Physical and chemical changes

A substance can change its appearance, shape or state without becoming a different substance. A physical change changes its form or state but does not produce a new substance. For example, when ice melts, solid water becomes liquid water. The water looks different, but it is the same substance before and after melting.

A chemical change produces one or more new substances with properties different from those of the starting substances. For example, when magnesium burns in oxygen, it forms magnesium oxide, a new substance.

Distinguishing physical and chemical changes

The key question is whether the substances present after the change are the same as those present before it. A change in appearance alone does not establish that a chemical change has occurred: melting ice changes its appearance without producing a new substance.

Reversibility is not a reliable test. Some physical changes are difficult to reverse, such as tearing paper, while some chemical changes can be reversed by further chemical changes. The classification depends on whether new substances form, not on how easily the change can be undone.