Is there another way to condense a gas besides cooling it off?

Let's think about this. If students have a picture of all substances being made of tiny particles called molecules, the molecules must be very far apart in a gas since a gas occupies a very large volume compared to a liquid. The molecules must get closer together if you place them under pressure.

Sometimes you can just put it under pressure; for example, butane in a butane cigarette lighter.

Demo: Press the button on a cigarette lighter without igniting the flame. This is easier to do if you first break the safety mechanism to keep you from pressing the button accidentally.

Can you see the butane escaping?

• Probably not; at least not from any distance. Butane is a clear, colorless gas. You might see some wavy lines like you see while filling you car with gasoline.

What experiment can you do to help see the butane escape? How did you see N₂ escape from the boiling liquid N₂?

• Second question first: N₂ is a clear colorless gas, but you could see the bubbles of N₂ gas passing through the liquid N₂. You use the property that gasses can displace liquids to visualize the N₂ bubbles. Sometimes this property of gasses displacing liquids is described as gasses penetrating liquids.

Demo: Wedge the button on the lighter open and immerse the lighter in water to visualize the bubbles as they escape. You can even ignite the bubbles as they reach the surface.

How is liquid N₂ made?

• The answer to this question is somewhat complicated so I don't usually go into it unless asked. However, N₂ and many other gasses can not be liquefied be pressure alone. Yet, a combination of pressure and cooling can be used to liquefy any gas. In the case of N₂, the gas is compressed which causes it to become warm. It is cooled to room temperature and the pressure released. The gas cools on expansion. The cooled gas is compressed and again it becomes warm. The warmed gas from the second expansion is cooled by the gas from the first expansion to the temperature of the first expansion. The pressure is released from the second compression and the gas cools below the temperature of the first expansion. By having several stages of compression and expansion, the gas becomes colder after each stage until it finally liquefies.

Comment: All gasses can be liquefied by some combination of cooling a pressure. If I do the dry ice experiments, however, you will see not some gasses can not be liquefied at atmospheric pressure. Dry ice is solid CO₂, which must be held under pressure to exist in the liquid state. At atmospheric pressure, CO₂ exists only as either a gas or a solid, but never as a liquid.

feedback: bill_switzer@ncsu.edu