In another test, the valve of a 500-L cylinder full of the gas mixture at 2000 psi (gauge pressure) is opened wide so that the gas rushes out of the cylinder very rapidly. Why might some N₂O condense during this process?

(a) This is an isochoric process in which the pressure decreases, so the temperature also decreases.

(b) Because of the rapid expansion, heat is removed from the system, so the internal energy and temperature of the gas decrease.

(c) This is an isobaric process, so as the volume increases, the temperature decreases proportionally.

(d) With the rapid expansion, the expanding gas does work with no heat input, so the internal energy and temperature of the gas decrease.

One type of gas mixture used in anesthesiology is a 50%/50% mixture (by volume) of nitrous oxide (N₂O) and oxygen (O₂), which can be premixed and kept in a cylinder for later use. Because these two gases don’t react chemically at or below 2000 psi, at typical room temperatures they form a homogeneous single gas phase, which can be considered an ideal gas. If the temperature drops below -6°C, however, N₂O may begin to condense out of the gas phase. Then any gas removed from the cylinder will initially be nearly pure O₂; as the cylinder empties, the proportion of O₂ will decrease until the gas coming from the cylinder is nearly pure N₂O.