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Mentos Fountain

This explosive experiment explores heat production, change of state and pressure. Get set for a dynamite result!

What you need

  • 1 packet Mentos lollies
  • 2 L bottle of soft drink (cola works well and the diet varieties are easier to clean up)
  • Blu-tac
  • Hot water in a large bowl or sink

What to do

Safety: It is best to do this activity outdoors as it is very messy. This activity requires a large, open space to allow people to stand far away from the experiment.

  1. Sit the soft drink bottle in the hot water for 30 minutes.
  2. Remove the lollies from their wrapper and stick them together using small amounts of Blu-tack so they sit in a straight line (like they did in the packet).
  3. Place the bottle of soft drink on a flat surface outside.
  4. Unscrew the lid from the bottle, quickly drop in the stack of Mentos and stand back... way back!

What's happening?

There are many explanations for the ‘Mentos Fountain’. Some people say it’s caused by a chemical reaction, similar to vinegar reacting with bi-carbonate of soda, while others say it’s caused by a physical reaction, like ice melting and turning into water. The best explanation includes both ideas.

Soft drinks are made mostly of sugar, water and dissolved carbon dioxide. The carbon dioxide is held in the drink by strong chemical bonds between water molecules. The water acts like a net, holding each molecule of carbon dioxide in the cola so the molecules of carbon dioxide can’t join together to form bubbles of gas. 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 which can act as ‘nucleation sites’. Nucleation sites are places where nucleation can occur quickly and easily. Nucleation is a physical reaction and is the process by which a substance changes its physical state in a small area. The formation of an ice crystal in a glass of water, or the formation of a water droplet in a cloud of vapour, are both examples of nucleation.

When the Mentos lollies are dropped into the soft drink, tiny molecules of dissolved carbon dioxide rapidly clump together at the nucleation sites all over the surface of the lolly, forming bubbles. Almost immediately, these bubbles rapidly float to the surface of the soft drink, exploding into the sky and taking a lot of liquid with them.

Meanwhile, chemicals in the Mentos lollies and the soft drink may also help release the dissolved carbon dioxide through chemical reactions. Mentos lollies contain a substance called gum arabic which is theorised to break the surface tension of the drink, allowing carbon dioxide gas to bubble up and escape.

However, this theory is questionable as many objects which contain no gum arabic whatsoever (such as five cent coins), can also cause a similarly spectacular soft drink explosion. While gum arabic probably does contribute to the reaction, most likely it isn’t the only cause but it may work in conjunction with the physical process of nucleation.

Investigate!

How big was your fountain?

There are lost of things you can change in this experiment that might give different results. Why not try using a soft drink that has come straight from the fridge or one at room temperature? What about substituting the Mentos lollies with rock salt, silver coins or marbles? What happens if the Mentos lollies are coated in vegetable oil?

Did you know?

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 deposits on the crystals, coating them with ice and the ice keeps growing. Finally, the ice particles fall to the ground. If it’s warm they fall as rain and if it’s cold they fall as snow. This technique is used in the Australian alpine regions to help encourage snow falls.