PRESSURE DEMONSTRATIONS USING A VACUUM CHAMBER
Created by Dale Olive and Randy Scoville
Future Flight Hawai`i instructors
Demonstration at a glance:
Various Items such as a balloon, potato chip bag, marshmallows, water and shaving cream are placed in a vacuum chamber and the air evacuated.
Listed below are my recommendations for doing the vacuum demos. Be careful not to let certain demo's go too far, such as the shaving cream demo. You could actually suck the cream into your vacuum pump if it gets too close to the intake hole requiring a good cleaning afterwards. I find that I have to change my vacuum oil more frequently than normal when doing these demos as water vapor and small bits of balloon contaminate the oil after several trials.
Balloon and chip bag: The vacuum chamber removes air from around the objects placed in it. As for the balloon, when air is removed from its surroundings it is no longer being squeezed by the 14.7 pounds per square inch, 101 Kilopascals, 760 torr or millimeters of mercury or 1 atmosphere of pressure (there are so many different ways to express pressure - to save confusion I find that the kids understand 14.7 psi better because 15 lbs is the weight of a bowling ball and I can say it's a bowling ball of pressure on every square inch!) Once the surrounding pressure is removed, the force of the air molecules slapping the inside of the balloon is no longer being opposed and the balloon bursts at many different points causing its disintegration. This also explains why the potato chip bag explodes open. This occurs along its weakest point at the ends of the bag.
Marshmallows: The marshmallow is very interesting and more complex. At first, the air from around the marshmallow is removed causing the air trapped inside the marshmallow to push outwards thus expanding it. Eventually the vacuum is strong enough to pull air from inside the marshmallow causing it to shrink. When air is let back into the chamber, you end up with a "marshmallow grape" because the air has been removed from inside the marshmallow.
Water: Boiling is influenced by two factors. The one the kids always think of is heat. But, pressure is also a major influencing power. Normally, a liquid at sea-level requires a lot of heat to get it to boil because the pressure of the air above the liquid is forcing escaping water molecules back into solution. When we heat the liquid, this gives the water molecules more kinetic energy and they have an easier time escaping the onslaught of the air above them trying to slap them back as well as overcoming the attraction water molecules have for eachother. The bubbles that form in a boiling liquid is caused by the fact that some molecules separate away from eachother in the liquid forming steam pockets. When the pressure is lowered on the surface of the liquid the kinetic energy of the liquid (how fast the water molecules are moving) becomes sufficient enough at lower and lower temperatures for the molecules to escape. This is so because there aren't as many air molecules above the liquid trying to slap them back in. Since there isn't as much pressure on the surface of the liquid, bubbles of "steam" (water in a gaseous state) also form easier within the liquid.
Shaving cream: Shaving cream is actually made of millions of tiny bubbles. Each bubble acts like little balloons and expand as the outside pressure is lowered thus the cream expands filling the chamber.
There are some great extension questions and discussionS that can take place after each demonstration.
What would happen to your face in space without a helmet?
For the balloon we often get into what would happen to an astronaut if he took his helmet off in space. Contrary to popular belief, perpetrated in the movies, ones head does not expand and explode. I tell my students the reason why is that Most of them are not air-heads. We don't contain a large pocket of air to expand in our heads! Your lungs are another story. To prove it I sometimes put a water filled balloon in the chamber and to their surprise, nothing happens. Water doesn't have the same expansion properties of a gas and the skin of the balloon exerts enough pressure to stop it from boiling so the movies are wrong - but neat!
Why do your ears pop when you drive up into the mountains or fly?
We are used to the air slapping our bodies; this includes the hole in our heads called ears. At altitude, there is less pressure pushing into our ears and so the pressure between our inner and outer ears needs to be equalized. Even though planes are "pressurized" we are sensitive to the small changes that still occur.
Why do I have to boil my eggs longer if I live in Denver?
Because the City of Denver is a mile up into the atmosphere, there is less pressure exerted on boiling liquids. Like the water in the vacuum chamber, less pressure lowers the boiling point. On top of Mauna Kea on the Big Island of Hawaii (almost 14,000 feet) water boils some 10 degrees Celsius lower than at sealevel. I always tell the kids, if I took a hot air balloon up really high I might be able to get my water to boil at room temperature and thus my eggs would never cook!
How can there be water on the Moon and Mars if water boils in a vacuum?
The kids will remember the astonishing announcement of water existing on the Moon and Mars. You've just shown them that water boils in a vacuum. Imagine you get your big gulp at the lunar 7-11 (you know they'll be there someday) and forget to put on your cosmic lid . . .What will happen? Your big gulp will boil away on the surface of the moon because it's a vacuum. This is true for all LIQUIDS - and thus the answer. Water in a solid state does not boil! To prove it I sometimes put some ice cubes in the vacuum chamber and they sit there without changing. Frozen water is the form they have discovered on the surface of the Moon and Mars. What if a future astronaut cuts himself on Mars in a new suit that exposes the skin to the surface? Yep, your blood would boil away as it hit the surface of the cut. Would it still clot then?
Communications: Hawaii Space Grant Office
This activity is featured in Future Flight Hawaii, a K-12 Education Project of Hawaii Space Grant Consortium.
FEB 27 2001.