Foam eruption

Foam erupts out of the flask like real lava!

10 minutes
Foam eruption



  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
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.

Step-by-step instructions

Prepare the solution of anthocyanin, a natural dye.


Prepare the dry reagent mixture, which will act as a source of carbon dioxide.


Add the liquid soap, which will later foam up the mixture.


Pour the dye solution into the dry reagent mixture to initiate the process.



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

Scientific description

What does this foam consist of?

The foam is simply a lot of carbon dioxide gas CO2 which makes the liquid soap form bubbles. And, of course, the food coloring anthocyanin, which can be found in grapeskin, and which makes the foam much more spectacular. You might also notice some particles of a white solid in the foam - these are small amounts of the initial powders (citric acid and sodium carbonate) which have neither reacted with one another nor dissolved in the water.

But how does this foam form?

As we’ve already figured out, the foam forms when a large amount of carbon dioxide gas bubbles vigorously through the liquid soap. But how does the carbon dioxide form?

Initially, we mix two substances: citric acid H3C6H5O7 and sodium carbonate Na2CO3. As solids, they don’t interact with each other. They react when we add water (the anthocyanin solution)!

Both citric acid and sodium carbonate split into ions in water. Citric acid yields citrate anions C6H5O73– and hydrogen ions H+, while sodium carbonate yields carbonate anions CO32– and sodium cations Na+.

The carbonate anion CO32– reacts with two hydrogen ions H+ to form carbonic acid H2CO3, which is unstable and easily decomposes, yielding water H2O and gaseous carbon dioxide CO2↑. These processes happen rather rapidly, so a lot of gas forms in a very short time, causing the foam to erupt!

What is anthocyanin and what do we use it for?

Anthocyanins are natural dyes that are often used as food coloring. They are usually extracted from grapeskin, but can also be found in many other fruits, berries, and plants. Raspberries, blueberries, blackberries, and cranberries all owe their color to anthocyanins!

We add anthocyanins to our experiment just for fun. Everything would work just as well without them, but our bubbles would be totally colorless if we were to skip the first step (adding and dissolving some anthocyanins).

Why isn’t the color of the foam more uniform?

This is also the anthocyanins’ doing. The foam can actually have several different colors, predominantly pink, violet, and blue. Anthocyanins can react to the presence of some acids and bases in the media they are dissolved in. Indeed, citric acid is acid and causes anthocyanins to turn pink or even red when the two interact. And, as sodium carbonate is a base, anthocyanins turn blue when they encounter sodium carbonate particles. You can learn more about this phenomena here in the «Anthocyanins» experiment.