Why is liquid nitrogen so cold? -Aaron, 9, Seattle, WA 

Dear Aaron,

When I got your question, I headed straight for my friend Jake Leachman’s lab at Washington State University. He’s an engineer who knows a lot about what happens to things when they get super cold.

He showed me a thermos full of colorless, liquid nitrogen. It was about -321 degrees Fahrenheit.

We can’t see it with our eyes, but I found out about 78 percent of our air is made up of nitrogen in its gas form.

You may have heard about states of matter, like liquids, solids and gases. Liquid nitrogen is so cold because of the way molecules change as a gas turns to liquid.

Nitrogen doesn’t naturally occur in a liquid form here on Earth. Humans have to make it from air. Since air is everywhere, it’s pretty cheap. In fact, some people have said making liquid nitrogen is cheaper than making soda pop.

We funnel air into a big compressor where it undergoes a lot of pressure. The compressor pushes the molecules, or those building blocks that make up air closer together.

This compression causes the gas to heat up. While keeping the pressure high we cool it down to the temperature of the lab. Next, we allow the gas to drop in pressure. This is known as expansion.

To expand the gas, sometimes scientists will force the gas through a packed bed of sand, called a throttle. Other times they will push it through a small hole called a Joule-Thomson valve. What works best for cooling though is to have the gas do useful work during the expansion, like spinning a turbine or pushing against a piston.

When the high-pressure gas expands, or relaxes, considerable cooling happens and eventually the gas becomes a very cold liquid. In fact, most gases turn to a liquid when they cool down.

“A liquid is a state of matter where atoms and molecules are continuously bumping into and communicating with their neighbors,” Leachman said, as we put on our safety goggles and gloves. “Gases chill by relaxing. This happens when there’s freed up space and reduced pressure, or stress on them.”

In the lab, Leachman filled a balloon with air and dropped it in the liquid nitrogen. At first, I thought it would pop. But the balloon actually shriveled up, as the air inside turned to liquid oxygen and nitrogen.

When the air inside the balloon got really cold, the particles started to slow down and take up less space.

When he took the balloon out the process reversed. The balloon went back to its original shape. When the liquid boils in room temperature, the molecules in it move faster. When the molecules move faster they take up more space and the balloon gets big again.

“(Liquid nitrogen) also makes great ice cream and frozen marshmallows,” Leachman adds. He dropped soft, fluffy mini marshmallows into the thermos. When he took them out, they were crunchy frozen. We taste tested them. In the name of science, of course.


Dr. Universe

Got a science question? E-mail Dr. Wendy Sue Universe at Dr.Universe@wsu.edu. Ask Dr. Universe is a science-education project from Washington State University.