States of Matter

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Introduction to States of Matter

Properties which are studied in the laboratory are not the properties of individual atoms or molecules but are the collective properties of the matter taken in bulk that is, a collection of a large number of atoms, ions or molecules. A few such properties are pressure, volume, temperature, melting point, boiling point, vapour pressure, density, surface tension etc. For Example, boiling point of a single molecule is meaningless. To study the boiling point, we have to take certain amount of the liquid which may contain millions of molecules of that liquid. Further, these physical properties depend upon the state in which that substance exists that is, solid, liquid or gas (though the chemical properties of the substance remain the same, only the rate of chemical reaction is affected by the physical state).
In this article we will discuss some important aspects of each of the three states of matter one by one. Let us first see what we mean by a solid, a liquid and a gas and in what are important characteristic properties the three states differ from each other.

Differentiating the Three States of Matter in terms of Melting Point and Boiling Point

A substance is said to be solid if its melting point is above room temperature under atmospheric pressure; a liquid if its melting point is below room temperature and boiling point is above room temperature and a gas if its boiling point is below room temperature under atmospheric pressure.

Existence of the same substance in different states

It has been found that in most cases, a given substance can be made to exist in any one of the three states when treated with different temperature and pressure. For Example, water which is a liquid under ordinary conditions of temperature and pressure, can be converted into steam (gas) at 100°C and under one atmosphere or into ice (solid) by cooling to 0°C under one atmosphere pressure.

A given substance can also exist simultaneously in all the three states under certain specified conditions of temperature and pressure. For Example, Water exists in all the three phases- ice (solid), water (liquid) and water vapours (gas) simultaneously at 0.01°C (273.16 K) and 4.58 mm pressure.
The temperature at which all the three phases exist together is called Triple Point

Water existing in all the three statesFig. No. 1 Water existing in all the three states

The change of state is often accompanied by either absorption or evolution of heat. For Example, a solid can be converted into a liquid or a liquid into a gas by absorption of heat. This implies that a substance possess least energy in the solid state but maximum in the gaseous state.

Some Important Points of difference in the Three States of Matter

Comparison of the characteristics of solid, liquid and gas

Molecules are packed very closely. (the order of intermolecular distances is 10-8 cm)Molecules are packed less closely. (Intermolecular distances is larger than that of solids but less when compared with gases).Molecules are sufficiently apart from one another (Intermolecular molecular distance are of order 10-7 to 10-5 cm)
Strong intermolecular forces of attraction exist between them.Weak intermolecular forces of attraction are present when compared with those of solids.Negligible amount of intermolecular forces of attraction is present.
Solids have high density when compared with liquids and gases.Liquids possess much lower density than that of solids but is much higher than that of gases.Densities of gases is much lower than solids and liquid.
Solids do not have translatory or rotatory motion because the positions of molecules in the crystal lattice are fixed and therefore they only possess vibratory motion.Liquids experience some translatory and rotatory motions in addition to the Vibratory motion as molecules of a liquid have greater freedom of movement.Molecules of a gas have large rotatory, Vibratory and translatory motions.
Molecules of a solid possess least energy.Molecules of a liquid have higher energies than those of solids.Gas molecules are most energetic.
Solids have both a definite shape and a definite volumeLiquids do not have definite shapes but have definite volumes.Gases have neither definite shapes nor definite volumes.
Solids possess least
compressibility and thermal expansion
Liquids have slightly higher compressibility and thermal expansion than those of solids.Gases possess high compressibility ‘and thermal expansion

Definition of Solid, Liquid and Gas

Based on the simplest observable characteristics states may be defined as follows:

The state of matter which possesses a definite shape and a definite volume Solid is called Solid.

Molecules in SolidFig. No 2 Molecules in Solid

Liquid is defined as that state of matter which has a definite volume but no definite shape. They take up the shape of the vessel in which they are put.

Molecules in LiquidFig. No. 3 Molecules in Liquid

Gas is defined as that state of matter which has neither definite shape nor definite volume.

Molecules in GasFig. No. 4 Molecules in Gas

Two More States of Matter

Besides the above three states of matter, two more states of matter have been found to exist and discovered recently.


One of these (fourth state) is called Plasma State which consists of a mixture of electrons and positively charged ions formed due to superheating of the gaseous state, For Example in the sun or in stars. It is hot ionized gas consisting of approximately equal number of positively charged ions and negatively charged electrons. The characteristics of plasma are significantly different from those ordinary neutral gases therefore they are considered as separate states of matter. Plasma state consist of enough energy that it free electrons from the molecules or atoms because of which both species (electrons and ions) exist simultaneously. Scientists say that approximately 99% of observable matter in the universe is made up of plasma state.

Plasma StateFig. No. 5 Plasma State

Bose-Einstein Condensate

Another state (fifth state) consists of a super cooled solid in which the atoms lose their individual identity and condense to form a single super atom, and this state is called Bose-Einstein Condensate. It was discovered by scientist Satyendra Bose and Albert Einstein in 1920s, however this state was not created experimentally until 1990s. In this state a group of atoms is cooled till the temperature of absolute zero and at this temperature they possess no energy because of which they are hardly in a position to move relative to each other. And at this point they combine together and thereby entering the same energy states and result in formation of Bose-Einstein Condensate. This is the point where they become identical and start behaving as a single atom.

Bose-Einstein CondensateFig. No. 6 Bose-Einstein Condensate

The physical properties of different states of matter differ mainly due to the difference in the intermolecular forces of attraction.

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