Changes that make something new
Chemistry · Key Stage 2 · The School
Melted chocolate sets again and dissolved salt comes back when the water evaporates. Burnt toast never comes back. The difference is that burning made a new material, and the change that made it cannot simply be undone by cooling or evaporating.
Bubbles of gas where there was no boiling, a colour change that does not reverse, heat given out, or a smell that was not there before. Any of these is a clue that the substances have become different ones rather than merely changing state or mixing.
Mix them and the froth is carbon dioxide, a gas that was in neither before. Let it settle and you cannot get your vinegar and powder back — the new substances are the evidence. This is why the change is called irreversible rather than just messy.
Iron left damp turns to rust: a new, flaky, orange material that is not iron. It takes weeks rather than seconds, but like burning it combines the metal with oxygen and cannot be reversed by drying. Painting a gate works by keeping the oxygen and water away.
A raw egg is clear and runny; a cooked one is white and firm and will never go back. Bread dough rises because yeast makes carbon dioxide, and toasting browns the surface into something that was not there before. Every one of these is a new material formed by heat, which makes a kitchen the most familiar chemistry lab most people ever use.