Thermodynamic properties of classical and quantum simple Boltzmann systems are discussed. It is pointed out that the standard formulas of a classical ideal gas actually describe a quantum Boltzmann gas. The heat capacity of classical and quantum Boltzmann systems is analyzed. The melting of classical systems and Boltzmann quantum hard spheres is considered.

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Keywords: ideal gases, Sackur—Tetrode equation, heat capacity of hard and soft spheres, Boltzmann system of quantum hard spheres PACS: 05.20.−y, 05.30.−d, 64.70.D− (all) DOI: 10.3367/UFNe.2024.06.039700 URL: https://ufn.ru/en/articles/2024/9/f/ Citation: Stishov S M "On the thermodynamics of simple systems" Phys. Usp. 67 (9) (2024) Received: 14th, February 2024, revised: 19th, April 2024, accepted: 19th, June 2024 Оригинал: Стишов С М «К термодинамике простых систем» УФН 194 967–973 (2024); DOI: 10.3367/UFNr.2024.06.039700