Great question! Marshmallows aside from being a
tasty treat, are composed of dense polymer
strands with air pockets. The air inside the
pockets wants to match the pressure of the
external environment, causing the marshmallow to
expand or contract.
I am going to address nitrogen gas first
since it is the easiest. The Earth's atmosphere is
composed mostly of nitrogen gas. If you were to
fill a container with just nitrogen gas and the
pressure matched the Earth's atmosphere pressure
the marshmallow would not change. If you decreased
the pressure, the air inside the marshmallow would
want more space causing the marshmallow to expand.
If you increased the pressure past atmospheric
pressure, the air inside the marshmallow would
want less space causing the marshmallow to
The effect of carbon dioxide and helium is a
bit more subtle. If the pressure is the same
as atmospheric pressure, the marshmallow should
not see a large change. But, a molecule of carbon
dioxide is much heavier than s molecule of
N2 which is the majority of air. The
air in the marshmallow will act like a balloon in
a CO2 environment. The air will want to
expand to leave the marshmallow. The opposite will
occur for marshmallow in helium because helium is
much lighter than nitrogen gas.
If your school science teacher can get ahold of
dry ice, I highly recommend you try putting a
marshmallow in a container and seeing what
happens. Make sure to wear safety glasses and
thermal gloves when handling dry ice!
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