clapeyron equation

[asdf1]

why's it positive for most materials?

[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.

[asdf1]

thank you very much!!! :)