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| What is Vapor Pressure |
What is Vapor Pressure?
The pressure exerted by vapors of liquid at given temperature called vapor pressure.
Explanation :
Placed some liquid into closed container due to evaporation molecules of liquid are converted into vapors.
When these vapors collide with each other, they lose a part of their kinetic energy and again exerted into liquid state. This is called condensation process.
Initially rate of evaporation is higher than rate of condensation. But after sometime when surface above the liquid become saturated then rate of evaporation becomes equal to the rate of condensation and dynamic equilibrium is established.
Like gases the molecules of liquid collide with each other and exerted pressure. This pressure is known as vapor pressure of liquid.
Factors Affecting Vapor Pressure :
- Nature of liquid
- Temperature
- Strength of inter-molecular Forces
Nature of liquid
The vapor pressure of liquid depends upon the nature of liquid. High boiling liquid exert low evaporation Pressure and low boiling liquid exert more vapor equilibrium pressure.
Strength of Inter-Modular Forces :
Greater the inter molecular forces lower will be the evaporation pressure.
For example at 35 centigrade temperature water with strong forces exerted 43 torr pressure while either with weak forces exert 442 torr pressure.
Temperature :
Greater temperature higher the kinetic energy of molecules and evaporation is also increase so vapors exert more pressure.
Example: Water exert vapor pressure 4.5 to 9.2 torr when temperature increase from 0 centigrade to 10 centigrade and vapor pressure of water increase from 527 torr to 760 torr when temperature increase from 90 c 100 centigrade.
Boiling Point
Boiling Point definition :
The boiling point refers to the temperature at which the vapor pressure of a liquid matches the pressure exerted by the surrounding atmosphere or external environment.
When temperature increase the vapor tension of liquid also increase. After sometime when vapor pressure becomes equal to atmospheric pressure at this stage liquid start boiling. This is called boiling point of liquid.
Some liquid boils at low temperature and some are boil at high temperature this is due to strong a week in the inter-molecular forces.
If inter-molecular forces are strong than boiling point of liquid is high. For example at same temperature the vapor pressure of Ether is 200 torr while vapor pressure of H2O is 4.9 torr.
Due to week forces in ether its vapor pressure becomes equal to atmospheric pressure easily at low temperature. That's why boiling point of ether is lower.
Boiling Point and External Pressure:
when external pressure increase the boiling point of substance is also increase and vice versa.
We know that when external pressure increase than liquid absorb more heat to equalized its vapor equilibrium pressure to the external pressure.
Hence boiling point increase when external pressure decrease the liquid absorb less amount of heat to equalized its evaporation pressure with external pressure. Hence boiling point of liquid decrease.
Example : At 760 torr or mm of HgH2O boils at 100 centigrade while at 1489 torr or mm HgH2O boil at 120 centigrade.
