There are mainly three different kinds of the physical states of matter, namely:
- Solid
- Liquid
- Gas

Plasma a fourth class of matter has also been identified. These states of matter are also termed as phases.

1) Solid : Solids are characterized by their definite shape and also their considerable mechanical strength and rigidity. Solids tend to resist the deformation of their shape due to strong intra molecular forces and absence of the translatory motion of the structural units (atoms, ions etc). A solid is relatively non compressible, i.e. temperature and pressure have only a slight effect on its volume.

Solids are broadly classified as crystalline or amorphous.

Crystalline solids : Here the atoms are arranged in a definite pattern which is constantly repeated.

Amorphous solids : These have no definite geometrical form.

2) Liquid : A liquid has no definite shape and it takes the shape of the vessel containing it. Like solids, the volume of a liquid is slightly altered by variations in temperature and pressure. Liquids have three typical physical properties, namely:

i) Vapor pressure : A Liquid when kept in a closed container vaporizes into the free space above it. The process of vaporization will continue till the equilibrium is reached between liquid and vapor. The pressure at which the liquid and vapor can co-exist is called the vapor pressure of the liquid at a given temperature.

ii) Surface tension : The surface of a liquid is always in a state of tension because a molecule at the surface is attracted towards the bulk by a force much greater than that drawing it toward the vapor where the attracting molecules are more widely spread. Due to this, a certain force is required to penetrate along any line in the surface. This force is called surface tension.

iii) Viscosity : It determines the flow of the liquid. It is the internal friction between layers of the liquid. Higher the rate of friction, greater the viscosity of the liquid and its flow will be retarded. Conversely, a lower rate of friction lessens the viscosity and makes the liquid more fluid.

3) Gases : A gas has no bounding surface at all and will occupy completely any vessel in which it is filled. It has no definite volume or shape and can be easily expanded or compressed.

Water is the ideal example to show the different states of matter.

Water when cooled to 0⁰C becomes solid. When the temperature of solid water is raised it becomes liquid. If the liquid is heated to 100⁰C it gets converted to steam or vapor (The Gas phase).

Almost all chemical substances can exist in more than one physical state (phase) depending on external pressure and temperature.

The following table illustrates different states of matter and their physical properties.

Plasma : This is the fourth state of matter. It is a type of gas containing positively and negatively charged particles in approximately equal numbers and present in the sun and most stars.

When a change in the state of a system occurs, energy is transferred to or from the surroundings. Energy consumed or lost is commonly studied as heat which is a measure of motions of molecules in a given system. Heat is best understood in terms of rise or fall of temperature of a system.

Units of heat : The unit of heat, which was used for many years is calorie (Cal). It is defined as the quantity of heat required to raise the temperature of one gram of water at 14.5⁰C by one degree.

Heat Capacity : It is the amount of heat required to raise the temperature of the system by one degree.

If the system is one gram of pure substance it is called specific heat, but if one mole of the pure substance is considered, it is called molar heat capacity.

Units of Heat Capacity : The usual units of the molar capacity are calorie per degree per mole (Cal K^-1 mol^-1 ) or joules per degree per mole ( J K^-1mol^-1 ) the latter being the SI unit.

The following chart shows the heat capacities of several elements and compounds in calories per degree per gram as well as per mole and their inter conversion.

Based on the above table let’s calculate heat capacity value.

Amount of heat required in calories to heat 100 gm of H₂O_(S) from 0.2 to 0.8⁰C is represented as follows :

= (0.8 - 0.2) deg ´ 100 grams ´ 0.485 Cal deg^-1gram^-1

= 0.6 ´ 100 ´ 0.485

= 29.1 calories

Similarly heat released into the surrounding while cooling 100 grams of H₂O_(S)from 0.8 to 0.2⁰C will be 29% calories.