# Can a first order phase transition have an order parameter?

Order parameter is used to describe second order phase transition. It seems that in some papers it is used in the first order phase transitions. Can first order phase transition have an order parameter? If so, how can we define the order parameter in liquid-gas transition (first order)?

• Dear hlew, the difference between $a$ and $b$ is the expression denoted $a-b$. Does it answer your question? I didn't say that the order parameter behaves in the same way as it does in the 2nd order phase transition and indeed, it doesn't behave in the same way. – Luboš Motl Jan 16 '13 at 15:05
• Dear hlew, in the presence of 1st order phase transitions, there is always (at any combination $p,T$) the other phase as well except that it is unstable. Maybe my approach was just a wrong way to deal with the question. Maybe I should have just written that the order parameter is only useful or "routinely used" for 2nd order phase transitions and the right theory for 1st order phase transitions is of course completely different than Landau's theory etc., so any attempt to apply 2nd order phase transition concepts will lead to confusion or chaos. – Luboš Motl Jan 17 '13 at 8:13