Boiling point

“The temperature at which vapour pressure of a liquid becomes equal to atmospheric or external pressure is called the boiling point”.

The phenomenon of conversion of liquid at its boiling point is called boiling.

With an increase in temperature vapour pressure increases and ultimately reaches atmospheric pressure or external pressure, at this stage, boiling starts. At boiling temperature remains constant although heat is continuously supplied. Before boiling point heat supplied is partly used to increase P.E and partly used to increase K.E. Due to the increase in K.E temperature increases. But at boiling point, all the heat supplied is used to increase P.E and K.E remains constant so the temperature remains constant.

Liquid	B.P (oC)	Liquid	B.P (oC)	Liquid	B.P (oC)
Carbon disulphide (CS2)	46.30	Ethanol (C2H5OH)	78.40	Water (H2O)	100.00
Carbon tetrachloride (CCl4)	76.80	Benzene (C6H6)	80.10	Acetic acid (CH3COOH)	118.10

Factors Affecting Boling Point:

Intermolecular forces:

The strength of intermolecular forces influences the boiling points of liquids. Stronger are the intermolecular forces low will be the vapour pressure and high will be the boiling point of the liquid and vice versa. Thus the boiling point is the measure of hydrogen bonding. e.g. boiling point of H2O is higher than that of H2S, H2O has strong hydrogen bonding while H2S has dipole-dipole forces which are weaker than hydrogen bonding, so the H2O of water is higher.

Vapour pressures of some compounds:

Compound	Formula	Vapour pressure at 293K/20oC
Isopentane	CH3-CH2-CH (CH3)-CH3	580 Torr
Chloroform	CHCl3	170 Torr
Carbon tetrachloride	CCl4	87 Torr
Water	H2O	18 Torr
Mercury	Hg	0.0012 Torr

Order of Strength of I.M.F     Hg > H2O > CCl4 > CHCl3 > CH3-CH2-CH (CH3)-CH3

Order of Vapour pressures    CH3-CH2-CH (CH3)-CH3 > CHCl3 > CCl4 > H2O  > Hg

Order of Boiling points     Hg > H2O > CCl4 > CHCl3 > CH3-CH2-CH (CH3)-CH3

 This shows that liquids having higher vapour pressures have weak intermolecular forces and in turn low boiling points and vice versa.                        

External Pressure:                                      

The liquid boils when its vapour pressure becomes equal to external pressure, thus boiling point depends upon external pressure. When external pressure is high liquid requires more heating to equalize its vapour pressure to external pressure and thus boils at higher temperature and vice versa. Similarly, when external pressure is low, the liquid needs less heating to equalize its vapour pressure to external pressure and thus boils at a lower temperature e.g. (i) Water boils at Karachi (atmospheric pressure = 760mmHg) at 100oC while it boils at Murree hills (atmospheric pressure = 700mmHg) at 98oC. Mount Everest is still a higher peak where atmospheric pressure is around 225mmHg so water boils at 69oC there.  

Impurities:

The impure liquid boils at slightly higher temperatures i.e. presence of solute in a liquid increases its boiling point. An increase in boiling point depends upon the number of particles in the solute. e.g pure water boils at 100 ^oC but when 180g of Glucose or 342g of sucrose are dissolved in 1kg of water then it boils at 100.51 ^oC.

How is a boiling point used in real life?

Applications of Boiling:

Pressure cooker:

It is a closed container so the vapours as well as air in it cannot escape and thus pressure inside it increases. Due to an increase in pressure boiling point of water increases. So more heat will be absorbed by water and as a result, food will be cooked quickly.

Vacuum distillation:

The technique used to purify a liquid sample by first boiling and then condensing the vapours of to get pure liquid is called distillation. 

“The distillation that is carried out at reduced or lower pressure is called vacuum distillation”.

By lowering the external pressure boiling point lowers. Thus in vacuum distillation boiling takes place at a temperature lower than the normal boiling point by reducing the external pressure.

Vacuum distillation is carried out:

• To Avoid Decomposition: Some liquids decompose near their normal boiling point so their vacuum distillation is carried out. e.g. Glycerin boils at 290 ^oC at 760 mmHg pressure but it decomposes at this temperature whereas it boils at 120 ^oC at 50 mmHg so it is distilled at this pressure. Similarly, H₂SO₄ decomposes at its normal boiling point (330 ^oC) so its vacuum distillation is carried out.  
  • To Avoid Burning of Liquid: Some liquids are inflammable near their normal boiling point. e.g. Acetic acid is inflammable near its normal boiling point (118 ^oC) so vacuum distillation prevents it from catching fire.
  • To Save Fuel: Some liquids have high boiling points. e.g. mobile oil has a high boiling point so it is obtained by vacuum distillation of petroleum. It is boiled at a lower temperature by reducing pressure to save energy (fuel).
• To facilitate Distillation: Lower temperatures are easy to achieve so vacuum distillation is carried out at lower temperatures by reducing pressure.