Gas is that state of matter in which the atoms or molecules are very far apart and loosely held together. This is the reason that gases do not have any defined shape of their own. They are usually characterized and studied on the basis of their properties; pressure, temperature and volume. All gases obey these gas laws irrespective of their chemical properties (www.chemistry.bd.psu.edu).
In the 18th century chemists started their research on gases and devised some Gas Laws. These laws are formulated according to the relation between the properties of gases.