Abstract: Physical chemistry is the fundamental knowledge for chemistry, biology, and pharmaceutical sciences. Thermodynamics is the most important part of physical chemistry. Classic thermodynamics has 20 core equations composed of the four state variables (temperature T, pressure P, volume V, and entropy S) and the four thermodynamic energy functions (internal energy U, enthalpy H, Helmholtz free energy F, Gibbs free energy G). The memorization of the 20 equations has been the emphasis and the hard part in teaching thermodynamics. In this article, a “Double Square + Five Rules” mnemonic scheme is proposed to help students quickly and accurately memorize the 20 equations.