Modeling Outer Space

You’re carrying an awful lot of weight right now. It’s hard to tell, but every square inch of you is supporting fifteen pounds of atmospheric pressure! Multiply that by the number of square inches on the surface of a human body, and you’ll find you’re bearing a hefty load.
Atmospheric pressure results from the weight of the air above you. Credit: Peter Mulroy,
Our senses don’t usually register atmospheric pressure because it’s part of our natural habitat. It’s easier to observe the significance of the air around us by removing it and seeing what happens. You don’t know what you’ve got until it’s gone! The vacuum chamber is the perfect tool for exploring atmosphere-free science.
One of the most telling differences between solid, liquid, and gaseous states is the way each one occupies space. A solid object keeps both its size and its shape, no matter where you set it down. A liquid changes shape to match the container that holds it, but still takes up a fixed volume of space.  Gases follow no such rules. The molecules in a gas will spread out indefinitely, filling any space available. We can observe this by trapping some gas in a stretchy container and exposing it to vacuum conditions.
At the microscopic level, a marshmallow is a sugar-and-gelatin sponge full of tiny air pockets. In other words, it’s a stretchy (and delicious) container with gas trapped inside! When we remove the air around the marshmallow, the gas within expands to fill the surrounding vacuum. The bubbles stretch bigger and bigger until they pop and the air escapes. When the air returns, there is nothing inside the tiny bubbles of the marshmallows, so they shrink back down below their original size.
Water is another interesting low-pressure test subject. By removing the air nearby, we effectively lift fifteen pounds of weight from each square inch of the water’s surface. This does not mean that the water is getting hotter. Instead, the drop in pressure brings the boiling point of the liquid down below room temperature.
Water in a vacuum evaporates rapidly at room temperature. Newly formed water vapor condenses on the inside of the chamber, running down in droplets.