Thermodynamic systems in engineering: a gas, such as air, a vapor, such as steam; a liquid, such as freon(氟利昂,一种致冷剂); a solid, such as semiconductor(半导体);…… They are alse0 called as working substance(工作物质) working substance Thermodynamic system A system of gas will be studied in this chapter.Thermodynamic systems in engineering: a gas, such as air， a vapor, such as steam; a liquid, such as freon(氟利昂，一种致冷剂）; a solid, such as semiconductor(半导体）; …….. They are also called as working substance（工作物质）. Thermodynamic system working substance A system of gas will be studied in this chapter