29-01-2013, 12:14 PM
Effect of Impurities on the Boiling Point and Freezing Point of A liquid
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Introduction
Boiling Point and Freezing Point : Generally, on cooling a liquid its temperature falls gradually till it starts freezing. At this point the temperature remains constant till whole of the liquid changes into solid.
"The fixed temperature at which a certain liquid freezes, is termed as its freezing point." Freezing point of water is 00C or 273 K.
In the same manner, on heating a liquid its temperature rises gradually till a stage comes when the temperature does not rise further and the liquid starts boiling. The fixed temperature at which a certain liquid boils is termed as the boiling point of liquid. The boiling point of water is 1000C or 373 K.
In terms of vapour pressure (pressure exerted by the vapours of solution) Boiling point of a liquid is the temperature at which vapour pressure of the liquid becomes equal to the atmospheric pressure.
Freezing point of a substance is the temperature at which the solid and the liquid forms of substance have same vapour pressure.
Effect of impurities on Boiling Point
When an impurity is added its boiling point is elevated i.e. its boiling point is increased.
The elevation in boiling point increases with increase in concentration of the solute because on adding the solute vapour pressure of the solution becomes lower than pure solvent. Thus the solution has to be heated more to make the vapour pressure equal to atmospheric pressure. Thus the boiling point gets elevated.
For example boiling point of water is 100oC under normal atmospheric pressure. If we add sugar or salt to this water its vapour pressure becomes lower and boiling point increases.
Generally, when 1 mole of any non electrolyte is dissolved in 1 litre of water the elevation of boiling point is 0.530.
[attachment=48700]
Introduction
Boiling Point and Freezing Point : Generally, on cooling a liquid its temperature falls gradually till it starts freezing. At this point the temperature remains constant till whole of the liquid changes into solid.
"The fixed temperature at which a certain liquid freezes, is termed as its freezing point." Freezing point of water is 00C or 273 K.
In the same manner, on heating a liquid its temperature rises gradually till a stage comes when the temperature does not rise further and the liquid starts boiling. The fixed temperature at which a certain liquid boils is termed as the boiling point of liquid. The boiling point of water is 1000C or 373 K.
In terms of vapour pressure (pressure exerted by the vapours of solution) Boiling point of a liquid is the temperature at which vapour pressure of the liquid becomes equal to the atmospheric pressure.
Freezing point of a substance is the temperature at which the solid and the liquid forms of substance have same vapour pressure.
Effect of impurities on Boiling Point
When an impurity is added its boiling point is elevated i.e. its boiling point is increased.
The elevation in boiling point increases with increase in concentration of the solute because on adding the solute vapour pressure of the solution becomes lower than pure solvent. Thus the solution has to be heated more to make the vapour pressure equal to atmospheric pressure. Thus the boiling point gets elevated.
For example boiling point of water is 100oC under normal atmospheric pressure. If we add sugar or salt to this water its vapour pressure becomes lower and boiling point increases.
Generally, when 1 mole of any non electrolyte is dissolved in 1 litre of water the elevation of boiling point is 0.530.