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22/10/2014

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The science behind

Bubbles are made from a gas (either air or carbon dioxide). The bubbles make up the fountain so the more bubbles there are, the more explosive it will be!

Diet cola contains aspartame. This is what sweetens the cola instead of sugar and is the main ingredient that causes the explosion. Potassium benzoate is a preservative added to all fizzy drinks and this also contributes to the explosion.

Up close, mentos have a rough texture which provides lots of sites for bubbles to form. Gum arabic is used in mentos coating to provide the chewy texture. The sweet’s density is also important.

What are nucleation sites?

The cola has high surface tension, which means that its water molecules like to be next to each other and there are strong forces between them. To overcome this high-surface tension and create bubbles, you need nucleation sites. The gas molecules in the liquid gather together at the nucleation sites and break up the chain of water molecules, forming a bubble! To begin with, the bubble will be very small and stay at the nucleation site. Eventually, enough gas molecules will join the bubble and it will float to the top of the liquid.

Why does the density of the mentos matter?

Mentos are heavy enough to sink in the cola. The whole time they are sinking, nucleation is occurring. The bubbles also act as nucleation sites, so bubbles lead to more bubbles, making a huge explosion!

What are surfactants?

Surfactants are key to creating an explosive mentos fountain. Surfactants are long molecules with one end that likes water and one end that doesn’t like water. Surfactants reduce the surface tension of liquids by disrupting and weakening the forces between the water molecules, so there's less stress on bubble walls. The aspartame and potassium benzoate in the diet cola and gum arabic in the mentos make bubbles form more easily and last longer.

Are there other factors?

The higher the temperature of the cola, the more explosive the reaction is because the carbon dioxide wants to leave the liquid when it is hotter. The reaction is not effected by the presence or absence of caffeine or the cola’s pH.

Summary

  • Cola: Diet colas work best because they contain sweeteners which reduce surface tension more than the sugar in regular colas.
  • Mentos: Mentos have a rough texture that provides lots of sites for bubbles to form. In addition, the sweet's coating contains gum arabic, which reduces surface tension in the liquid.
  • Density of the sweet: Dense sweets sink rapidly and create bubbles that seed further bubbles as they rise.

What things have you discovered affect your fountain?

Email us your results at mentos@britishscienceassociation.org

 

Reference: Coffey, T.S. (2008). Diet Coke and Mentos: What is really behind this physical reaction? American Journal of Physics. 76, 551.


From the chemistry of gunpowder to the anatomy of high explosive detonations, the dissection of an internal combustion engine and the might of the hybrid rockets that launch man into space, KABOOM! uncovers the science behind the explosions that shape out modern world. Find out more at www.gregfoot.com/kaboom.

National Science & Engineering Week is a programme of science, engineering and technology events across the UK aimed at people of all ages. Anyone can organise an event or activity, from schools to community groups and from parents to large organisations. The resulting programme of over 4500 events and activities is a hugely varied and eclectic celebration of science. Visit www.nsew.org.uk for more information.