is one of my favourite equations in all of physics. I have used it many times, in particular to obtain the Helmholtz free energy; my favourite state function. One thing I am not 100% sure of though is whether this equation is still valid during phase changes. I believe it is, as I believe you should be able to take a revisable path, whence I think this equation should hold, but I thought I would ask.
Is this equation valid through phase changes? Or have I got it wrong?