Identification for compounds

the reason is very simple. first we should notice the magnitude of melting point is depend on the intermolecular force between the molecule. Stronger the intermolecular force, more heat need to supply to breask the bond and melt it. Take propanone and propanal as an example they both have the same functional group of carbonyl group as oxygen is an electronegativity atom thus oxygen has a partial negative charge. this contributr to the polar bond of c=o bond. that means both propanone and propanal are the polar molecule and the intermolecular force will be the same------------------dipole dipole interaction . thus they will have similar melting poing and the melting point cannot use ti identify them