Chemistry

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Video

Materials

• Dry ice - Either block or pellets.
• 1 towel
• 1 coin
• Safety equipment: 1 glove or 1 pair of tongs

Short explanation

Carbon dioxide sublimates at room temperature. That is, changes from solid to gas. When you press metal against the ice, this process speeds up. The coin pushes on, and the gas from the ice pushes away, resulting in vibrations.

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.

A coin is made of metal and metal is a great conductor of heat. The coin conducts heat from the air to the ice. Therefore, the ice sublimates quickly when it comes into contact with the coin. Carbon dioxide gas lifts the coin and the coin falls by gravity back down. In this way, it vibrates and hits the ice, which leads to the creation of sound.

The vibration of the coin against the ice propagates into the air. The air also begins to vibrate. When the ice or the coin pushes on the air, the air in front is compressed (the particles are packed more tightly). When the ice or the coin swings back, the air becomes thinner again. These alternating thickenings and thinnings spread in the air as waves. When the sound wave hits your ears, you hear the sound.

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 push harder?
• What if you use other metal objects?
• What if you use something made of wood or plastic?
• What if you place a coin in between two blocks of dry ice?
• What if place a block of dry ice on top of 10 coins?

Variations

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