This week’s science project is pretty simple and straightforward, but it’s another good illustration of how heat affects molecular movement. It’s based on this NASA demonstration.
To do the demonstration with your own students, you’ll need three basins, an empty plastic water bottle, an empty glass soda bottle, and two balloons.
Put a quart of ice water in the first basin, a quart of room-temperature water in the second basin, and a quart of hot water in the third basin. (I used my coffeemaker to heat the water for the third basin, but you could just as easily use a microwave. You want it hot but not boiling.) I let the kids get the ice water and room-temperature water for me, but I handled the hot water myself to make sure nobody got scalded.
Ask two students to prepare the bottles by stretching balloons over the mouths. Once the bottles are ready, ask the class to predict what will happen when you place each bottle in each basin.
Have a student start the experiment by holding the glass bottle in the cold water for a minute or two. Repeat the process with the room-temperature water and the hot water. Observe what happens each time, and let the students discuss their theories about what is happening and why. Do the same experiment with the plastic bottle, and discuss the results and the students’ theories.
The balloons will do nothing in the cold and room-temperature water, but they should begin to inflate in the hot water. This is because air molecules move faster and spread apart, causing the air to expand, when it is heated. Warmed by the water, the air will expand into the balloon, inflating it. (This is basically how hot-air balloons work — the heated air expands into the balloon, which rises because the hot air is less dense than the cooler air around it.)
The balloon stretched over the plastic bottle will inflate more quickly than the balloon stretched over the glass bottle. My kids correctly surmised that this is because the glass is much thicker than the plastic and thus insulates the air better and slows down the transfer of heat from the water outside the bottle to the air inside it.
Like most hands-on science projects, this one is great for your tactile and visual learners.