Physical and Chemical Change: One Test, Four Clues

One test, and four clues that point towards it

The test is single and it is not negotiable: is there a new substance present afterwards, with properties the original did not have? Everything else in this topic is evidence gathered in support of that one question.

Four observations usually mean a chemical change has happened: an unexpected colour change, a gas being produced, a temperature change that the surroundings did not cause, and a new permanent smell. Light, a precipitate forming in a clear solution, and a change that cannot be undone by heating or cooling belong on the same list.

Each of them has a physical impostor, and knowing the impostors is what separates a student who understands the topic from one who has memorised the list. Bubbles in boiling water are water vapour, not a new gas. Food colouring spreading through water is a colour change with no reaction. Water freezing releases energy to its surroundings. And a strong smell when a bottle is opened is a substance escaping, not a substance forming.

Six everyday changes, decided

The definitions get much easier to apply once you have argued through a set of real cases rather than a list of terms.

Two of these are worth extra attention. The kettle is the one students get wrong most often at Key Stage 3, because bubbling looks like a reaction — but the gas in those bubbles is water, and it condenses back to water on a cold surface. The baking cake is the one they get wrong at GCSE, because it feels like cooking rather than chemistry, and yet nothing about a baked cake can be returned to batter.

• Ice melting — physical. Still H₂O, and freezing it returns exactly what you had.
• Water boiling in a kettle — physical. The bubbles are water vapour, not hydrogen and oxygen; splitting water needs electrolysis.
• Paper burning — chemical. Carbon dioxide, water vapour and ash, and nothing brings the paper back.
• Iron rusting — chemical. Hydrated iron(III) oxide is brown, flaky and weaker than the iron it replaced.
• A cake baking — chemical. Raw batter and baked cake are different substances, and the rise comes from carbon dioxide released by a reaction.
• Bicarbonate of soda meeting vinegar — chemical. The fizz is carbon dioxide, and what is left in the glass is not vinegar.

Dissolving, where the rule of thumb collapses

Stir salt into water and it disappears. The word disappears is doing all the damage, because nothing that vanishes looks like it survived. Yet evaporate the water and the salt is back, in the same quantity, with all its original properties. No new substance was made — the ionic lattice separated into ions that were already there — and the change is physical.

Now dissolve carbon dioxide in water under pressure, which is what a fizzy drink is. Some of it simply dissolves, but a portion reacts with the water to form carbonic acid, which is a substance that was not there before and which makes the drink measurably acidic. That is chemical.

And drop magnesium ribbon into hydrochloric acid. The metal dissolves too, but it is gone for good: hydrogen bubbles off and magnesium chloride is left in solution, and no amount of evaporation returns the ribbon. Three cases, all described by the same everyday word, and the word tells you nothing about which kind of change occurred. Ask what is in the beaker afterwards instead.

The rusting nail that gets heavier

Weigh an iron nail, leave it in damp air until it is thoroughly rusted, and weigh it again. It is heavier. Students who have just been taught that mass is conserved find this alarming, and the explanation is the most useful thing in the whole topic.

Mass is conserved across the reaction, not across the nail. Oxygen from the air has joined the iron and become part of the rust, so the solid gains exactly the mass that the air has lost. The same thing happens when magnesium is burned in a crucible: the white ash weighs more than the ribbon did, because oxygen has been added.

Burning a candle looks like the opposite, and is not. The wax gets lighter because two of the products — carbon dioxide and water vapour — walk away into the room. Put the same reaction in a sealed container and the total mass does not move by a milligram. Whenever a mass appears to change in a chemical reaction, a gas has either joined in or escaped, and finding which is usually the whole of the question.