Science Experiment: Diet Coke and Mentos Geyser

My preschoolers love experimenting with reactions, and the classic Diet Coke and Mentos reaction is so popular it's been requested and repeated more than once!

The INGREDIENTS are simple:

• Diet Coke (you will want several bottles!)
• Mint-flavored Mentos  (at least two rolls!)
• Other mix-ins  (rock salt, nails, and other candy are interesting ones, but let your child choose what she wants to experiment with!)

The HOW TO is fun:

1. Plan how you will put 5 Mentos into a 2-liter bottle of Diet Coke at the same time.  I've used a roll of paper, but prefer to thread them onto a string with a needle.  It doesn't take long to thread, and is so much more effective.  
2. Drop 5 mint-flavored Mentos into a 2-liter bottle of Diet Coke at the same time.  Step back fast!
3. Oooh and aaah!!  Note: the reaction only lasts 5-6 seconds, and the kids usually want to do it again...and soon!
4. Talk about the reaction on whatever level your child can understand.  Then invite her to experiment with other reagents.  Is there anything else that makes as big a reaction as the Mentos/Diet Coke combination?

What does make the reaction?  Interestingly enough, there is still considerable debate about the cause (it's even been on Mythbusters!), but this is the most likely explanation.  As you know, Diet Coke is a very carbonated drink.  The molecules of carbon dioxide (a gas) are surrounded by molecules of water.  The surface tension of water is quite strong (strong enough for insects and even the basilisk to run across the top of water!), but there are lots of ways to break it.  If you shake the bottle before you open it, you release a lot of carbon dioxide that will rush out the top of the bottle when you open it.  When you drop certain objects (or candy) into the drink, you also break the surface tension and release carbon dioxide.  Mentos contain gum arabic and gelatin that seem particularly effective in breaking the surface tension.  Mint Mentos are especially exciting because they have many little holes on their surface (called nucleation sites) that the carbon dioxide will cling to, making the reaction even more explosive!  Fruit-flavored Mentos do not have the nucleation sites (you can feel how smooth they are), and do not make nearly as exciting a reaction.

So, to summarize, the Diet Coke/Mentos reaction seems like a physical reaction, not a chemical reaction.  The final products are the same as the initial ones, but they combine to form a geyser that is fun and exciting!


I'll be sharing this post at some of these linky parties!
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