The Mentos and Diet Coke Geysir

Last week touring the local schools in the Harstad-area, I demonstrated the Mentos eruption as part of the theme solids, liquids and gases. The experiment is simple; make a tube of paper where you can stack a handful of Mentos and place a card underneath the tube. Place the tube and the card on top of an opened Diet Coke (or similar), remove the card and see the Mentos fall into the soda. Evacuate the site and watch the geysir burst into the air.

The phenomena was first demonstrated by Steve Spangler, a science educator and television host from USA, back in 2005 and has since become a TV and internet phenomena. Steve Spangler lauched a geysir reaching 5. 5 m using a 2 litre coke bottle, live on TV. TV-shows like the Letterman Show and Mythbusters has blown away multiple bottles of Diet Coke, and lots of videos showing the eruption are posted at YouTube. Even at Flickr there is a group called Planet Mentos. Today's record is 10.4 m, set by the Mythbusters. They demonstated that freezing the Mentos increased the effect, because gum arabic expands when frozen, the mentos become more porous and cause the reaction to speed up.

Although many might think it is a waste of soda and candy, there is some good science in there. Who has not seen the bubbles formed in the soda when something is accidently dropped into the glass? When using the Mentos candy, you see the extreme version of the same phenomena.

The gas is trapped in the soda due to the water molecules. When Mentos is dropped into the Diet Coke, the gelatine and the gum arabic coating dissolves and breaks the surface tension of the soda. The mesh of water molecules is disturbed and the carbon dioxide easily form bubbles. Mentos has thousands of tiny pores on their surface, and these function as nucleation sites where the gas bubbles form. As the rather heavy candy falls to the bottom of the bottle, carbon dioxide is released and the suddenly increased pressure pushes the liquid out of the bottle.

As I discovered on my tiny tour, this experiment create enormous amounts of enthusiasm and curiosity among the kids. I encouraged them and their teachers to continue experimenting with this; trying other types of soda and Mentos, other candies and heavy oval objects, and varying the number of Mentos used. Exploring which ingredients causing the effect, is scientific investigation in it's full extent. I am awaiting feedback from the kids in the Harstad area, and hope others also wish to try this fun backyard experiment.