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Coke and Mentos Rocket Car

This demonstration unleashes the power of candy and soda to propel a rocket car.

Rocket propulsion works because of pressurized gas. When the gas escapes the rocket (going down), it pushes the rocket up.

If we turn the rocket on its side, the gas will push behind the rocket to propel it forward. Put some wheels beneath it and you have a rocket car! We can use a variety of methods to create pressurized gas, the following activity uses Mentos and Diet Coke.

Mentos and soda “Rocket Fuel”

Soda is fizzy because it contains molecules of carbon dioxide (CO2). When it’s poured into a glass, the carbon dioxide molecules separate from the water by forming bubbles—a process called nucleation—and escaping into the air.

After a while, the soda will go flat, but this process takes some time and isn’t powerful enough to propel our rocket car. By adding Mentos candy to the soda, we can speed up the nucleation process to cause a geyser powerful enough to spray high into the air or, in this case, move a rocket car.

Gum arabic on the Mentos’ surface breaks up the surface tension of the water in the pop. The surface of Mentos is rough and filled with tiny dimples and holes that are great spots for nucleation because of how they increase the surfacearea available for reaction. When Mentos and soda come into contact, lots of CO2 bubbles form very quickly and cause a "geyser"-like eruption.

Although any carbonated beverage will produce a similar effect, we recommend aspartame diet beverages for two reasons:

  1. Aspartame, the artificial sweetener , speeds up the reaction even more because water with aspartame in it has a lower surface tension than sugary water (this allows the bubbles to escape more easily)
  2. It’s much less sticky to clean up!

Objectives

  • Explore and demonstrate the effects of action and reaction forces.

Materials

  • Per Class:
    an outdoor space (e.g. playground or yard)
    1 L of diet Coke (room temperature)
    an old skateboard or piece of wood and four wheels
    duct tape
    masking tape
    5 Mentos candies
    hammer

  • Per student:
    safety goggles

Key Questions

  • What is making the rocket move forward?
  • How could we make the rocket move faster?

What To Do

Activity

Wear safety glasses, conduct in a wide open space.

  1. Using duct tape, tape the bottle onto a skateboard (or similar) so that the cap overhangs the end of the skateboard.
  2. Tear off a 20cm strip of masking tape.
  3. Stack 5 Mentos candies on the sticky side of the masking tape and wrap the tape lengthwise around the candies to create a package. The sides of the candies should be exposed.
  4. Remove bottle cap and use double sided tape to secure mentos package to the inside of cap. When you replace cap you don’t want the mentos to touch the coke before securing the lid so you may need to pour out a small amount.
  5. Replace cap tightly.
  6. Shake the bottle to build up even higher pressure inside. The bottle will feel very firm.
  7. Standing to the side of the skateboard, strike the bottle cap with the hammer (to disrupt the threads). The pressure inside the bottle should be enough to blow the cap off and propel the skateboard in the opposite direction.

Teacher Note: Warm diet coke works best

Extensions

  • Experiment with different kinds of candy and pop. Which combinations work the best? Which combinations don’t work as well?
  • Experiment with different ways to introduce the candy and seal the bottle.