Pure Substance A substance is uniform and fixed in chemical composition i.e.water,N2,He,CO2,etc. A mixture of chemical elements or compounds is treated as a pure substance as long as mixture is homogeneous i.e.gaseous Air (pseudo-pure substance) A mixture of two or more phases could still be a pure substance as long as chemical composition of each phase is same i.e.ice and liquid water Vapor Air but not liquid and gaseous air Liquid Liquid (a)H20 (b)Air 上游通大学 March 12,2019 2 SHANGHAI JLAO TONG UNIVERSITYMarch 12, 2019 2 Pure Substance A substance is uniform and fixed in chemical composition i.e. water, N2 , He, CO2 , etc. A mixture of chemical elements or compounds is treated as a pure substance as long as mixture is homogeneous • i.e. gaseous Air (pseudo-pure substance) A mixture of two or more phases could still be a pure substance as long as chemical composition of each phase is same • i.e. ice and liquid water • but not liquid and gaseous air