See explanation below.
Work Step by Step
Plan: A physical change is one in which the physical form of a substance, but not its composition, is altered. A chemical change is one in which a substance is converted into a different substance with different composition and properties. a) Bonds have been broken in three yellow diatomic molecules. Bonds have been broken in three red diatomic molecules. The six resulting yellow atoms have reacted with three of the red atoms to form three molecules of a new substance. The remaining three red atoms have reacted with three blue atoms to form a new diatomic substance. b) There has been one physical change as the blue atoms at 273 K in the liquid phase are now in the gas phase at 473 K.