MECHANICS & PROPERTIES OF MATTER - PART 4
MOLECULAR FORCES
• Surface Tension is essentially a molecular phenomenon. There are two types of molecular forces of attraction Adhesive force and Cohesive force.
• Forces between molecules of different substances are called adhesive forces.
• The adhesive force is different for different pairs of substances.
• Gum or glue is an adhesive. The force of attraction between gum and paper is an adhesive force.
• Forces between molecules of the same substances are called cohesive forces. The cohesive forces are short range forces and therefore they are effective only up to a very small distance.
• The adhesion of water to glass is stronger than the cohesion of water. On the other hand, the cohesion of mercury is greater than its adhesion to glass.
• The maximum distance at which the molecules can attract each other is called molecular range.
• The molecular range is of the order of 10-8
Explanation of surface tension on the basis of molecular theory :
• A sphere drawn with the molecule as centre and radius equal to the molecular range is called the sphere of molecular influence.
• The molecular forces are effective within this sphere of molecular influence. Therefore all the molecules lying within this sphere of molecular influence exert a force of attraction on the molecule at the centre.
• These molecular forces are responsible for surface tension.
ILLUSTRATIONS OF SURFACE TENSION
• The following activities illustrate surface tension of liquids.
CAPILLARY RISE
• A glass tube with a very fine uniform bore is called a capillary tube.
• When a capillary tube is dipped vertically into a liquid contained in beaker, the liquid immediately rises or falls in the tube.
• q is the angle of contact for the given pair of solid and liquid. The angle of contact is defined as the angle between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid.
• If the angle of contact is acute, the level of liquid inside the capillary tube is higher than that in the beaker.
• This capillary rise is observed in the case of water. If the angle of contact is obtuse, the level of liquid inside the tube is lower than that in the beaker.
• This capillary fall is observed in mercury ( 140°). For water in silver tube, = 90° and h = 0.
• The level of liquid remains the same. For pure water and clear glass = 0°.
• This phenomenon of rise or fall of liquid in a capillary tube is called capillarity and this capillarity is due to the property of surface tension of liquids.
Examples of capillary action
• The rise of sap in trees and plants.
• The rise of kerosene or oil in the wick of an oil lamp or stove. The absorption of ink in a blotting paper.
• Sandy soil is drier than clay : The interspaces between the particles of the clay form finer capillaries and water rises to the surface quickly.
Applications of Surface Tension
• Capillary rise is responsible for rising of water in plants. In an oil lamp or stove the oil rises up the wick due to capillarity.
• The purpose of applying soap to clothes is to spread it over large area
• When soap is dissolved in water the surface tension of water is lowered. Surface tension always opposes the spreading of a liquid. By reducing surface tension we facilitate the liquid to spread over larger surfaces. This is why soap is used for washing. For the same reason The paste spreads more freely in the mouth and facilitates cleaning of the mouth.
• When we pour oil on the surface of water it lowers the surface tension of water. Hence the mosquito breed sinks down and perishes.
• In voyage at the high seas, when there are violent waves the sailors pour tins of oil around their boats or ships. Due to oil the surface tension of sea water is reduced thereby the height of water waves is also reduced.
• A pen nib is split at the tip to provide the narrow capillary and the ink is drawn upto the tip continuously.
• When molten lead is allowed to fall through the end of a narrow tube, lead drops assume spherical shape due to surface tension. In factories lead shots are manufactured in this way. Rain drops assume spherical shape due to surface tension of water.
• Surface Tension is essentially a molecular phenomenon. There are two types of molecular forces of attraction Adhesive force and Cohesive force.
• Forces between molecules of different substances are called adhesive forces.
• The adhesive force is different for different pairs of substances.
• Gum or glue is an adhesive. The force of attraction between gum and paper is an adhesive force.
• Forces between molecules of the same substances are called cohesive forces. The cohesive forces are short range forces and therefore they are effective only up to a very small distance.
• The adhesion of water to glass is stronger than the cohesion of water. On the other hand, the cohesion of mercury is greater than its adhesion to glass.
• The maximum distance at which the molecules can attract each other is called molecular range.
• The molecular range is of the order of 10-8
Explanation of surface tension on the basis of molecular theory :
• A sphere drawn with the molecule as centre and radius equal to the molecular range is called the sphere of molecular influence.
• The molecular forces are effective within this sphere of molecular influence. Therefore all the molecules lying within this sphere of molecular influence exert a force of attraction on the molecule at the centre.
• These molecular forces are responsible for surface tension.
ILLUSTRATIONS OF SURFACE TENSION
• The following activities illustrate surface tension of liquids.
CAPILLARY RISE
• A glass tube with a very fine uniform bore is called a capillary tube.
• When a capillary tube is dipped vertically into a liquid contained in beaker, the liquid immediately rises or falls in the tube.
• q is the angle of contact for the given pair of solid and liquid. The angle of contact is defined as the angle between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid.
• If the angle of contact is acute, the level of liquid inside the capillary tube is higher than that in the beaker.
• This capillary rise is observed in the case of water. If the angle of contact is obtuse, the level of liquid inside the tube is lower than that in the beaker.
• This capillary fall is observed in mercury ( 140°). For water in silver tube, = 90° and h = 0.
• The level of liquid remains the same. For pure water and clear glass = 0°.
• This phenomenon of rise or fall of liquid in a capillary tube is called capillarity and this capillarity is due to the property of surface tension of liquids.
Examples of capillary action
• The rise of sap in trees and plants.
• The rise of kerosene or oil in the wick of an oil lamp or stove. The absorption of ink in a blotting paper.
• Sandy soil is drier than clay : The interspaces between the particles of the clay form finer capillaries and water rises to the surface quickly.
Applications of Surface Tension
• Capillary rise is responsible for rising of water in plants. In an oil lamp or stove the oil rises up the wick due to capillarity.
• The purpose of applying soap to clothes is to spread it over large area
• When soap is dissolved in water the surface tension of water is lowered. Surface tension always opposes the spreading of a liquid. By reducing surface tension we facilitate the liquid to spread over larger surfaces. This is why soap is used for washing. For the same reason The paste spreads more freely in the mouth and facilitates cleaning of the mouth.
• When we pour oil on the surface of water it lowers the surface tension of water. Hence the mosquito breed sinks down and perishes.
• In voyage at the high seas, when there are violent waves the sailors pour tins of oil around their boats or ships. Due to oil the surface tension of sea water is reduced thereby the height of water waves is also reduced.
• A pen nib is split at the tip to provide the narrow capillary and the ink is drawn upto the tip continuously.
• When molten lead is allowed to fall through the end of a narrow tube, lead drops assume spherical shape due to surface tension. In factories lead shots are manufactured in this way. Rain drops assume spherical shape due to surface tension of water.
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