Silicon dioxide ($Si{O}_2$) has a high melting point because strong covalent bonds throughout its giant macromolecular structure must be broken.

Magnesium has a higher melting point than sodium because the $M{g}^{2+}$ ion has a higher charge density than the $N{a}^{+}$ ion, leading to stronger metallic bonding.

Phosphorus ($P_4$) has a higher melting point than sulfur ($S_8$) because phosphorus molecules contain more electrons, resulting in stronger van der Waals forces.

Ice has a lower density than liquid water because hydrogen bonds hold the water molecules in an open lattice when solid.