. Suspend a paper flower covered with thymol blue solution over the emerging SO2 gas.


The reaction between Na2S2O5 and citric acid happens rather slowly at room temperature, barely releasing any SO2 gas. Heat the flask a little bit to speed things up.


The acid-base indicator coating the flower turns red, meaning that some acid was produced when the SO2 dissolved in the indicator solution.


To stop the production of SO2 gas, add some cool water to the flask.


An excess of SO2 will slowly escape the indicator solution, turning the indicator from red to yellow.



Dispose of solid waste together with household garbage. Pour solutions 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 it contains many hydrogen ions H+, or protons. 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. Without it, the reaction would proceed very slowly. You can check out how exactly heat speeds up the reaction here: Fox tail gas

Why do the flowers change from blue to red?

Thymol blue is a pH indicator, meaning that it changes colors 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. The thymol blue reacts to this change by transitioning from blue to red. This causes the flowers to change colors as well.

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

The gas flow stops for two reasons. Firstly, there is no further heat supply once we have blown out the candle and added cold water. Secondly, the addition of water makes the reaction mixture too diluted. Both of these factors contribute to stopping the gas release.

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 protons remain in the solution. In turn, the reduction in the number of protons creates a less acidic medium. This makes thymol blue change its color from blue to orange.

That's interesting!

Sulfur dioxide is a colorless gas with a pungent, suffocating odor. It is also highly toxic – inhaling its vapors will invoke a cough, runny nose, and a sharp tickling sensation in the throat. In light of this, incorporating it in food may not seem like the best idea! However, it is widely used in the food industry as a preservative. Interestingly, it was first incorporated in food in the Middle Ages. The modern era has seen the advent of legal limits regarding the maximum content of this substance in food products. Limited quantities of this substance can be found almost everywhere, including in fruits and vegetables (canned, dried, and frozen), meat, drinks, some sweets, sausages, and products made of mushrooms and potatoes. Moreover, sulfur dioxide is considered an excellent stabilizer and antioxidant. It prevents the proliferation of bacteria in meat products and is also used to prepare fruits, both fresh and dried, for storage. Juices and other non-alcoholic beverages are almost always treated with sulfur dioxide to ward off mold and bacteria. Moreover, sulfur dioxide is quite commonly added to wine to stabilize its microflora, prevent unwanted fermentation and souring, and “fix” its color.