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Natural petroleum is usually a pale yellow to black, fluid or semi-fluid viscous liquid; it can also be dark green or reddish-brown in color. It is generally lighter than water, with a specific gravity ranging from 0.8 to 0.98, though there are exceptions where it is heavier than water, with a specific gravity of up to 1.02. Oil is essentially composed of five elements: carbon, hydrogen, sulfur, nitrogen, and oxygen. The mass fraction of carbon is generally 83–87%; the mass fraction of hydrogen is generally 11–14%; the mass fraction of sulfur is generally 0.05–8%; the mass fraction of nitrogen is generally 0.02–2%; the mass fraction of oxygen is generally 0.05–2%. In addition to these elements, there are also trace amounts of metallic and non-metallic elements. Vapor pressure, boiling range, and average boiling point. Density ρ20 (standard density), relative density d420, characteristic factor (K), average relative molecular mass. (The average molecular mass of gasoline is 100–120; that of diesel is 220–240; that of wax oil from vacuum distillation fractions is 370–400; that of vacuum residue is 1000.) Viscosity of oils and fats. (Dynamic viscosity, kinematic viscosity) Critical properties, compressibility factor, and eccentricity factor. Thermal properties: enthalpy; specific heat capacity; heat of vaporization. Surface tension, interfacial tension, refractive index, thermal conductivity