What is Boiling Point ?

 1. Boiling point is defined as the temperature at which vapour pressure(VP) of liquid is equal to atmospheric pressure (760mm of Hg). It represents dynamic equlibrium i.e escaping tendency of molecules from liquid to vapour state is equal to that of molecules from gaseous state to liquid state.

2. When is heat is transferred to a liquid , vaporization takes place. This process continues until the pressure exerted by vapour is equal to the atmospheric pressure.B.pt of liquid depends of atmospheric pressure.

3. At a given pressure, different liquids will boil at different temperatures.

4. Presence of non volatile impurities increases the b.pt of a liquid 

5. It provides brief indication of attractive forces of liquids.

6. It rises with increase in molecular mass due to enhanced vander waals forces.

7. Branching of compounds decreases the boiling point .

8. B.pt of liquid is measured by using Ebullioscope

Examples –

Boiling point of water is 100°c

Ethanol boiling point 78.37°c

Ethylene glycol boiling point 197.6°c

Reduction of external pressure results in decrease of boiling point of liquid .

Example-

At sea level the b.pt of water is 100°c , but at higher altitude the b.pt is lower than at sea level (at 6,600 feet the b.pt is 93.4 °c)

Boiling point elevation

Solvent with impurity has higher boiling point than a given pure solvent.

Example –

Boiling point of solution [ water+salt] is higher when compared to pure solvent [Water] .

Boiling point formula

The molality of the solute particles (in solution) is directly proportional to boiling point elevation

Boiling point of solution is given by

B.pt = ∆T + b.pt of the solvent

Or

T solution = Tpure solvent + ∆T

Here ,

∆T = Km

∆T = Amount by which the b.pt is elevated.

K = molal b.pt elevation constant(solvent dependent)

m =molality (moles of solute particles / kg of solution)

Difference between Boiling and Evaporation

1.In boiling liquid is converted in to vapour when it is heated to its b.pt  whereas in evaporation liquid is converted in to vapour at a temperature below its boiling pt.

2.Boiling  is a quick process compared to evaporation.

3.In boiling energy is supplied through a energy source and in evaporation energy is supplied by surrounding atmosphere.

4.Boiling occurs only at the b.pt of liquid and evaporation occurs at all temperatures below b.pt