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Boiling Point -Factors on which boiling point depends

Boiling point is the temperature at which vapour pressure of liquid is equals to atmospheric pressure.

Boiling point of any liquid depends on the length of carbon-carbon chain(parent chain) ,  functional group and branching in the parent chain. 

Length of parent chain

Boiling point increases with increase in the length of carbon-carbon  chain .This is because when length of chain increases , it increase the surface area of the chain. More the surface area , more will be the van der Waal's force of attraction and more energy requires to break this bond . 
Boiling point of the member of same family and having same functional group increases with the length of chain .

For example , boiling point of 
Propane is more than ethane and boiling point of ethane is more than Methane.
i.e Propane > Ethane > Methane .

Branching in a  parent chain .

Boiling point decreases with branching . This is because branching in the parent chain decreases the surface area of the chain . Due to this can der Waal's force of attraction decreases in the Molecule . So , it become easier to break the bond between the Molecules 

Polarity in the parent chain

Boiling point also increases with polarity of Molecules . More the polarity , more will be the boiling point of liquid . For example , alkyl halides are more polar than hydrocarbons (Alkanes , Alkenes and Alkynes ). Therefore it requires more energy to break the bonds between these Molecules . 


Types of intermolecular forces 

Boiling point of liquid depends on the intermolecular force of attraction between the Molecules . Increasing order of the intermolecular force of attraction ,
Ion dipole > hydrogen bonding > dipole-dipole > van der waal

Polar Molecules containing ions have high boiling point than just a polar Molecule  for example R-COONa+ is an example of ion-dipole interaction . 

Increasing order of boiling point due to intermolecular forces :

R-COONa > R-COOH > R-O-R > R-R 
(R - represent alkyl group )

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