### Question

How does one 'liquify' helium? How is it cooled enough to become a liquid?

Asked by: Shannon Range

### Answer

To answer this question, I must start with the ideal gas equation:

**PV=nRT **

This equation
states that Pressure(P) times Volume(V) equals the number of moles(n) times a constant(R) times
Temperature(T). The number of moles is just a measure of how many atoms or molecules you have
and is sometimes more convenient to use than mass. The constant is different depending on
the units you use for your pressure (atm, Torr, Pa). If you keep the number of moles
constant and remove n and R (you can do this with constants as long as you change the
equals sign to a proportion), the relation is then Pressure times Volume is proportional to
the Temperature.

This means that as temperature decreases, the pressure and/or volume
decrease and vice-versa. According to this, you can either cool the helium to liquid or
decrease the volume. Decreasing the volume most of the time means applying pressure.

Answered by: Travis Rappleye, Chemistry Undergrad, San Jose State University