How do covalent bonds affect physical properties?
Covalent bonds are formed by sharing of electrons between atoms. Compounds that are made of covalent bonds usually have low melting and boiling points,are nonconductive and not soluble in water.
Covalent bonds are relatively weak than ionic bonds. That's the reason behind the low melting and boiling points of covalent compounds.
Secondly, during a covalent bond formation, no ions are formed. This is why covalent compounds are nonconductive. They don't conduct electricity or heat. This is also the reason behind their insolubility in water (because no ions are formed). Covalent compounds are more likely to dissolve in organic solvents (e.g. chloroform, benzene).