Physical properties tend to be determined by a molecules mass and whether it is polar or non-polar.  Chemical properties are determined primarily by the nature of the bonds which hold atoms together in the molecule.

Alkanes and alkenes tend to be very similar in mass and polarity due to the fact that alkenes are created by removing two lightweight hydrogen atoms from an alkane and creating a double bond between carbons.  The loss of the mass of two hydrogens compared to the overall mass of the molecule does not significantly change the the mass.  Replacing the single bond with a double bond also does not significantly change the non-polar nature of the molecule.  Consequently physical properties remain very similar.

However, a double bond is a significanlty different chemical bond than a single bond requiring a considerable more energy to break.  Therefore, the chemical properties of the alkenes are considerably more different than the alkanes due to the presence of the double bonds.