The group of molecules is called matter. Matter is made up of small particles. Matter is in three states, Solid, liquid and gas. The other two states are known as Plasma and Bose, Einstein condensate. Thephysical state of the matter changes by changing temperature. The physical properties of a subtance are changed by changing its physical state but the chemical properties do not change, sometimes the rate of chemical reaction changes by changing the physical state. During the chemical calculation, it is most essential to have the information about the physical state of substances (reactant or product) and hence it is essential to study the physical state of matter, factors affecting and related some important laws. The deciding factors of the physical state of matter are intermolecular forces, molecular interaction and the effect of thermal energy on the motion of particles.

     The Dutch scientist van der Waals suggested that the weak forces of attraction exist between the molecules, which cannot be explained by any other chemical attraction is known as van der Waals attractive forces. This force is universal. This force of attraction is exerted upto 4.50Å distance in substance. van der Waals forces depend upon the shape of molecules, number of electrons present in molecules, contact surface of molecules and average intermoleculer distance. The van der Waals forces of attraction are different like (1) Dispersion forces or London forces. (2) Dipole-dipole forces and (3) Dipole-induced dipole forces.

     Dispersion forces of attraction was first of all proposed by the German scientist Fritz London so it is known as London forces. This type of force of attraction is observed in atoms or molecules, there is a temporary dispersion in electron density that affect the electron density of nearby atom or molecule so the force of attraction is developed and so such effect is called dispersion force. The dipole-dipole forces are observed in permantently dipolar molecules. Such dipolar molecules also have interactive London forces so the cummulative effect of both forces are observed. The dipole-dipole force is stronger than London forces. The dipole-induced dipole forces are observed when dipolar molecules come near to non-polar molecules. This type of molecules also have London forces and hence the cumulative effect of both forces are observed. The hydrogen bonding is an important intermolecular force. The first elements of groups 5, 6 and 7 due to their high electronegativity combine with hydrogen to form hydride compounds, in which hydrogen bond is observed. There also exists an intermoleculer repulsive forces; and based on that the effect of pressure on solid, liquid and gaseous state explained very easily. The most important factor which decides the physical, state of matter is the effect of thermal energy, on motion of molecules due to this motion of molecules or atoms the energy produced is called thermal energy to keep the molecules near to each other while the thermal energy has tendency to keep the molecules away from each other. By balancing combination of the two opposite factors, the physical state of matter as solid, liquid or gas is decided. Due to weak forces of attraction between molecules of gaseous state have some characteristics.

     The behaviour of gas is described by the quantitative relation between mass, volume, temperature and pressure and these relations are discovered by experimental observations and such relations are called laws of gases.

     The relation between pressure and volume of a gas was studied and it is known as Boyle’s law. At constant temperature for a fixed amount gas, pressure (P) varies inversely with its volume (V). Mathematically the Boyle’s law is written as \(PV = K\) or \(P_{1}V_{1}=P_{2}V_{2}\). The equation \(\frac{d}{P}=K\) devised from Boyle’s law where d is the density. The Kelvin temparature is accepted as an SI unit. The relation \(T=(t+273.15)K\) is obtained.

     On the basis of experimental observations a relation between absolute temperature and volume is obtained, which is known as Charles’ law. Mathematically it is written as \(\frac{V}{T}=K\) or \(\frac{V_{1}}{T_{1}}=\frac{V_{2}}{T_{2}}\).

     The relation between pressure and absoulte temperature (T) is obtained on the basis of experimental orbservations by scientist Gay Lussac and is known as Gay Lussac’s law. Mathematically it is written as \(\frac{P}{T}=K\) or \(\frac{P_{1}}{T_{1}}=\frac{P_{2}}{T_{2}}\).

     The relation between volume of a gas and number of molecules was given by Avogadro, which is known as Avogadro’s law. The mathematical form of it is V = K•n. The \(0^{\circ}C\) or 273 K temperature and 1 bar pressure is accepted as a standared value by SI system and hence these values are known as standard temperature and pressure (STP). 1 mole of gas at STP is having volume 22.4 litre and number of molecules equal to 6.022 × 1023 known as molar volume and Avogadro’s number respectively.

     Combining Boyle’s law and Charles’ law, the relation obtained \(\frac{PV}{T}=K\) or \(\frac{P_{1}V_{1}}{T_{1}}=\frac{P_{2}V_{2}}{T_{2}}\) is known as combined gas equation.

     The ideal gas equation, PV = nRT is also known as equation of state and R is called universal gas constant which has different values in different units. The real gas behaves as ideal gas at high temperature and low pressure and are called ideal gases.

     The behaviour of real gas is deviated from ideal gas and its study came from the study of effect of pressure and temperature and so the ideal gas equation is written as \(\left ( P+\frac{an^{2}}{V^{2}} \right )(V-nb)=nRT\) and this equation is also known as van der Waals equation.

     The gas can be liquefied by lowering the temperature and increasing pressure at which gas get liquified is known as critical temperature \((T_{C})\) and critical pressure \((P_{C})\) respectively and at critical temperature and critical pressure the volume occupied by 1 mole of gas is called critical volume \((V_{C})\) and this state is called critical state. The \(P_{C}\), \(T_{C}\) and \(V_{C}\) values are constant so they are known as critical constants. The liquefication of gas is explained by isotherm. Maxwell and Boltzmann had studied the distriubution of molecules between different possible and plotted graph which is known as Maxwell’s distribution curve.

     The total pressure of the mixture of two or more than two gases is obtained by the Dalton’s law. Total pressure \((P)=p_{A}+p_{B}+p_{C}+p_{D}+......\) and the partial pressure \((p)\) is calculated from total pressure by equation \(p_{1}=X_{1}\times P_{total}\). If the % by volume is given then the partial pressure of gas is caculated using equation Partial pressure \(p_{A}=\frac{\%\ by\ volume\ of\ gas\ A\times total\ pressure}{100}\).

     The Graham’s law of gaseous diffusion is \(r\alpha \frac{1}{\sqrt{d}}\) and using formula the ratio of rate of diffuson of \(NH_{3}\) and \(HCl\) gas was obtained practically as \(1.46\pm 0.01\). The application of Graham’s law of gaseous diffusion are as given in the text. The Avogadro’s hypothesis is useful to calculate the number of molecules, atoms and total number of atoms in given amount of gas.

     The liquid state has its physical properties like fixed volume, fluidity, non-compressibility, diffusion, evaporation, vapour pressure, surface tension and viseosity.