Science: How Warm and Cold Air Behave

23 05 2010

In science, we’re still studying air (in our Childcraft science text). Today, we read about how  powerful air is (demonstration: blow up a balloon and let it go. See? Powerful!) and how people use it (exhaust fans, clothes dryers, wind turbines, food preservation, how helicopters fly, etc.). We also learned about storm fronts, and how air masses move around the planet, and what happens when warm and cold fronts collide. Finally we did an experiment to illustrate the differences in how warm and cold air behave.

To do the experiment, you’ll need four identical bottles (we used brown glass beer bottles courtesy our brewer friend Charles Bristle), four identical balloons (we used green water balloons), equal amounts of very hot water, room temperature water, and ice water, and three identical bowls big enough to hold your water plus a bottle standing upright.

The experiment is simple. You stretch a balloon to fit over the neck of each bottle. You pour one temperature of water into each bowl. Then, all at once, you stand each balloon-covered bottle upright in a bowl of water, leaving one balloon-covered bottle outside a water bath as a control. What happens next is quite impressive:

The bottle in the middle is in room temperature water, and it looks the same as the bottle that was not in the water at all, and also the same as all the bottles looked before going into their baths — the balloon is taught and upright, but not expanded. The bottle on the right is sitting in near-boiling water, and it immediately puffed up and expanded. The bottle on the left is, as you can see, in ice water, and as the air sunk and shrunk, the air was drawn out of the balloon and it collapsed. This all happened immediately and was dramatic and cool to watch.

Here’s how Floppy recorded it in his lab notebook:

The text reads: “5-23-10. The boiling hot water’s balloon puffed up so fast and the 2nd one did it slower and the third one got smaller. This happened because it’s hot air and hot air expands. It [the cold air] went into the bottom of the bottle and got smaller. And the air in the controls didn’t do anything.”

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