Physical changes involve alterations in the physical properties of a substance, such as shape, size, state, or appearance, without forming any new substances. These changes are generally reversible. For example, melting of ice is a physical change because water remains H₂O, just in a different state. Similarly, tearing a piece of paper changes its size but not its chemical composition. Chemical changes, on the other hand, involve the formation of one or more new substances with entirely different properties from the original ones. These changes are usually irreversible and are often accompanied by energy changes, such as heat or light production. An example is the burning of wood, which produces ash and smoke, substances completely different from wood. Another example is the rusting of iron, where iron reacts with oxygen and moisture to form iron oxide, a new compound.