1.0 Introduction

  Gaseous State

Generally, there are three states of matter: solid, liquid and gas. The states of matter actually differ in their intermolecular distances. In the solid state, distance is minimum. When solids change to liquids upon heating, these distances become more and maximum when the liquids change to gases.

To begin with, it is necessary to understand the nature of intermolecular forces, molecular interactions and effect of thermal energy on the motion of particles because a balance between these determines the state of a substance.

Intermolecular forces: Intermolecular forces are the forces of attraction and repulsion between interacting particles (atoms and molecules). This term does not include the electrostatic forces or force due to covalent bonding. There are four type of intermolecular forces:

1. Dispersion Forces: Atoms and non-polar molecules are electrically symmetrical and have no dipole moment because their electronic charge cloud is symmetrically distributed . Due to some external factors, these atoms or molecules develop instantaneous dipoles due to distortion of electron clouds around them . The neighbour atoms or molecules, as a result, also develop instantaneous dipole. Hence, a weak force of attraction start operating between two of these constituent particles. Forces which are generated are momentarily and interaction energy is inversely proportional to the sixth power of the distance between two interacting particles( i.e. $1/{r^6}$) . Magnitude of these forces depend on degree of polarisation and distance between two constituent species.

2. Dipole-dipole forces: These forces act between the molecules possessing permanent dipole . These polar molecules interact with each other - positive end of one molecule attracts negative end of other molecule and vice versa. This interaction is stronger than the Dispersion forces but is weaker than ion-ion interaction because only partial charges are involved. The attractive force decreases with the increase of distance between the dipoles. The interaction energy is inversely proportional to distance between polar molecules.

3. Dipole–Induced Dipole Forces: These forces act between the molecules possessing permanent dipole and molecule possessing temporary dipole. When any neutral molecule comes in close vicinity of of a polar molecule then it gets polarized thus developing a temporary dipole. Hence, force of attraction starts operating between these two species. Magnitude of this type of force is less than dipole-dipole forces but greater than dispersion forces.

4. Hydrogen bonding: These type of forces operates in highly polar molecules. Basically, only hydrogen can form hydrogen bonds with some of the most electro-negative elements such as fluorine, oxygen and nitrogen. Chlorine sometimes also participates in hydrogen bonding. Hydrogen bonds play a major role in deciding certain properties of water. For example, density of ice is less than water. Hydrogen bond is the strongest among all the intermolecular forces.

Thermal Energy: Thermal energy is the energy of a body possessed by virtue of its temperature. With increase in thermal energy molecular motion of a substance increases. It is the measure of average kinetic energy of the particles of the matter and is thus responsible for movement of particles. It also gives knowledge about randomness of a body.

Existence of states of matter: Different states of matter exist because of competition between interaction forces and thermal energy. The more the magnitude of interactive forces of a substance the more is its tendency to transform from gas to liquid and then to solid. This is because interactive forces bind the constituent species . But in case of thermal energy the case is just reversed i.e. more the thermal energy of a substance the more is its tendency to transform from solid to liquid and then to gas, This is thermal energy imparts motion to atoms and molecules thus increasing their degree of freedom.

Gaseous State: It is the simplest form of matter. It forms as a result of weak interactive forces and high thermal energy. Some of the general properties of this state are as follows:

• gas has neither fixed shape nor fixed volume.
  

• gases are highly compressible.

• gases have low density than solid and liquid.

• gases mix evenly and completely in all proportions without any mechanical aid.

• gases exert pressure equally in all the directions.