Mentos Mayhem

    Kids, why do Mentos mints dropped into a can of soda make a foamy fountain?   One might guess that the acid in the soda might be reacting with some kind of carbonate in the mint coating to create CO2 carbon dioxide fizz.  Mentos have a strange chalky color and texture and they do taste a bit like antacid (calcium carbonate) tablets.  However, the ingredients do not include carbonates or, for that matter, any other significantly alkaline material.   Mint Mentos contain sugar (sucrose), glucose, coconut oil, starch, emulsifiers, natural flavor, and gum arabic.  They are pretty much just big pellets of flavored sugar with gummy stuff added to give them structural integrity.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    Drop a Mentos directly into a freshly opened full can of soda.  But wait!   First, make sure that the can is in a sink or tray to collect the significant amount of foam that will spill over.  In our labs, a mint Mento and a diet cola provided the most foam, causing about half of the soda to be lost.  Using the can makes the foaming more spectacular than if you poured the soda into a glass because of the small opening.

    So why do Mentos make soda foam up?  It's a physical reaction, not a chemical one.  Ordinarily, water resists the expansion of bubbles in the soda.  Water molecules attract each other strongly, forming a tight mesh around each bubble.   It takes energy to push water molecules away from each other to form a new bubble, or to expand a bubble that has already been formed.  The property is called surface tension.  The oils, emulsifiers, and gum arabic from the dissolving candy disrupt the water mesh, so it takes less work to expand bubbles.   At the same time, the roughness of the candy surface provides many little nooks and crannies (more surface area) that allow new bubbles to form more quickly.  As more of the surface dissolves, both processes accelerate, and foam rapidly begins to form.

    You can see a similar effect when cooking potatoes or pasta in a pot of boiling water.  The water will sometimes boil over because organic materials that leach out of the cooking potatoes or pasta disrupt the tight mesh of water molecules at the surface of the water, making it easier for bubbles and foam to form.   Root beer can also foam over if a scoop of ice cream is added, for essentially the same reason.  The surface tension of the root beer is lowered by gums and proteins from the melting ice cream, and the CO2 outgassing from the root beer blows the foam.

    Test this hypothesis by dropping a Mentos into orange juice or any acidic but non-carbonated drink, or by dropping a Mentos into completely flat soda.   What happens?  Why?  (Mentos is a registered trademark of Van Melle USA Inc.) 


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    September 2004


    Experiment suggested by Steven S. Trail (BP Chemicals).
    As an interesting sidelight, teacher Lee Marek of Naperville North High School, Naperville, IL, developed this into a demo for the Letterman show.

    Fred Senese,