Physical change or chemical change?

Melting an ice cube and burning a log are both ‘transformations’, but they have nothing to do with one another. Knowing how to tell them apart is the first step before we can talk about chemical reactions.

The key criterion

The only question you need to ask yourself is: have any new substances been formed?

	Physical transformation	Chemical transformation
New substance?	no, the matter remains the same	yes, new substances are formed
Reversible?	often easily	rarely, or with great difficulty
Examples	melting of ice, dissolving of sugar	burning of wood, rusting of iron

Two detailed examples

When an ice cube melts, you get liquid water: it is still water, H₂O, both before and after. Nothing new has been formed — only the arrangement of the molecules has changed. This is a physical change.

When iron rusts on contact with moist air, iron oxide forms: a reddish-brown, brittle substance that no longer has the properties of the original shiny metallic iron. A new substance has been formed: this is a chemical transformation, also known as a chemical reaction.

The classic trap

Dissolving sugar in water produces a clear, sweet solution: one might think that a new substance has been created. But if you allow the water to evaporate, you’ll find the sugar intact and unchanged. No new substance has been created; the sugar has simply dispersed amongst the water molecules: this is a physical transformation, despite the misleading appearance of a new ‘mixture’.