Question

Place the following substances in order of increasing viscosity.

CH3CH2CH2OH, CH3OH, CH3CH2CH(OH)2, CH3CH3

Could you also explain how you got your answer, it would help tremendously!

Answer 1

The correct order for increasing viscosity:

CH3CH3 < CH3OH < CH3CH2CH2OH <   CH3CH2CH(OH)2

Actually, the viscosity refers to a liquids resistance to flow and this increases with increase in the intermolecular forces of attraction. The force of arrtaction maybe in the form of hydrogen bonding (dipole-dipole interactions), dipole induced interactions or other molecular forces like van der Waals forces. In ethane there is only  van der Waals forces which is very weak and hence it is a gas, methanol has one polar OH group causing intermolecular H-bonding and small CH3 group for other molecular interaction hence has more viscosity. Further, CH3CH2CH2OH has one polar OH and long chain hence, has more viscosity than CH3OH. Last molecule CH3CH2CH(OH)2 the inter molecular hydrogen bonding increases many times because of two OH groups.