Chemistry

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Video

Materials

• Dry ice - About 1 tablespoon (15 mL) 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 tall and narrow container (such as a vase or a large graduated cylinder)
• Water
• Food coloring
• 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 in warm water, and lots of bubbles of carbon dioxide gas are formed. The carbon dioxide gas is extremely cold and cools the air. This causes water vapor in the air to condensate, that is change from gas to liquid, and become visible as smoke.

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.

When carbon dioxide ends up in water, sublimation goes faster than in air. That's because water conducts heat better than air. Large amounts of carbon dioxide gas are quickly formed, which are visible as bubbles in the water.

Smoke is also produced. It is not carbon dioxide, but water. The carbon dioxide gas that ends up in the air is extremely cold, which causes water vapor in the air to condensate into small water droplets, which are visible as smoke. This smoke is cold, which means that, just like carbon dioxide gas, it flows over the edge of the table down to the floor.

In the water, carbonic acid is formed. This is because a small percentage of the carbon dioxide gas reacts with the water and forms carbonic acid. If you would taste the water, it would taste a bit sour - just like in a carbonated beverage.

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 more dry ice?
• What if you use cold water?
• What if you use boiling hot water?
• What if you use milk?
• What if you place some lit tealights around the bowl?

Variations

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