  Equation of state (EOS) describes the thermodynamic properties of substances.
It has important applications in many fields such as power mechanics,
geophysics, astrophysics, and detonation physics. Currently, most EOSs have
been constructed using the ideal gas EOS as the base framework. However, this
is inappropriate for the substances with high pressures or low temperatures
that are far from the ideal gas state. Here we extract a concept of "ideal
solid" that is symmetrical with ideal gas and propose its EOS. We verify that
the ideal solid EOS represents the thermodynamic properties of the
high-pressure and low-temperature limiting state. It indicates that the
thermodynamic properties of substances have symmetrical characteristic. Then we
develop a universal model by interpolation of the ideal gas and the ideal solid
limits. We verify that the universal EOS has high description accuracy in a
wide region. The concept of ideal solid and its EOS potentially provides a new
direction for the development of the EOS theory.
