Page 19 - ICSE Chemistry 8

• The collisions are elas c in nature, i.e., they take place within a negligible span of me and no loss
of kine c energy occurs.
• The average kine c energy of the par cles is directly propor onal to the temperature of the gas.
• The intermolecular space in gases is maximum.

Take a Break!

Write true (T) or false (F).
1. The par cles in solids majorly vibrate at their own posi ons. ________
2. Solids possess weak interpar cle force of a rac on. ________

3. Liquids have fi xed volume and take up the shape of a container. ________
4. The average kine c energy of molecules in liquids is inversely propor onal
to the temperature of the liquid. ________

5. A gas consists of a large number of molecules moving in random direc ons. ________

CHANGE OF STATE OF MATTER

The change of substance from one physical state to another is known as change of state.

Change of state of any substance depends upon the mo on of its molecules and the forces which hold
the molecules together.

When a solid is heated, its molecules gain kine c energy and become ac ve. The molecules try to
overcome the intermolecular force of a rac on. So, they vibrate faster and start to move and the
intermolecular space between them increases. As the
temperature increases so does the intermolecular space Liquid
and the intermolecular force of a rac on decreases.
Eventually, the solid changes to a liquid, and on further Fusion Vaporisation
hea ng, the molecules gain more kine c energy. The Solidifi cation Condensation
intermolecular space between the molecules of the liquid
increases even more. The molecules move randomly and Solid Sublimation Gas
the intermolecular force of a rac on decreases gradually. Deposition
A er a point, the intermolecular force becomes so weak Fig. 1.7: Interconversion of state of matter
that liquid changes to gas.

Similarly, when the temperature is decreased, the molecules of a substance lose kine c energy.
As a result, the intermolecular force of a rac on between them increases and the molecules come close
together, leading to a decrease in the intermolecular force. With con nued cooling the intermolecular
space decreases to reach an extent where gas changes to liquid and liquid to solid.

Terms Related to Change of State of Matter

The diff erent terms related to the processes involved in change in state of ma er are as follows.

Boiling: The process of change of a liquid into its gas state at a fi xed temperature is called boiling.

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