Liquefaction
According to kinetic theory, the kinetic energy of the molecules is low for lower temperature. These slower moving molecules become subject to inter molecular attraction. At a sufficiently low temperature these attractive forces are capable of holding the molecules with one another so the gas is changed into liquid and the process is called liquefaction.
Liquid State
It is one of the state of matter. In this state, the kinetic energy of the molecule is very high due to which the molecules of the liquid are able to move but due to compact nature liquids are not compressible. On this basis we can say that the volume of a liquid is always constant but its shape can be changed.
Behaviour of Liquids
The main properties of liquids are as follows.
Diffusibility
The diffusion of one liquid into another liquid is possible but its rate is slow as compared with the rate of diffusion of gases. Example of diffusion of liquids is mixing of alcohol in water.
Explanation of Diffusion in Terms of Kinetic Energy
As the molecular of a liquid are in cluster form they are very close to each other but these molecules are movable so they can mix with the other molecules. Since the intermolecular distance are smaller due to which the rate of diffusion of liquids is slow.
Compressibility
The space between liquid molecules are very small due to strong Van der Waals forces. When the pressure is applied, they can be compressed but to a very little extent.
Expansion
When a liquid is heated, the kinetic energy of its molecules also increases so the attraction between the molecules becomes weaker due to which they go further apart and hence the liquid expands.
Contraction
When a liquid is cooled its kinetic energy is lowered and the attraction among the molecules becomes stronger so they comes close to each other and hence the liquid contract.
Viscosity
Definition
The internal resistance in the flow of a liquid is called viscosity.
Liquids have the ability to flow, but different liquids have different rates of flow. Some liquids like honey mobil oil etc. flow slowly and are called viscous liquids while ether, gasoline etc. which flow quickly are called less viscous.
Explanation
The viscosity of liquid can be understood by considering a liquid in a tube, a liquid in a tube is considered as made up of a series of molecular layer. The layer of the liquid in contact with the walls of the tube remains stationary and the layer in the center of the tube has highest velocity as shown.
Each layer exerts a drag on the next layer and causes resistance to flow.
Factors on Which Viscosity Depends
1. Size of Molecules
The viscosity of a liquid depends upon the size of its molecules. If the size of the molecules is bigger the viscosity of the liquid is high.
2. Shape of Molecules
Shape of the molecules affects the viscosity. If the shapes of the molecules are spherical they can move easily but if the shapes of the molecules are irregular such as linear or trigonal then the molecules will move slowly and its viscosity will be high.
3. Intermolecular Attraction
If the force of attraction between the molecules of a liquid is greater the viscosity of the liquid is also greater.
4. Temperature
Viscosity of a liquid decreases with the increase of temperature.
Units of Viscosity
Viscosity of a liquid is measured in poise, centipoise or millipoise & S.I unit.
1 poise = 1 N.s.m(-2)
1 centipoise = 10(-2) N.s.m(-2)
Surface Tension
Definition
The force acting per unit length on the surface of a liquid at right angle direction is called surface tension.
Explanation
Consider a liquid is present in a beaker. The molecules inside the liquid are surrounded by the other molecules of the liquid. So the force of attraction on a molecule is balanced from all direction. But the force of attraction acting on the molecules of the surface from the lower layer molecules is not balanced.
The molecules lying on the surface are attracted by the molecules present below the surface Due to this downward pull the surface of the liquid behave as a membrane which tends to contract to a smaller area and causes a tension on the surface of the liquid known as surface tension.
Diagram Coming Soon
Factors on Which Surface Tension Depends
1. Molecular Structure of the Liquid
If the force of attraction between the molecules is greater, the surface tension of the liquid is also greater. Those liquids in which hydrogen bond formation take place will have more surface tension.
2. Temperature
Surface tension of a liquid is inversely proportional to the temperature.
Units
1. Dynes / cm
2. Ergs / cm2
Capillary Action
The fall or rise of a liquid in a capillary tube is called capillary action.
When a capillary tube is dipped in a liquid which wets the wall of the tube, the liquid will rise in the capillary tube, to decrease the surface area due to surface tension. The liquid will rise in the capillary tube until the upward force due to surface tension is just balanced by the downward gravitational pull. This is called capillary action.
Vapour Pressre
Definition
The pressure exerted by the vapours of a liquid in its equilibrium state with the pure liquid at a given temperature is called vapour pressure.
