Project Report 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.
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.
To determine the effect of impurities on the freezing point of water.
500 ml beaker, boiling tubes, 1100 thermometer with 0.10C calibration, cork, iron stand, stirrer. MATERIAL REQUIRED:- Glucose, ice, water and KNO3.
To determine the effect of impurities on the boiling point of water. APPARATUS REQUIRED : – Three boiling tubes each fitted with cork with two holes, 1100 thermometer with 0.10C calibration, sand bath, burner, iron stand. MATERIAL REQUIRED :- Glucose, water.
In the expression of Boiling Point i.e. D Tb = Kbm it is clear that the elevation in boiling point depends on modality i.e. the number of moles of solute dissolved in 1000 g of the solvent and not upon the nature of solute. Hence a colligative property.
Same in case of freezing point.
Application of Depression in Freezing Point :
The running of a car in sub zero whether even when the radiator is full of water (which freezes below 273 K) has been possible due to fact that depression in freezing point of water takes place when appropriate amount of a suitable solute (usually ethylene glycol) called antifreeze is dissolved in water.
When an impurity is added its freezing point is lowered i.e. its freezing point decreases. The depression in freezing point increases with the increase in concentration of the solute because on adding the solute the vapour pressure of solution becomes lower than that of pure solvent. Since freezing point is the temperature at which vapour pressure of liquid and solid phase are equal, therefore, for the solution, this will occur at a lower-temperature. For example freezing point of water is OoC under normal atmospheric pressure. If we add sugar or salt to this water its vapour pressure lowers and freezing point decreases. Generally, when 1 mole of any non-electrolyte is dissolved in 1 litre of water the depression in freezing point of water is 1.860C.
The impurities present in a liquid pull its two fixed points away from each other i.e. the freezing point is lowered while the boiling point is raised. The depression in freezing point and the elevation in boiling point increases with increase in the concentration of the solute or impurity i.e. these are the colligative properties that depends only on the no. of moles of the solute. They are independent of the nature of the solute.