The phase diagram of helium is shown here: Helium has two different liquid phases, referred to as Helium I and Helium II.
Triple point: 10°C, 29.7 bar
Solid
Liquid He [Lambda line]
Critical point: 5.195 K, 2274 bar
Liquid He II
Lambda point: 2177 K, 0.050 bar
Temperature (K)
i) Why does the critical point mark the end of the curve between Liquid Helium and Vapor? (5 points)
ii) Based on what you have studied so far, explain the fact that no solid helium can exist under atmospheric pressure. (5 points)
Vapor