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Chapter: 11th 12th std standard Class Physics sciense Higher secondary school College Notes

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Surface tension: Intermolecular, Cohesive, Adhesive force

Intermolecular forces : The force between two molecules of a substance is called intermolecular force. This intermolecular force is basically electric in nature.

Surface tension

 

Intermolecular forces

 

The force between two molecules of a substance is called intermolecular force. This intermolecular force is basically electric in nature. When the distance between two molecules is greater, the distribution of charges is such that the mean distance between opposite charges in the molecule is slightly less than the distance between their like charges. So a force of attraction exists. When the intermolecular distance is less, there is overlapping of the electron clouds of the molecules resulting in a strong repulsive force.

The intermolecular forces are of two types. They are (i) cohesive force and (ii) adhesive force.

 

Cohesive force

 

Cohesive force is the force of attraction between the molecules of the same substance. This cohesive force is very strong in solids, weak in liquids and extremely weak in gases.

 

Adhesive force

 

Adhesive force is the force of attraction between the moelcules of two different substances. For example due to the adhesive force, ink sticks to paper while writing. Fevicol, gum etc exhibit strong adhesive property.

 

Water wets glass because the cohesive force between water molecules is less than the adhesive force between water and glass molecules. Whereas, mercury does not wet glass because the cohesive force between mercury molecules is greater than the adhesive force between mercury and glass molecules.

 

Molecular range and sphere of influence

 

Molecular range is the maximum distance upto which a molecule can exert force of attraction on another molecule. It is of the order of 10-9 m for solids and liquids.

Sphere of influence is a sphere drawn around a particular molecule as centre and molecular range as radius. The central molecule exerts a force of attraction on all the molecules lying within the sphere of influence.

 

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