In other words, the system does work to expand by doing work in the surroundings, to move the surroundings away so that the system could expand. Same thing with compression. Therefore, when you're dealing with reactions of gases, this equation holds the change in enthalpy is equal to a change in internal energy plus p times change in volume. But when you're dealing with reactions in liquid and solid phases, you have to realize that these type of systems are not compressible. If you can't compress them or expand them, that means no PD work is done. Okay?