In deriving the chemical potential and also in discussion of intensive properties chemistry books will often do something like this. Say you have a gas and liquid isolated in a container, the total internal energy is equal to the energy of the gas plus the energy of the liquid. Why? I would think they can't be seperated in that way, isn't the liquid exerting electrostatic forces on the gas and vice versa. If you had the same amount of liquid and half the gas surely the internal energy of the liquid would change. What am I not seeing.