Mass is always conserved in a physical change. Energy may be released or absorbed when a substance changes from one physical state to another. In a chemical change, a chemical reaction yields a completely new substance. A substance's particles are changed during a chemical reaction.
Bromine is less electronegative than chlorine, yet methyl bromide and methyl chloride have very similar dipole moments. This is because the bond distance in methyl bromide is more due to the large size of bromine atom.
Dipole moment is calculated by multiplying the charge on the atom with the bond distance.