Sulfur dioxide bleaches a petal!

30 minutes
Experiment's video preview



  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray and in a well-ventilated area.
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 flowers to use for this experiment?

This experiment works best with pale red, blue, and violet petals. The gentler are the flowers, and the thinner are the petals, the better the experiment turns out. For very bright flowers, it can take up to several hours to bleach the petals. And for yellow flowers, the experiment wouldn’t work at all.

I’ve waited, but the petal didn’t bleach. What to do?

Perhaps, you chose a very bright flower with quite thick petals. Wait 20–30 minutes more and see what happens. If waiting longer doesn’t help, take another flower with thinner petals.

Step-by-step instructions

Anthocyanins are substances which give many flowers their colors, ranging from red to blue.


Sodium pyrosulfite Na2S2O5 decomposes in acidic solutions, giving off sulfur dioxide gas—SO2.


Anthocyanins react with SO2 and lose their color, making the petal’s color fade. The speed of this reaction depends on the petal chosen—the more intense the color, the more time needed.


Wash the Petri dish to repeat the experiment.


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

Scientific description

What happens in the Petri dish?

In the presence of sodium hydrogen sulfate NaHSO4, sodium pyrosulfite Na2S2O5 decomposes and releases sulfur dioxide gas SO2. This chemical transformation proceeds in two steps. First, sodium pyrosulfite reacts with water yielding sodium hydrogen sulfite NaHSO3:

Na2S2O5 + H2O → 2NaHSO3

Second, the sodium hydrogen sulfite decomposes into Na2SO4, H2O and SO2↑ in the acidic medium created by sodium hydrogen sulfate:

NaHSO3 + NaHSO4 → Na2SO4 + H2O + SO2

Learn more

In the same manner, we synthesize SO2 in the corresponding experiment in which we study the properties of sulfur dioxide. Reagents for this experiment are included in the set «Sulfur and nitrogen dioxides»).

Why does the petal turn colorless?

Sulfur dioxide SO2 penetrates the petal tissues and dissolves in the intracellular fluid. As a result bisulfite anions are formed:

SO2 + H2O → HSO3- + H+

This intracellular fluid contains dissolved anthocyanins – the pigments responsible for the lilac color of flowers. Upon reaction with HSO3, these pigments become colorless, and as a result, the petal turns white.

This process is reversible. However, completely clearing the pigments of the bisulfite takes much longer than their discoloration. Restoring the original color of the petals would take at least 24 hours.

Learn more

Apple and rose hip flesh contain colorless anthocyanins – so-called leucoanthocyanins. When treated with an acid, leucoanthocyanins will turn into true anthocyanins. As a result, these anthocyanins will also respond to changes in acidity or react with sulfur dioxide.

Thus, petals of some flowers can gain color when exposed to an acid. A good example is the white Dianthus flower which has a thin red trim on its petals. If you immerse this flower in vinegar, in a few moments its petals start turning red due to the oxidation of leucoanthocyanins.

What petals would work best for this experiment?

For best results, pick a flower with thin and tender petals of delicate colors: soft red, blue, violet or lilac. For this experiment, avoid thick petals with bright coloration – such as irises or dark roses – it would be quite difficult to completely remove their color. Yellow flowers are unsuitable – their color is due to the presence of different pigments – xanthophylls and carotenes.