Chemistry

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Video

Materials

• Dry ice - About the amount of a 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 glass
• Phenolphthalein
• Water
• Maybe: Something alkaline (such as dishwasher detergent or drain cleaner)
• Safety equipment: 1 glove or 1 pair of tongs

Short explanation

Carbon dioxide sublimates at room temperature. That is, changes from solid to gas. Some of the carbon dioxide gas dissolves in the water and causes the water's pH value to drop. Phenolphthalein is a so-called pH indicator, and shows that the pH value has dropped by changing color.

Long explanation

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 this experiment, room temperature water is used. Hot water causes the ice to evaporate quickly and a lot of bubbles and smoke are formed, obscuring the view. Therefore, you should also not use too large a piece of ice.

Carbon dioxide makes water acidic. When carbon dioxide dissolves in water, carbonic acid is formed:

CO₂ + H₂O → H₂CO₃

However, it is only a small part of the carbon dioxide that reacts chemically with the water and forms carbonic acid. The majority (more than 99 %) of the carbon dioxide does not react with the water but remains carbon dioxide. However, the word carbonic acid is also often used colloquially for carbon dioxide dissolved in water, or for carbon dioxide bubbles in water.

Carbonic acid has two hydrogen ions that can detach. Like this:

H₂CO₃ → HCO₃^- + H⁺ HCO₃^- → CO₃^2- + H⁺

Phenolphthalein works well as an indicator for carbon dioxide experiments, as only a small pH change is needed for a big difference in color (from pink to transparent). But this only applies when the pH is just above 8.2 before adding the ice. That is because it is at 8.2 that phenolphthalein loses its pink color and becomes transparent. That's when the phenolphthalein molecule takes two hydrogen ions from the water and becomes a new variant of itself - a variant that no longer reflects light but instead is transparent.

The pH value is a measure of how many hydrogen ions (H⁺) there are in a solution. The more hydrogen ions, the more acidic a solution is said to be. Anything below 7 on the pH scale is acidic and anything above 7 is basic. or alkaline. The pH value is very important in many ways. That is because hydrogen ions are small, charged particles that have the ability to change all kinds of molecules, either by getting stuck to them or by breaking them.

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 larger piece of dry ice?
• What if you use hot water?
• What if you use ice cold water, with water ice cubes in it?
• What if you add dishwater detergent, right after the liquid has become transparent

Variations

Of course, you can use hot water for more bubbles and smoke, although it can be difficult to see the glass then.

Since you exhale carbon dioxide, you can also blow into the liquid using a straw to make it transparent!

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