This paper was prepared for the One-Point Lectures of the TED-Conference, and deals with the fundamental knowledge of Thermodynamics for handling mixtures, so called a part of Chemical Thermodynamics. The main contents of this paper are the fundamental thermodynamic theories for mixtures, phase diagrams of binary mixtures such as H_2O-NH_3 and H_2O/CO_2/hydrate, and remarks for calculations of enthalpy and entropy. Many kinds of mixtures are lately used for the thermal and energy equipments, machines, plants and others, but most of mechanical engineers are not so much concerned with Thermodynamics of mixtures. For this reason, this presentation is a warning to the young students in the Department of Mechanical Engineering.