#### Topics

1. What is the cause for surface tension?

The resultant force experienced on a molecule on the liquid surface is not zero. It acts downwards. As a result of it the liquid surface will act as a stretched membrane. This has a tendency to reduce its surface area. Thus  the molecules at the surface attracts each other with a force. This is the reason for the surface tension.

Molecules are packed closely in solids so cohesive force exists among themselves.

3. Discuss the facts given and note down the reasons.

5. What is the cause for capillarity?

The capacity of a liquid to rise up against the force of gravity is known as capillarity.Capillary action is a result of adhesion and surface tension.

6. When a blade or stitching needle is placed cautiously on the surface of water ,it will float on the surface of water. Describe this phenomenon.

When we kept blade or needle on the surface of the liquid  without any disturbance, it will float on the liquid surface .It is because of surface tension. The surface of liquid act as a stretched membrane and it supports the blade or needle and keep them floating. The blade creates a depression in the liquid surface so that the surface tension forces now have an upward directed component which is capable of supporting the weight of the blade.

Surface tension is less for hot water and soap water. So soap solution can be enter into the minute portion of the clothes and can remove the dirt particles.

8. What is the difference between adhesion and cohesion force?

Cohesive force is the attractive forces exist between molecules of the same substance. Adhesive force exists between unlike molecules. This is caused by forces between substances such as mechanical forces and electrostatic forces. Cohesive force is maximum in solids, lesser in liquids and least in gases.

9. When we put a drop of blue ink in a glass of water,the whole water turns blue after sometime. Why?

The drop of blue ink undergoes a process of diffusion .The particles of ink enter into the space between the particles of water . As a result the whole water returns blue in colour .

13. A bloating paper absorbs ink. What is the principle imply in it?

Bloating paper absorbs ink by capillary action. The pores in  the bloating paper act as capillaries. Even though these holes cannot be seen with naked eyes, due to capillary action ink or water rises through these holes. Like this ink or other substances are absorbed.

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