Air is a Mixture of Gases

This year we have switched over to Building Foundations of Scientific Understanding. We are focusing on Chemistry topics, and so are working through primarily A thread in Volumes I and II.

This past week we’ve been working on topic A7,  Air: A mixture of gases (mixtures and chemical reactions).

Air is an excellent example for understanding molecules, mixtures and chemical reactions. Air is made up of nitrogen, oxygen and carbon dioxide. Oxygen is required by animals for respiration, and carbon dioxide is produced as a waste product. The oxygen and carbon dioxide in air are held constant in the atmosphere by green plants, which use carbon dioxide and emit oxygen. We’ve covered photosynthesis previously, in Biology as well as earlier this year when learning about energy, so this cycle is familiar around here. Here is a video that sums covers the Carbon Cycle.

We also love the app My Incredible Body  and learned all about respiration to get a better understanding of how we as animals breathe in air to bring oxygen into our body, and breathe out to expel carbon dioxide.

Air is made up of matter and has weight

Sometimes when we can’t see something, like air, it can be a little difficult to understand how it is made up of matter. To help visualize this, we set up this simple experiment. We started by tying string around the center of a wooden dowel, making sure the dowel was balanced. We then blew up two balloons, one much fuller than the other. We tied each balloon to an end of the dowel, spacing them the same.

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Kyri is a pretty smart cookie, so she expected the balloon that was inflated more to weigh more as well since it contained more air. As expected, the heavier balloon pulled its end of the dowel down lower!

Burning Requires Oxygen

We next set up a simple demonstration. Placing a tea light in a shallow plate of water (be sure to not cover the candle!), we then covered the candle with a jar and observed how, after just a short time, the flame sputtered and then was extinguished. I explained that the wax was the fuel (potential energy) and burning released this potential energy as kinetic energy in the form of heat and light. Oxygen is necessary for the fire to burn the fuel. Air is made up of approximately 20 percent oxygen, so in an enclosed jar, the oxygen is used up fairly quickly. Once the oxygen is used up, the flame goes out!

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Mixtures and Chemical Reactions

When we started discussing mixtures, we used some simple examples to understand – a mixture of coins or a bag of mixed candy. In those simple examples, the individual components didn’t change and were identifiable. But sometimes when components are mixed, a reaction occurs and the components are broken apart and new components formed. Our second demonstration involved the release of carbon dioxide during a chemical reaction.

Baking soda and vinegar can be mixed, and when they react carbon dioxide is released. This is a fun chemical reaction because of the intense fizzing!

Burning Releases Carbon Dioxide

Similarly, when a candle is burned (also a chemical reaction!), carbon dioxide is also released. Carbon dioxide, like air, is not visible. But we used a fun demonstration to produce and observe carbon dioxide.

We lit a tea candle in a shallow plate, and in a half-pint mason jar combined 1 Tbsp baking soda and 1/4 cup vinegar (Pour slowly to prevent fizzing over!). The carbon dioxide that is produced from the reaction remained in the jar and because it is heavier than air, can actually be “poured” like a liquid. We carefully tipped the jar toward the flame, as if we were pouring but taking care to not pour out any of the liquid contents. While we couldn’t see the carbon dioxide directly, we could observe the flame sputtering and finally going out, as the carbon dioxide poured over the wick and prevented the flame’s access to oxygen.

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