Explanation
Consider a liquid is present in a bottle as shown.
Diagram Coming Soon
In the beginning the atmosphere above the surface of liquid is unsaturated but due to continuous evaporation the molecule of the liquid are trapped in the bottle and the air present above the surface of the liquid is becomes saturated and after it the molecules present in the vapour state may hit the liquid again and rejoin it by condensing into liquid. Thus in this closed vessel two process are going on simultaneously which are evaporation and condensation of vapours. When the rates of these two processes becomes equal at this point the pressure exerted by vapours is called vapour pressure.
Units of Vapour Pressure
The units for vapour pressure are
1. Millimeter of Hg
2. Atmosphere
3. Torr
4. Newton / m(2)
Factors for Vapour Pressure
1. Nature of Liquid
Vapour pressure of a liquid depends upon the nature of the liquid. Low boiling liquid exert more vapour pressure at a given temperature.
2. Temperature
Vapour pressure of a liquid also depends upon temperature. The vapour pressure of the liquid increases with the increase of temperature due to the increase of average of kinetic energy.
3. Intermolecular Forces
Those liquids in which the intermolecular forces are weak shows high vapour pressure.
Explanation of Evaporation on the Basis of Kinetic Theory
According to this theory the molecules of a liquid collide with each other during their motion. Due to these collisions some of the molecules acquire greater energy than Van der Walls forces which binds the molecules of the liquid together so these molecules of higher energy escapes from the surface into the air in the form of vapours.
Evaporation is a Cooling Process
In liquids, due to collision between molecules some molecules acquire higher energy and escapes from the surface of the liquid in the form of vapours. The kinetic energy of the remaining molecules decreases due to which the temperature of the liquid also decreases and hence we can say that evaporation is a cooling process.
Boiling Point
Definition
The temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure is called boiling point.
When a liquid is heated the rate of evaporation of the molecules also increases with the increase in temperature. When the pressure of the vapours becomes equal to the atmospheric pressure the liquid starts boiling and this temperature is known as boiling point.
If the external pressure on a liquid is changed the boiling point of the liquid also change. The increase in external pressure on a liquid increases the boiling point while the decreases of external pressure decrease the boiling point.
According to kinetic theory, the kinetic energy of the molecules is low for lower temperature. These slower moving molecules become subject to inter molecular attraction. At a sufficiently low temperature these attractive forces are capable of holding the molecules with one another so the gas is changed into liquid and the process is called liquefaction.
Liquid State
It is one of the state of matter. In this state, the kinetic energy of the molecule is very high due to which the molecules of the liquid are able to move but due to compact nature liquids are not compressible. On this basis we can say that the volume of a liquid is always constant but its shape can be changed.
Behaviour of Liquids
The main properties of liquids are as follows.
Diffusibility
The diffusion of one liquid into another liquid is possible but its rate is slow as compared with the rate of diffusion of gases. Example of diffusion of liquids is mixing of alcohol in water.
Explanation of Diffusion in Terms of Kinetic Energy
As the molecular of a liquid are in cluster form they are very close to each other but these molecules are movable so they can mix with the other molecules. Since the intermolecular distance are smaller due to which the rate of diffusion of liquids is slow.
Compressibility
The space between liquid molecules are very small due to strong Van der Waals forces. When the pressure is applied, they can be compressed but to a very little extent.
Expansion
When a liquid is heated, the kinetic energy of its molecules also increases so the attraction between the molecules becomes weaker due to which they go further apart and hence the liquid expands.
Contraction
When a liquid is cooled its kinetic energy is lowered and the attraction among the molecules becomes stronger so they comes close to each other and hence the liquid contract.
Viscosity
Definition
The internal resistance in the flow of a liquid is called viscosity.
Liquids have the ability to flow, but different liquids have different rates of flow. Some liquids like honey mobil oil etc. flow slowly and are called viscous liquids while ether, gasoline etc. which flow quickly are called less viscous.
Explanation
The viscosity of liquid can be understood by considering a liquid in a tube, a liquid in a tube is considered as made up of a series of molecular layer. The layer of the liquid in contact with the walls of the tube remains stationary and the layer in the center of the tube has highest velocity as shown.
Each layer exerts a drag on the next layer and causes resistance to flow.
Factors on Which Viscosity Depends
1. Size of Molecules
The viscosity of a liquid depends upon the size of its molecules. If the size of the molecules is bigger the viscosity of the liquid is high.
