MadSci Network: Chemistry

Re: why does a marshmallow get bigger when it's in a jar without air?

Date: Mon Feb 5 16:02:31 2001
Posted By: Steve Williams, Staff, Science Demonstrator, Pacific Science Center
Area of science: Chemistry
ID: 980119299.Ch

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 
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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