What is the cause of metallic luster?

Explain the repulsive forces between two molecules.

Suggest the most important type of intermolecular attractive interaction in the following pairs.
$(i)$ $n$-hexane and $n$-octane
$(ii)$ $I_{2}$ and $CCl_{4}$
$(iii)$ $NaClO_{4}$ and water
$(iv)$ methanol and acetone
$(v)$ acetonitrile $(CH_{3}CN)$ and acetone $(C_{3}H_{6}O)$

Metallic lustre is due to

Which intermolecular force is most sensitive to distance?

The ionic compound $BaSO_4$ is insoluble in water due to