Chemistry

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Materials

• 1 roll of Mentos mint (it must be mint)
• A 1.5 or 2 L bottle of Diet Coke (Coca-Cola Light)
• 1 tube, open in one end - This tube must fit all of the Mentos stacked on top of each other, just like in the Mentos roll. And when open in one end, all the Mentos should quickly slide out. In some countries, there are these aspirin tubes that are perfect. Otherwise, make a tube out of paper and tape, slightly wider than the Mentos roll.
• 1 plastic card, such as a playing card or a discount card

Short explanation

When a soda bottle is opened, the pressure in it decreases, which leads to carbon dioxide leaving the soda in the form of bubbles. This bubble formation occurs to a greater extent on uneven surfaces. A Mentos candy is full of small craters and is therefore an excellent place for carbon dioxide to form bubbles. Bubble formation occurs so quickly that almost all the carbon dioxide leaves the soda at once, at the same time as it pushes the soda away in front of it.

Long explanation

Who would have thought that Mentos and Diet Coke would create such a violent eruption? This demonstration has become a modern hit, like the baking soda and vinegar of our time.

Carbon dioxide (CO₂) can be dissolved (mixed) in water. This happens to a greater extent if the carbon dioxide molecules are pushed in between the water molecules under high pressure. Otherwise, the carbon dioxide molecules have a tough time penetrating the dipole bonds that water molecules have between each other. But in the manufacturing process of carbonated drinks, high pressure is used to counteract this.

When carbon dioxide is mixed with water, a small part of the carbon dioxide chemically reacts with the water and becomes carbonic acid (H₂CO₃). But more than 99 % of the carbon dioxide doesn't react with the water and remains as carbon dioxide. However, the word carbonic acid is sometimes used for carbon dioxide dissolved in water.

Carbon dioxide, like all pure compounds, can exist in different states of matter. In everyday life, the four states of matter that are commonly observed are solid, liquid, gas and plasma. In nature, you usually encounter carbon dioxide as a gas, more specifically as part of air. When carbon dioxide is dissolved in water, it can be said that it changes from a gas to a liquid. But this is to put it simply.

The correct answer is a little more complicated. Carbon dioxide in water is a mixture, and it's the mixture whose state of matter is liquid. But the carbon dioxide no longer has its own state of matter. Sounds strange? Yes, and that is because the concept of states of matter is not enough here. Instead, one needs to use the more accurate chemical term phase. There are only a few states of matter, but there are countless phases. A phase is a uniform region of a material, where the constituents are homogeneously distributed and there is no division into parts. Gaseous carbon dioxide is an example of a phase - here all carbon dioxide molecules are evenly distributed and there is no division of the carbon dioxide into two different parts. Likewise, the mixture of water, carbon dioxide and the other substances dissolved in the soda is another phase. Here, all molecules (and ions) are evenly distributed and there is no division of the soda into different parts. If, on the other hand, you put ice cubes in the soft drink, there would be two phases present - the soft drink mixture and the ice cubes. When carbon dioxide is dissolved in water, there is a phase change - from the phase "carbon dioxide in gaseous form", to the phase "mixture of water and carbon dioxide (and other substances) in liquid form".

Anyway, when you open a soda bottle, the pressure in the bottle decreases, causing carbon dioxide to leave the water. Remember - this is because the water in the soda doesn't really like carbon dioxide being there. Water molecules have quite strong dipole bonds between them, and the only way to get carbon dioxide in between them was to push them in there under high pressure in the manufacturing process and then put the cap on. But now, the pressure drops, and water "spits" them out. This process starts as soon as you open the bottle. But why does it happen so fast when you drop in a Mentos?
The expulsion of carbon dioxide is seen as bubbles rising to the surface. Bubbles form most easily on surfaces. Such as on a bottle's inside or on microscopic dirt particles in the liquid. These sites are called nucleation sites, because the first seed of a bubble is called the nucleus in Latin. A good nucleation site has a lot of surface (for example of a bottle) around it, and not a lot of liquid around it. This means that pits are the very best nucleation sites. The explanation for this is that, the smaller the contact surface between the liquid and the bubble is, the less energy is required to separate and move the water molecules in that contact surface (which are joined together by dipole bonds). Especially good are pits that the water, because of its surface tension, doesn't even get into. Here, carbon dioxide molecules can escape the water and germinate into a bubble in peace and quiet. When the bubbles are large enough, the lift force, which occurs due to the water and the bubbles having different densities, lifts them towards the surface.

A Mentos mint candy has a very uneven surface with plenty of places for nucleation to take place. That is thought to be the main reason for the Diet Coke and Mentos eruption.

There are also other factors that might contribute to the strong effect. Gum arabic and gelatin in the candy dissolve quickly in water, i.e. settle between the water molecules, and interfere with their dipole bonding. This happens quickly because the uneven surface of the Mentos candy gives it a large contact surface. In addition, potassium benzoate and aspartame in Coca-Cola light are also thought to contribute to the effect in some way.

Exactly why Diet Coke seems to work best is still under debate (this is a pretty important issue). Also, it's actually not entiry clear that Diet Coke is the best. Maybe some other diet soda with aspartame works just as well?

An advantage of diet sodas is that they are not as sticky to clean up as regular sodas.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• How is the eruption affected by changing the Mentos mint to other candys?
• How is the eruption affected by changing the Diet coke to other drinks?
• How is the eruption affected by the temperature of the Diet Coke?

Gilla:		Dela: