Melting of Covalent Networks
A chemical change is defined as when your substance reacts with another substance to form some products, or decomposition into other products. In a chemical change, there will be formation of new chemicals that are different in terms of chemical formula.

Thus, breaking of intramolecular bonds will not be a chemical change in the case of C-C bonds in diamond/graphite, because there is no formation of new chemical substances with an alternative formula. Similarly, ionization (without formation of complex ions) would be physical change