Conceptual computer artwork of molecules in space The theory of panspermia states that the molecules that form the building blocks of life are found throughout the universe. They arrived at earth from extraterrestrial sources early in its history and may still be arriving today!

When we boil water in a tea kettle, the increase in temperature produces steam that whistles out of the spout at high pressure. If we forget to poke holes in a potato before baking it, the high pressure steam produced inside it can cause it to explode messily. These examples show the relationships among the macroscopic properties of a substance, such as pressure 'P', volume 'V', temperature 'T' and mass 'm' of the substance. But we can also describe a substance using the microscopic properties such as, the mass, speed, kinetic energy and momenta of the individual molecules of the substance.

In fact the microscopic and the macroscopic properties are inter-related. For example, the microscopic collision forces that occur when air molecules strike a solid surface cause atmospheric pressure which is a macroscopic property.

So to study the thermal properties of matter, first we shall look at the macroscopic properties of the substance using the ideal gas laws. Then using the knowledge of momentum and kinetic energy, we can relate them to the microscopic properties. It is valuable to determine a relation between the macroscopic properties, volume, pressure, temperature and the mass of the substance. Such a relation is called as 'equation of state' and the variables in the equation are called 'state variables'. State represents the physical condition in which a particular substance exists.