effect of impurities on the

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Effect of Impurities on the

Boiling Point and Freezing Point of A

liquid

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 theliquid 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 isincreased.

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.

Experiment No.1

AIM :- To determine the effect of impurities on the freezing point of water.

APPARATUS REQUIRED : - 500 ml beaker, boiling tubes, 1100 thermometer

with 0.10C calibration, cork, iron stand, stirrer.

MATERIAL REQUIRED :- Glucose, ice, water and KNO3.

PROCEDURE :-

1) Take three clean and dry boiling tubes and label them as 1,2,3.

2) Take 20ml distilled water in each boiling tube. Add 1g of glucose to boilingtube no.2 and 2g of glucose to boiling tube no. 3. Shake till the glucose is

dissolved.

3) Put a mixture of ice and KNO3 in a beaker and fix test tube no. 1 in it and dip

a stirrer. Insert a thermometer in a bored cork and fix in a stand in such a way

that its bulb dips in water in the boiling tube.

4) Water is stirred in the boiling tube and watch the temperature which is

falling gradually. Note the temperature when it remains constant and water

starts freezing. This is the freezing point of the pure water.

5) Take away boiling tube no. 1 from the beaker and replace it by the boiling

tube no. 2. Repeat the experiment and note the freezing point of glucose

solution (it is 5% since 1g is dissolved in 20ml water).



6) Now place boiling tube no. 3 in position and repeat the experiment to note its

freezing point. (10% glucose).

Observation :-

FREEZING POINT

PURE WATER

10% GLUCOSE

20% GLUCOSE

RESULT :-

1) Freezing point of water decreases due to the presence of dissolved glucose.

2) Depression in the freezing point increases as the quantity of dissolved

glucose increase.

Experiment no.2

AIM :- 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.

PROCEDURE :-

1) Take three clean and dry boiling tubes fitted with a cork with two holes

(each) one for the thermometer and other for a delivery tube.



2) Take 20ml distilled water in each boiling tube. Add 1g of glucose to boiling

tube no.2 and 2g of glucose to boiling tube no.3. Shake till the glucose is

dissolved.

3) Fit boiling tube no. 1 with a thermometer, keeping its bulb above the level of

water. Fit it with a delivery tube, fit it in an iron stand and start heating it on asand bath.

4) Heat and watch the temperature which rises gradually and note it when

becomes constant, i.e. when water starts boiling. This is the boiling point of

pure water.

5) Now fit the boiling tube no. 2 in the above described manner and heat it.

Note the boiling point of 5% glucose solution.

6) Similarly, note the boiling point of 10% glucose solution.

Observation :-

BOILING POINT

PURE WATER

10% GLUCOSE

20% GLUCOSE

RESULT :-

1) Boiling point of water increases due to the presence of dissolved glucose.

2) Elevation in the boiling point increases as the quantity of dissolved glucose

increase.

Boiling Point & Freeezing Point as Colligative Property :



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.

Effect of impurities on freezing point

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 whichvapour 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.

Conclusion

1) 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.



2) 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

effect of impurities on the

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