Steve demonstrated the special effect of making clouds with liquid
nitrogen. Combined with hot water, liquid nitrogen creates clouds of
water vapor. To make a really big cloud, Steve instead poured hot water
into liquid nitrogen and the result was explosive. Sometimes physicists are interested in finding out what
happens to things when they get really cold. What’s the coldest thing you can think of? (ice is +32 F, dry ice [CO2]
is –109 F) What we have here
is liquid nitrogen, its temperature is minus 320 degrees Fahrenheit (pour some
liquid nitrogen into a container.
Discretely put a banana and racquetball in liquid nitrogen for later
use). First off, who knows where
you can find nitrogen? (air). The
air we breathe is about 80% nitrogen gas.
So what we have here is liquid air. It took a really powerful refrigerator to make the air so
cold that it turns into a liquid.
Most gases turn into a liquid when they get cold enough. What’s all this bubbling, does it
remind you of anything? (boiling water).
It’s actually a lot like boiling water. When you boil water, you make the pot really hot until it
gets to a specific temperature, called the boiling point, then the liquid
starts turning into a gas. For
water the boiling point is 212 degrees, that’s why we need to heat it on a
stove. For nitrogen the boiling
point is minus 320 degrees, so the liquid nitrogen is boiling at room
temperature to become air. What is
this smoke coming off the nitrogen, does that remind you of anything? (fog). It actually is the same as fog. The liquid nitrogen makes the nearby
air very cold. The air can’t hold
as much water vapor when it’s colder, so some of the water vapor condenses into
small water droplets that we see as fog, just like you can see over the ocean
in the morning.