The part of the universe which is under investigation in an experiment and which can be separated by real or imaginary boundaries from the rest of the universe is called the system.
1. Types of System
Broadly speaking system are classified into two categories:
(1) Homogeneous System: A one phase system which is uniform in composition an and has similar properties in all its parts is called homogeneous system.
(2) Heterogeneous System: A system consisting of more than one homogeneous system (phase) is known as heterogencous system.
These systems are further sub-classified as follows:
(i) Isolated system: It is that system which can neither exchange matter nor energy with the surroundings, e.g. a thermos flask with its contents.
(ii) Open system: It is that system which can exchange both matter and energy with the surroundings e.g. boiling water in a beaker.
(iii) A Closed system: It is one which can exchange energy with the surroundings but not the matter, e.g. a gas in a closed container.
2. Properties of the system
There are two important properties of a system:
(i) Intensive Properties
(ii) Extensive Properties

(i) Intensive Properties: The properties like pressure and temperature which do not depend on the amount of matter in the system are called intensive properties. Whether the system has one mole or 1000 moles of a gas, it may have the same pressure and the temperature in both the cases. When two systems have identical intensive properties, they are said to be in the same state. Concentration, density, viscosity, surface tension and refractive index etc. are other examples of intensive properties, but volume and mass are not.
(ii) Extensive Properties – Properties which depend on the mass or the number of moles in the system are known as extensive properties. Volume, mass, heat content, internal energy and entropy etc. are the examples of extensive properties.