7.2 Physical vs Chemical Change

What Counts as a Change?

Almost everything that happens to matter can be sorted into two kinds of change, and ICAS questions on this topic - physical and chemical change is a named topic on Paper D - nearly always come down to one golden test: is a new substance formed?

A physical change alters how a substance looks or feels - its state, shape, size, or whether it is dissolved - but the substance itself stays the same. No new substance is made. Melt an ice cube and you still have water. Dissolve salt in water and the salt is still there; evaporate the water away and you can get the salt back. Tear a piece of paper and every scrap is still paper.

A chemical change, also called a chemical reaction, rearranges particles into one or more new substances with different properties from what you started with. Burn that same piece of paper and you end up with ash, smoke and invisible gases. You cannot un-burn it, because the original substance no longer exists.

Sorting everyday changes

The table hides a famous trap: boiling water looks dramatic, with bubbles and steam, but it is a physical change - steam is still water. Meanwhile an egg boiling in the same saucepan is undergoing a chemical change, because the heat permanently rearranges its proteins. Drama is not the test; new substances are.

Dissolving is another favourite trap. Salt dissolving in water is physical because no new substance forms and the salt can be recovered. But dropping an antacid tablet into water involves both dissolving and a chemical reaction - the fizzing is a new gas being made. Read the stimulus carefully and look for evidence of something new.

Signs That a Chemical Change Has Happened

You cannot watch particles rearranging, so scientists look for observable clues:

• a colour change that is not just mixing, such as a cut apple turning brown
• bubbles of gas appearing without boiling, such as vinegar poured on bicarbonate of soda
• a temperature change the mixture produces on its own, such as a hand warmer heating up
• a precipitate, meaning a solid forming when two liquids are mixed
• a new smell, such as milk turning sour

One sign alone is not proof - boiling water makes bubbles too, and that is physical. But several signs together, or a change you cannot undo, strongly suggest a chemical change.

Reversibility: a clue, not a rule

Physical changes are usually easy to reverse: freeze the melted water and you have ice again. Chemical changes are usually hard or impossible to reverse: you cannot turn ash back into paper. Use this as a supporting clue rather than the deciding test, because it has exceptions - and ICAS knows it. The deciding test always remains: was a new substance formed?

What happens to the particles

From Paper F onwards, ICAS expects you to describe chemical processes with the particle model, and Paper E already asks about the chemical processes involved in food production. In a chemical reaction the particles themselves are not created or destroyed - they are rearranged into new combinations. Every particle in a piece of firewood ends up somewhere in the ash, the smoke and the gases. That rearrangement is why the products have completely different properties from the reactants, even though they are built from the same particles.

Conservation of mass

In a closed system - one where nothing can enter and nothing can escape - the total mass before any change exactly equals the total mass after it, whether the change is physical or chemical. Melt 10 grams of ice inside a sealed plastic bag and the bag still reads 10 grams. React vinegar with bicarbonate of soda inside a sealed flask sitting on a balance and the reading never moves, even though a gas is being made. Open the flask, however, and the gas escapes - the mass appears to drop, which is a favourite ICAS trap. The mass has not vanished; it has simply left the system.

Senior students should know that Paper I formalises this idea as the law of conservation and the law of constant proportion: matter is neither created nor destroyed in a reaction, and a pure compound always contains the same elements in the same proportions. For now, the closed-system picture is all you need.

A decision routine you can apply

When a question shows you a change, walk through these steps in order. First, name what you start with and what you end with. Second, ask whether anything new has appeared - a gas, a solid in a liquid, a new colour, a new smell, or heat given off or absorbed. If yes, call it a chemical change. Third, if nothing new appears, ask whether you could easily get the original substance back - by cooling, freezing, evaporating or filtering. If yes, call it a physical change. Fourth, if you are still unsure, look for the one clue the question-writer always plants: there will be at least one new substance mentioned somewhere in the stimulus if the intended answer is chemical.