Properties of the System

A system's properties -- the measurable quantities that describe its current state -- split cleanly into two categories depending on whether they depend on how MUCH of the system you have.

Extensive properties scale directly with the amount (mass or size) of the system: double the quantity of substance, and an extensive property doubles too. Volume, number of moles, mass, and internal energy are the obvious examples; enthalpy, entropy, free energy and heat capacity (all introduced later in this chapter) are also extensive, because each is defined per mole of a definite total quantity of substance present.

Intensive properties are completely independent of how much substance you have -- they describe the character of the substance rather than its quantity. Refractive index, surface tension, density, temperature, boiling point, freezing point, molar volume, molarity, mole fraction, molality and specific heat capacity are all intensive: a litre of water and a bucket of water boil at the same temperature and have the same density, even though their masses/volumes (extensive properties) differ enormously. …