The temperature change produced during a Joule–Thomson expansion is quantified by the Joule–Thomson coefficient, μ μ J T . This coefficient may be either positive (corresponding to cooling) or negative (heating); the regions where each occurs for molecular nitrogen, N 2, are shown in the figure. Note that most conditions in the figure correspond to N 2 being a supercritical fluid, where it has some properties of a gas and some of a liquid, but can not be really described as being either. The coefficient is negative at both very high and very low temperatures; at very high pressure it is negative at all temperatures. The maximum inversion temperature (621 K for N 2) occurs as zero pressure is approached. For N 2 gas at low pressures, μ μ J T is negative at high temperatures and positive at low temperatures. At temperatures below the gas-liquid coexistence curve, N 2 condenses to form a liquid and the coefficient again becomes negative. Thus, for N 2 gas below 621 K, a Joule–Thomson expansion can be used to cool the gas until liquid N 2 forms.