Saturday, February 14, 2009

Science Saturday Challenge #3

Expanding and contracting air has myriad applications; in our home lab we'll use it to inflate and deflate a balloon. Adding heat energy to air causes it to expand and displace cooler air, thus decreasing its density and causing it to rise. It follows that cooling the air has the opposite effect.


Plastic soda bottle



Fill the soda bottle with very hot tap water and let it sit for around 5 minutes to warm it up; empty the hot water.

Put the opening of the balloon over the opening of the bottle. Run the bottle, cautiously, under the hot tap water. You will see the balloon inflate a little.

Now run cold tap water over the bottle. The balloon will not only deflate, but may also be pulled into the bottle quite a bit. Run the hot water again to re-inflate the balloon.

Here are three videos demonstrating what happens:

Sometimes this is done by putting burning paper into the bottle, and instead of a balloon you use a hard boiled egg on top of a jar with just the right sized opening, sucking the egg into the jar. But this can mislead some to think that the flame is creating a vacuum by consuming the oxygen (which is replaced by other gases so no vacuum is formed.) And this version can be reversed in both directions repeatedly, making it a hit if you have multiple children that want to try it out.

Here's a PBS interactive demonstration of what is going on.


dstb said...


Thanks for posting the Science Saturday Challenges. They are just what we were looking for.

Over a month ago, my husband and I had thought we should do some simple experiments once a week with the kids. Something fun, but easy, to get used to using the scientific method a little bit. We never actually implemented it though, because we never got around to picking out experiments. Then along came "Science Saturday"!

We did the balloon experiment tonight. My youngest recalled that when it is warm, our bathroom door swells and won't close properly, so he thought cold air would contract the bottle and push air into the balloon. The boys did not think the hot water would have an impact on the bottle or balloon. In their initial comments they noted that hot air rises, but that did not really figure into their hypothesis.

They thought it was very cool that the balloon inflated and then got sucked into the bottle when switching between hot and cold water. Thanks for a great experiment!

BTW, I couldn't view the videos. May just be my computer, but I wanted to let you know in case it was something that wasn't working right.

Thanks again, we look forward to more Science Saturdays!

Laurel said...

Hi - Thank you for Science Saturdays. We will look forward to it. It gives me something to do with the kids on quiet weekend afternoons.

We did the balloon experiment today. I had both of my girls write down their hypothesis and we tried it. My girls were so excited when the balloon was sucked into the bottle.

The experiment reminded me of the egg in the bottle experiment I have seen Steve Spangler do on TV. I did a little research tonight and found he sells a balloon in the bottle experiment for those who want to take this one a step further.

Kris said...

Thanks, Sarah--I will check the videos!

I didn't know Steve Spangler was on TV. I've come across his great site many time while Googling science stuff, I even have it linked on this site. I should check out his profile, too, LOL!