Simple Solar Thermal Projects for Kids - Expanding Air, Balloon Blow-up

While we were looking over the instructions for a solar water purifier on E is for Explore! I noticed they also had a suggestion for using solar energy to blow-up a balloon. 

All it called for was an empty 2 liter bottle, a small balloon, and a sunny day.

We had sun, and an empty bottle (once we poured the rest of our tonic water into our "purifier").

We didn't have a small balloon, but we cut the neck off of a larger one, stretched it over the bottle, and called it good.

It's a nice, simple, quick experiment.

Put a small balloon on top of an empty 2 liter bottle, place it in the sun, and watch as the warming, expanding air fills the balloon.  The only problem we had...

...is that it didn't work.  Instead of filling with air, our balloon was being sucked back into the bottle.

The E is for Explore! experiment used one white bottle, and one black bottle for comparison.  We didn't feel like painting our bottle, but we did find a green bottle under the sink, so we pulled it out too, just to mix things up, a little...

...but the results were the same.  Outside, in the sun, the balloons were sucked back into the bottle.  When we brought them back inside - the balloons filled with air.

I asked the children what they thought was happening.  They suggested, that since we had put the balloons on bottles inside, where it was warm, and then moved them outside, where it was sunny, but cold, that the air in the bottles was cooling, and not heating.  The sun might be warming the air, but not as quickly as it was being cooled by the air outside.  Cold air takes up less space than warm air, and so the balloons were getting pulled in to fill the empty space.

Then, when we brought the bottles back inside, the air warmed and expanded, and the balloons filled.

We gave it another try - completely inside, where we could control the temperature of the room.  We took the balloon off the bottle, let the air out, and then put it back on.

Then, we placed the bottle in the sun...

...and watched as the balloon fill up before our eyes.

Success!