(i) State variables : The measurable properties required to describe the state of the system are called state variables. For example temperature, pressure, volume, composition etc. are state variables.

(ii) State functions: A state function is a property of the system whose value depends on only upon the state of the system and is independent of the path or manner by which the state is reached. For example, pressure, volume, temperature, internal energy (E), enthalpy (H), entropy (S) etc. are state functions.

Homogeneous system: A system is said to be homogeneous if all the constituents present are in the same phase and the composition of the system is uniform throughout.

Heterogeneous system: A system is said to be heterogeneous if it consists of two or more phases and its composition is also not uniform.

Types of systems. The exchange of energy between the system and its surroundings usually takes place in the form of heat or work or both. Based upon this, the systems have been classified into three types:
(i) Open system (ii) Closed system (iii) Isolated system.

(i) Open system: A system is said to be an open system if it exchanges the matter (mass) as well as the energy with its surroundings. All chemical reactions carried in open containers constitute the open system. For example,
(a) Combustion of carbon in an open tube.

(b) Tea placed in a cup.

(ii) Closed system: A system is said to be a closed system if the only exchange of energy is possible between system and surroundings and exchange of matter (mass) is not possible. All chemical reactions carried in closed containers constitute closed system e.g.

(a) Decomposition of calcium carbonate in a closed tube.

(b) Tea placed in a tea-pot.

(iii) Isolated system: A system which can neither exchange energy nor matter (mass) with its surroundings is called an isolated system. All chemical reactions carried in a closed container insulated from all sides represent the isolated system. For example.

(a) Neutralisation reaction between NaOH and HCl carried in a thermos flask.



(b) Tea placed in a thermos flask.