|MadSci Network: Chemistry|
Hi Anne! You have just had an encounter with air pressure. Before I answer your question about the expanding marshmallow, though, we should discuss this air pressure thing. Make a ring with your forefinger and thumb about an inch around. Put it on your head. Right there, in that ring on your head, is a bowling ball of air pressure! Air pushes down at about 14.7 pounds per square inch at sea level. This means that every square inch of your body has a bowling ball pushing on it. Everything at sea level has one of these bowling balls pushing on every inch of the ground and every inch of every object near the ground. Now, the obvious question is, why isn't everything crushed by all of this pressure? There must be thousands of bowling balls pressing down on you and you are probably just fine! Well, the answer lies in our response to air pressure. Everything that exists at sea level is designed to withstand all of that pressure. In humans, we push right back out and that keeps us equilized with the pressure pushing in. In marshmallows, there is a lot of air inside pushing out. It is equilized with the air pressure outside the marshmallow pushing it in (crushing it). If you removed this air pressure outside, then the air pushing out on the inside would push the walls of the marshmallow out, causing it to expand. A marshmallow is a gooey substance normally, and it is quite compact. When they manufacture a marshmallow, they insert a tube and blast quite a bit of air inside while it is still really hot. You can think of this marshmallow as a sponge, filled with air pockets. When they finish making the marshmallow, it hardens with an airtight (or just about airtight) covering. Now there is 14.7 pounds per square inch of air inside pushing out of a finished marshmallow. When you set up your contraption, I am assuming you "suck" on the straw to make the marshmallow expand. What you are doing is lowering the air pressure inside the jar, around the marshmallow. Less air in the jar means less air pressure inside. The air trapped inside the marshmallow is still pushing out, however, and begins to push the walls of the marshmallow, making it larger. Since our lungs get tired here at the Pacific Science Center, we use a vacuum generator to do the same effect. We have inflated a miniature marshmallow almost ten times it's original size and shrunk it back down by putting the air (and air pressure) back in. It still tasted great later. Take Care and Be Safe, Steve E. Williams Rock Star and Science Demonstrator Pacific Science Center, Seattle, Washington, USA
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