Vacuum Chamber
Explanation:
A partially inflated balloon is placed in a vacuum chamber. It expands to more than double its initial size as the pressure in the chamber decreases.
Lesson:
Even though we can't see air, we shouldn't think of it as empty space. In fact, it is a gas made up of molecules which are in constant motion. When placed in a container, these molecules exert a force on the container walls.
At first, there's air on the inside of the balloon pushing out and air on the outside of the balloon pushing in. When we remove some of the air from the outside of the balloon, it doesn't push as hard on the balloon but the inside air is still pushing out at full strength. As we remove more and more air, the balloon expands to nearly fill the chamber.
A Marshmallow is a puffed candy that seems to melt in your mouth as you eat it. This is because the sticky, sugary backbone of the treat traps tiny pockets of air on the inside. These pockets of air behave like balloons in the vacuum chamber.
Advanced Lesson:
We are roughly at sea level here in Austin, Texas. So we'll assume the air pressure is one atmosphere (atm) or 760 torr. This amounts to a force of roughly 14.7 lbs pushing on every square inch of your body (and everything else in the room). When I blow up the balloon, it naturally assumes the volume which matches the inside and outside air pressures. Let's assume for a moment that the balloon is a perfect sphere with a 2-inch radius. This gives us a surface area of 16π (SA=4πr2). So, the total force that the inside air exerts on the inside of the balloon is 738.9 lbs (14.7×16πr), which meets the condition of inside/outside pressure balance. If the vacuum pump works to decrease the pressure in the chamber to .5ATM, that's now 370lbs pushing in and still 738 lbs pushing out. To re-equilibrate, the pressure inside must decrease. Using the ideal gas law (PV=nRT), we can see that the volume of the balloon must double (radius increase to 2.52 in). Now, assume the vacuum chamber was as large as this room. The pump is rated to achieve pressure as low as 10 mtorr (.01 torr) or 1/76000 of an atmosphere. What would be the new radius of the balloon, assuming it didn't break?