# Why isn't Argon's heat capacity ratio constant with temperature?

The Wikipedia Heat Capacity Ratio page claims that Argon has a different ratio at -180°C, that would make it the only noble gas that doesn't always hold a ratio of ~1.660, why is that ?

• Why should it not be some function of temperature? Just because the others are not listed as having different heat capacities does not mean that they don't. In particular, compare the low temperature listed for Ar with its melting point. – Jon Custer Apr 25 at 14:50

## 1 Answer

It's because the other heat capacity ratios are for 19°C, while Argon is for -180°C.

For an ideal monatomic gas (the noble gasses being monatomic, but not strictly ideal), $$\gamma = \frac 5 3 = 1.6666...$$. For a real monatomic gas, these values will vary somewhat with temperature and pressure.

Because Argon is measured at so much lower of a temperature, one could expect that the heat capacity ratio will not be identical to a monatomic gas at room temperature, which is what we see here.

• And -180C is 9C above the boiling point... – Jon Custer Apr 25 at 15:36