Page 16 - ICSE Chemistry 8

KINETIC MOLECULAR THEORY OF MATTER

The kine c molecular theory of ma er helps us to explain why ma er exists in diff erent states (i.e., solid,
liquid, gas) and how it can change from one state to another. The kine c molecular theory of ma er is
based on assump ons such as ma er consists of very ny par cles that are constantly in mo on and
possess kine c energy.

There exist intermolecular forces of a rac on between the molecules and the distance between them is
called intermolecular space (distance).

Postulates of Kinetic Molecular Theory of Matter

Following are the postulates of the kine c molecular theory of ma er.
• Ma er consists of a large number of ny par cles which could be atoms or molecules.

• The atoms/molecules are in constant mo on and hence possess kine c energy.
• The atoms/molecules are in random mo on resul ng in collision with other atoms/molecules of
same ma er and also of any other ma er they come in contact with. As a result of these collisions,
the exchange of energy occurs between the colliding atom/molecules.
• A change in phase may occurs when the energy of the par cle is changed.

• There are empty shapes between atom/molecules of ma er. These empty spaces are called
intermolecular spaces.
• There are a rac ve forces between the molecules of a substance. The force of a rac on between
the molecules is called intermolecular force of a rac on. The intermolecular force of a rac on
depends on the following.

 Intermolecular space: The intermolecular force of a rac on decreases if there is more
intermolecular space between the molecules and vice versa.
 Nature of molecules: The force that binds the molecules of a substance in known as the force
of cohesion. Molecules of a similar type exert the force of cohesion. Molecules of diff erent
type exert the force of adhesion.

STATES OF MATTER BASED ON KINETIC MOLECULAR THEORY

The state of ma er is decided on the basis of intermolecular force of a rac on and intermolecular spaces
between the par cles of ma er.

Kine c molecular theory explains the behaviour of ma er in diff erent states.

Solids

The molecules are closely packed to each other in solid and the intermolecular
space is negligible. Due to this, the intermolecular force of a rac on between
the par cles is the strongest. The molecules are arranged in a well-defi ned
order and remain fi xed in their posi ons. The interpar cle force of a rac on
between the molecules is so strong that the kine c energy is not enough to
break the force. Hence, the molecules in solids hardly move about, they only Fig. 1.1: Solids consisting of
vibrate about their posi on. tightly packed molecules

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