well, i've some questions abt basics of thermodynamics (1)what is the meaning of enthalpy(h) i know h=u+pv but i don't know what it means, i know what internal energy means but i don't know whats the difference between internal energy and enthalpy,what's the meaning of quantity (pv) what does it indicate for? i know p is pressure and v is volume nut my question how can it be an energy? as i think it doesn't represent work because h is property for state and u cant say there's work in the same state ?so i cant inteprete what's the meaning of h or what's the difference between u & h? (2)2nd question abt diagrams of pure substances (t-v diagram, p-v diagram) what is the meaning of critical point in these diagrams? (3)3rd question abt ideal gases,my question here is when can i consider the gas is ideal? i can consider it the gas behaves an ideal gas when its density is low?or when its pressure is low and its temperature is high relative to its critical point(indeed cant get what critical point has to do with this because basically i dont get exactly physical meaning of critical point)?