We know that the term energy is broadly defined as the capacity of doing work. During different processes occurring in the universe, the energy changes constantly in various forms such as heat, radiation, mechanical, electrical, nuclear, sound light and chemical energy. During these changes, energy is neither created nor destroyed. Therefore, ,the total amount of energy in the universe is always constant. Heat is considered as a major form of energy because, there is a natural tendency of all the other forms of energy to get finally converted into heat energy.

Though the derivations and discussions in thermodynamics are based on mechanics, it is closely related to chemical reactions. So it can be termed as the branch of physical science which deals with quantitative relationship between heat and other forms of energy. Thermodynamics is macroscopic in character, as it is mainly concerned with the macroscopic properties of the matter. A system which consists of a large number of chemical species (molecules, atoms or ions) is called a macroscopic system and the properties associated with this type of system are known as macroscopic properties. The common macroscopic properties are pressure, temperature, volume, density, viscosity, surface tension, refractive index and composition etc.

Most of the important laws of physical chemistry like phase rule, distribution law, law of flowering of vapor pressure can be derived from laws of thermodynamics. Under a given set of conditions of temperature, pressure and concentration it is possible to guess whether the reaction will occur or not. It also helps in predicting how far a physical or chemical change can proceed until equilibrium conditions are established. It is concerned with those concepts, which are related to the observable features of matter in bulk. It is not applicable to study the properties of microbodies like protons, electrons, neutrons, positrons etc. It is concerned with only the initial and final states of the system. It does not describe or explain the atomic and molecular structures of the atoms and molecules of the system. It does not explain the mechanism of the chemical processes taking place thermodynamically. A system is a specific portion of the universe which is under thermodynamics study or consideration and the region which is found to be present outside the boundary of the system is called its surroundings. A system is always separated from the surroundings by a boundary. Therefore the portion of the universe, which is not a part of the system is called the surroundings. A boundary may be defined as the real or imaginary surface through which, the energy can be transferred from a system to the surroundings or from the surroundings to a system.