Which has the weakest bond?

In silicon dioxide $(SiO_2)$:

Give reasons: Graphite is used as a lubricant.

Potash alum is a

What is the dependence of interaction energy on the distance $(r)$ between molecules for the following forces?
$(i)$ London forces
$(ii)$ Dipole-dipole forces (stationary)
$(iii)$ Dipole-induced dipole forces

Diamond is extremely hard whereas graphite is soft. This is because