2. Shape of Molecules
Shape of the molecules affects the viscosity. If the shapes of the molecules are spherical they can move easily but if the shapes of the molecules are irregular such as linear or trigonal then the molecules will move slowly and its viscosity will be high.
3. Intermolecular Attraction
If the force of attraction between the molecules of a liquid is greater the viscosity of the liquid is also greater.
4. Temperature
Viscosity of a liquid decreases with the increase of temperature.
Units of Viscosity
Viscosity of a liquid is measured in poise, centipoise or millipoise & S.I unit.
1 poise = 1 N.s.m(-2)
1 centipoise = 10(-2) N.s.m(-2)
Surface Tension
Definition
The force acting per unit length on the surface of a liquid at right angle direction is called surface tension.
Explanation
Consider a liquid is present in a beaker. The molecules inside the liquid are surrounded by the other molecules of the liquid. So the force of attraction on a molecule is balanced from all direction. But the force of attraction acting on the molecules of the surface from the lower layer molecules is not balanced.
The molecules lying on the surface are attracted by the molecules present below the surface Due to this downward pull the surface of the liquid behave as a membrane which tends to contract to a smaller area and causes a tension on the surface of the liquid known as surface tension.
Diagram Coming Soon
Factors on Which Surface Tension Depends
1. Molecular Structure of the Liquid
If the force of attraction between the molecules is greater, the surface tension of the liquid is also greater. Those liquids in which hydrogen bond formation take place will have more surface tension.
2. Temperature
Surface tension of a liquid is inversely proportional to the temperature.
Units
1. Dynes / cm
2. Ergs / cm2
Capillary Action
The fall or rise of a liquid in a capillary tube is called capillary action.
When a capillary tube is dipped in a liquid which wets the wall of the tube, the liquid will rise in the capillary tube, to decrease the surface area due to surface tension. The liquid will rise in the capillary tube until the upward force due to surface tension is just balanced by the downward gravitational pull. This is called capillary action.
Vapour Pressre
Definition
The pressure exerted by the vapours of a liquid in its equilibrium state with the pure liquid at a given temperature is called vapour pressure.
Explanation
Consider a liquid is present in a bottle as shown.
Diagram Coming Soon
In the beginning the atmosphere above the surface of liquid is unsaturated but due to continuous evaporation the molecule of the liquid are trapped in the bottle and the air present above the surface of the liquid is becomes saturated and after it the molecules present in the vapour state may hit the liquid again and rejoin it by condensing into liquid. Thus in this closed vessel two process are going on simultaneously which are evaporation and condensation of vapours. When the rates of these two processes becomes equal at this point the pressure exerted by vapours is called vapour pressure.
Units of Vapour Pressure
The units for vapour pressure are
1. Millimeter of Hg
2. Atmosphere
3. Torr
4. Newton / m(2)
Factors for Vapour Pressure
1. Nature of Liquid
Vapour pressure of a liquid depends upon the nature of the liquid. Low boiling liquid exert more vapour pressure at a given temperature.
2. Temperature
Vapour pressure of a liquid also depends upon temperature. The vapour pressure of the liquid increases with the increase of temperature due to the increase of average of kinetic energy.
3. Intermolecular Forces
Those liquids in which the intermolecular forces are weak shows high vapour pressure.
Explanation of Evaporation on the Basis of Kinetic Theory
According to this theory the molecules of a liquid collide with each other during their motion. Due to these collisions some of the molecules acquire greater energy than Van der Walls forces which binds the molecules of the liquid together so these molecules of higher energy escapes from the surface into the air in the form of vapours.
Evaporation is a Cooling Process
In liquids, due to collision between molecules some molecules acquire higher energy and escapes from the surface of the liquid in the form of vapours. The kinetic energy of the remaining molecules decreases due to which the temperature of the liquid also decreases and hence we can say that evaporation is a cooling process.
Boiling Point
Definition
The temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure is called boiling point.
When a liquid is heated the rate of evaporation of the molecules also increases with the increase in temperature. When the pressure of the vapours becomes equal to the atmospheric pressure the liquid starts boiling and this temperature is known as boiling point.
If the external pressure on a liquid is changed the boiling point of the liquid also change. The increase in external pressure on a liquid increases the boiling point while the decreases of external pressure decrease the boiling point.
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