Explain the following briefly: (a) Sodium chloride dissolves in water but carbon tetrachloride is insoluble in water. (b) Helium does not form He2 molecule. (c) Pure water does not conduct electricity, but on adding sodium chloride to it, it starts conducting electricity. (d) Cl2 is a non-polar molecule, while HCl is a polar molecule. (e) Metals are electropositive.