State and Properties of a System

The state of a system is described by its measurable properties. For example, you can describe the state of a gas by specifying its pressure, volume, temperature, etc. These variable properties are called state variables or state functions. Their values depend only on the initial and final state of the system and not on the path taken by the system during the change. When the state of a system changes, the change depends only on the initial and the final state of the system.

State functions are those functions which depend only on the state of the system.

Properties of a System

State variables can be divided into two main types:

1. Extensive property (variable) is one whose value depends upon the size of the system. For example, volume, weight, heat, etc.
2. Intensive property (variable) is one whose value is independent of the size of the system. For example, temperature, pressure, refractive index, viscosity, density, surface tension, etc.

An extensive property can become an intensive property by specifying a unit amount of the substance concerned. For example, mass and volume are extensive properties, but density (mass per unit volume) and specific volume (volume per unit mass) are intensive properties.