Which of the following most likely happens during a physical change? a) an irreversible process takes place. b) substance changes its form but not its molecular composition. ( i think maybe it's this one) c) new atoms are introduced in the molecules of the substance. d) a substance reacts differently with a chemical before and after the change. which of the following represents a chemical change? a) souring of milk (maybe this one? ) b) melting of chocolate c) condensation of water d) breaking of an egg shell