Categories of physical quantities

Physical quantities may be divided into different categories, according to their nature. The following groups may be distinguished  

Extensive quantities, which are proportional to the extension of the system considered. Examples are mass, volume, enthalpy, entropy, etc. When subsystems are combined, the values of the extensive quantities are summed up. 

Intensive quantities, these are independent of the extension of the system, but, as the name suggests, determine an intensity or a quality of the system. Examples are temperature, pressure, density, heat capacity, compressibility, field strength, etc. When subsystems are combined, the intensive quantities are averaged in accordance with the composition. An intensive quantity may nearly always be regarded as the quotient of two extensive quantities. 

Specific quantities. These, two, are independent of the extension of the system under consideration. They result from extensive quantities when these are related to the unit of mass. So these quantities are also quotients of two extensive quantities and consequently have all the characteristics of intensive quantities. For mixtures the numerical value of these specific quantities is determined by the composition and averaged in accordance with it. Examples  

Molar quantities are related to the specific quantities, but now numerically related to one mole as unit of amount of substance. These quantities are, inter alia, obtained by multiplication of the specific quantities (related to the unit of mass) by the molar mass. These molar quantities will play an important role in the considerations that are to follow. 

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