# Homework Help: Clapeyron equation

1. Jan 10, 2006

### asdf1

why's it positive for most materials?

2. Jan 10, 2006

### Michael_McGovern

I'm not sure exactly what you mean. The equation says, for a phase transition:
$$\frac{dP}{dT}=\frac{\Delta H}{T\Delta V}$$
I think you're talking about dP/dT, but correct me if I'm wrong. Suppose you have a liquid that is being converted into a gas. This process is endothermic so $\Delta H>0$ and since the volume of the gas is greater than that of the liquid $\Delta V>0$. Since the LHS has all positive terms, then $\frac{dP}{dT}>0$. If you go in reverse and condense a gas then both $\Delta H$ and $\Delta V$ change sign so the RHS remains positive.

If you want to know if it is positive for most substances, you have to think of it as "are delta H and delta V the same sign for most substances and most phase transitions". There are exceptions, like the liquid-solid transition of water, but think about most substances and the answer will come to you.

3. Jan 13, 2006

### asdf1

thank you very much!!! :)