The classification of matter into three different states, namely solid, liquid and gaseous state is termed as the physical classification of matter. Most properties of solid, liquid and gases that can be observed with our sense organs are called as 'macroscopic' properties. The description of the behaviour of the three states of matter in terms of atomic theory is called 'microscopic' description of matter. From the study of the observable properties of different states of matter one can understand the microscopic nature of matter in terms of the behaviour of constituent particles.
The important characteristics of the three states are listed out below:
Solid state
A solid possesses a definite size (volume) and a definite shape under ordinary conditions; and tends to maintain these even under deforming conditions. The substances in solids are closely packed and bound by strong inter particle attraction, making them rigid and geometrical. Some common examples of solids are iron, silver, common salt, etc.
Liquid state
A liquid possesses a definite volume but not definite shape. The substances in liquids have particles, which are loosely packed and bound to each other by forces weaker than those of solids. This makes a liquid mobile and shapeless resulting in its taking up the shape of the container in which it is placed. A liquid also has a tendency to flow. For example, water, alcohol, milk, oil, etc.
Liquid state
A gas neither possesses a definite volume nor a definite shape. The substances in a gas have particles that are separated by great distance, having virtually no force of attraction between them. A gas occupies the whole of the volume of the vessel in which it is placed. It also takes up the shape of the container. For example, air, carbon dioxide, oxygen, hydrogen, etc.
The three states of matter are inter-convertible. This can be done by heating or cooling. Heating increases the inter particle spacing and kinetic energy of the particles. So, a solid on heating gets converted into a liquid, and a liquid into a gas.
The various points of distinction between the three states
| Solids | Liquids | Gases |
| particles are very closely packed | Particles are loosely packed | particles are very loosely packed |
| Voids are extremely small | voids are relatively larger | voids are very large |
| Particle motion is restricted to vibratory motion | particle motion is very slow | particle motion is very rapid and random |
| Inter particle forces are large | Inter particle forces are intermediate | Inter particle forces are negligible |
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