The author shows that in general, the real gas state equations are identical to the constitutive volume deformation equations of deformable material. On this basis, a fundamental concept of deformation mechanics based on the division into free and elastic is applied. As a result, the volume deformation equation is generalized to the ideal gas law. Peculiarities of the gaseous state are established and compared to the solid and liquid states. In particular, the continuum physical meaning of ideal gas and that of universal gas constant are shown.