Question

Which liquid among water and alcohol, has a lower boiling point (boils first) and can be used for distillation?
A.Alcohol
B.Water
C.Both have the same boiling points
D.None of these

Answer 1

Hint: The boiling point of a substance depends on its molecular weight as well as the force of attraction in between the constituent particles. Ionic molecules have higher boiling point than covalent molecules.

Complete answer:
Among water and alcohol, the boiling point may vary. Water has the molecular weight of 18 while the simplest alcohol, methanol has the molecular weight of 32. So, on the basis of molecular weight, the boiling point of methanol should be greater than that of water. But then the “hydrogen bonding” comes into picture.
The hydrogen bonding exists in between the molecules of both methanol and water, but due to the presence of the alkyl groups in the alcohols, only the end of the molecules is available for bonding which is the end containing the hydroxyl group. Hence the strength of hydrogen bonding is relatively weak in case of methanol as well as the rest of the alcohols.
Hence, the alcohols should have lower boiling point than water, except for very high molecular weight alcohols. So as alcohols boil faster, they can be used for distillation.

So, the correct answer is option (A).

Note:
Hydrogen bonding can be defined as a special type of dipole-dipole attraction between molecules that contain strong electronegative atoms such as nitrogen, oxygen, and fluorine attached to a hydrogen atom. Due to the electronegativity differences, the hydrogen atom of one molecule acts as the positive dipole and gets attracted to the electronegative atom of the next molecule.