You’ll need

  • A 2 L bottle of soft drink – diet cola works best, but any kind will do
  • Hot water in a large bowl or sink
  • 1 roll of Mentos lollies
  • Blu Tack
  • A large, open, outdoor space with a flat surface (use a flat plastic container if the ground isn’t flat)

What to do

Warning!

The experiment can get messy! Do it in a large, open space people can move away from the experiment, like a backyard.

  1. Sit the soft drink bottle upright in the hot water for 30 minutes.
  2. Remove the Mentos from their wrapper. Stick about 7 of them together using bits of Blu Tack so they sit in a straight line (like they did in the roll).
  3. Move the soft drink bottle from the hot water to a flat surface outside. Put it in the plastic container if the ground is uneven.
  4. Unscrew the lid from the bottle, drop in the stack of Mentos and quickly move back … way back!
  5. Watch what happens!

Questions to ask

How big was your fountain? Can you change the results?

Try using a soft drink that has come straight from the fridge or one at room temperature.

Try using rock salt, silver coins or marbles instead of Mentos.

Try coating the Mentos lollies in vegetable oil.

What's happening

Soft drinks are mostly made of sweetener, water and dissolved carbon dioxide (also written by its chemical formula CO2). The carbon dioxide stays dissolved in the water for a few reasons. One is pressure. When the lid is on, the gas cannot expand and the pressure helps keep the carbon dioxide in the water. When you open the bottle you hear a fizz of escaping gas, even if you don’t shake it, because the pressure is released.

A second reason is that water works to keep the carbon dioxide molecules apart. Water molecules (also written by its chemical formula H2O) join together to form a network. This works like a net, stopping many carbon dioxide molecules from collecting into bubbles. Once they form, bubbles can float up and escape. This is why a soft drink doesn’t go flat as soon as you open the bottle.

The surface of each Mentos lolly has many tiny bumps and gaps that act as nucleation sites. (Nucleation is a physical reaction in which a change in physical state, like melting or boiling, begins at a small site and then spreads.) Carbon dioxide molecules like to gather at these sites. This creates a lot of bubbles. These bubbles rush to the surface of the soft drink, explode into the sky, and take a lot of liquid with them.

Also, chemicals in the Mentos lollies and the soft drink may react to help release the dissolved carbon dioxide. Mentos lollies contain a substance called gum arabic, which is thought to break through the surface of the drink. This allows carbon dioxide gas to escape faster. But, many objects that don’t contain gum arabic, like 5-cent coins, can also cause a spectacular soft drink explosion. While gum arabic probably does help the chemical reaction, it is likely working with the physical process of nucleation.

Did you know

A chemical called silver iodide has a crystal structure similar to ice particles. Silver iodide can act as a nucleation site for water vapour in clouds. In a process called cloud seeding, people put silver iodide crystals in clouds. Water collects on the crystals, coating them with ice. The ice keeps growing until, finally, the ice particles fall to the ground. If it’s warm, they melt and fall as rain. If it’s cold, they fall as snow. This technique is used in Australian mountain regions to help make snow.

Transcript

Steve: Hi, my name’s Steve and I’m from the Questacon Science Squad and today we’re going to do one of my favourite activities, the Mentos Fountain.

You need:

  • a bottle of cola that has been sitting in a bucket of hot water for about 30 minutes. Now, I find that diet cola works much better than regular cola
  • some blu-tac
  • some Mentos and,
  • a bucket to catch all of the mess

The first thing you want to do is get a piece of blu-tac, stick it onto one of your Mentos. Do that about seven times and stick them all together so that you get this tower of Mentos like I have here.

(Steve shows his completed example)

Now comes the fun part, but we’d better head outside to minimise the mess. Let’s go!

(Steve moves to his outside location)

Carefully sit the warm bottle of cola in the container.

(Steve places the cola into his container)

Open the bottle. Drop in the Mentos and… you get a HUGE FOUNTAIN!

(The camera zooms out as Steve places the Mentos into the bottle of cola to capture the cola exploding)

To find out what’s happening with the Mentos fountain, head to the Questacon Science Squad website. See you next time.