Chemistry

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Video

Materials

• Dry ice - About the amount of one sugar cube is needed for this experiment. Either block or pellets.
• If you buy a block:
  • 1 towel
  • 1 hammer (or something else to break the ice)
• 1 balloon
• 1 bowl
• Water
• Safety equipment: 1 glove or 1 pair of tongs

Short explanation

Carbon dioxide sublimates at room temperature. That is, changes from solid to gas. This happens faster the warmer it is, and lots of carbon dioxide gas is formed. Here you capture the gas in a balloon that inflates. This demonstration also shows how the solid carbon dioxide just "disappears", without leaving any wet traces behind.

Long explanation

Every pure substance can exist in different forms, each with distinct properties. These different forms are called states of matter. In everyday life, the three states of matter that are commonly observed are solid, liquid and gas. Then there are a few more that exist at extremely high or low temperatures, such as plasma, Bose-Einstein condensate, and quark-gluon plasma.

In a solid state, the pure substance's particles (atoms, ions or molecules) are tightly packed and stuck together. In a liquid state, they are still close together, but can move relative to each other. In a gaseous state, they have completely separated from each other. The factors determining the state of a pure substance is pressure and temperature. At high pressure and/or low temperature, a pure substance is solid. At low pressure and/or high temperature, a pure substance is a gas. In between, the pure substance is a liquid.

There are words for when a substance changes from one state of matter to another:

• Melting: solid → liquid
• Sublimation: solid → gas
• Vaporization: liquid → gas
• Condensation: gas → liquid
• Deposition: gas → solid
• Freezing: liquid → solid

Liquid carbon dioxide can be formed at pressures of 5.1 atm or higher. But under normal pressure, carbon dioxide changes directly from a gaseous to a solid state (deposits) and vice versa (sublimes). This takes place at 78.5 °C (-109.3 °F). This is the reason solid carbon dioxide is called dry ice, due to the fact that is doesn't melt.

The reason carbon dioxide "skips" its liquid state and sublimates/deposits is that the molecule is completely symmetrical and thus not a dipole. This means that the separate molecules are only held together by weak van der Waals bonds (and not also dipole bonds like water molecules). As soon as they get enough kinetic energy to break free from each other, they do so completely.

Carbon dioxide as a gas is odorless in low concentrations. In higher concentrations, however, it smells sour. The gas is always colorless. The density of carbon dioxide gas is 1.67 times that of air, which means it can flow down the edge of a table and settle down on the floor.

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.

• What if you use a smaller or larger bowl?
• What if you use colder or warmer water?
• What if you use more or less dry ice?
• What if you don't use a bowl of water, but instead just place the balloon on the table?
• What if you don't tie the balloon?

Variations

This demonstration can be done without hot water. It just takes longer.

It's a bit tricky and risky to get an ice cube into a balloon (especially for younger children). A version of the demonstration is to put the ice cube in a bottle and thread a balloon over the opening.

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