Classifying changes

Every classification question collapses to one test: has a new substance, with different chemical properties, been formed?

• Physical change — no new substance appears. Only the state, shape, size, or physical form of the substance changes; its chemical composition is untouched. Because nothing chemically new has appeared, the original substance can usually be recovered by a suitable physical method (cooling, evaporation, filtration) — a helpful clue, though recoverability itself is not the official test.
• Chemical change — a new substance appears, with properties genuinely different from what you started with. Simple physical steps like heating, cooling, or filtering cannot get the original substance back; a separate chemical process would be needed.

You can write the whole rule as a compact rule of thumb:

where: "new substance" means matter with a different chemical formula and different chemical properties (colour, smell, reactivity, and so on) from the starting material — not merely a different state or shape of the same substance. Four everyday clues often accompany a chemical change, though none alone is proof: a colour change that is not just melting or dissolving, a gas being given off (bubbles, a smell, a hiss), a precipitate — an insoluble solid — appearing in a liquid, or heat/light being released or absorbed. You need at least one of these plus the actual identity test to be confident. Running underneath both types of change, always, is the Law of Conservation of Mass: the total mass of everything you start with equals the total mass of everything you end up with, counting any gas absorbed from or released into the air. Classifying a change never changes how much matter exists — it only decides whether that matter is chemically the same stuff or something new.

The goal here is to see why "new substance formed?" is the only test that never fails you, even when a change looks small (dissolving sugar) or looks huge (burning paper), and why looser tests such as "does it look different?" or "can I undo it?" both break down on ordinary examples.

Step 1. Start from what "substance" means in chemistry: a specific kind of matter with its own fixed set of properties — melting point, colour, smell, how it reacts with other things. A substance's state (solid, liquid, gas) and its shape or size are not part of that identity. Ice, water, and steam are the three states of exactly one substance, water; cutting a sheet of paper into confetti changes its size, not its identity as paper.

Step 2. Apply this to physical changes. Melting, freezing, boiling, condensing, sublimation, dissolving, cutting, and stretching all rearrange the state, shape, size, or spread of a substance without altering its chemical composition. Salt dissolved in water is still salt — you can prove it by evaporating the water and weighing the same mass of salt crystals back out. Since the identity never changed, no new substance ever formed, so the test correctly labels all of these as physical.

Step 3. Apply it to chemical changes. Rusting, burning, cooking, curdling milk, and a fizzy tablet reacting in water each produce matter with a genuinely different set of properties from what you started with — rust is not iron, ash is not paper, curd is not milk. Because those properties differ chemically, not just in appearance or state, the test correctly labels these as chemical, even in cases like curdling where the new substance still looks like a liquid or solid similar to the original.

Step 4. See why the weaker, tempting tests fail. "Looks different" fails because melting ice looks nothing like ice, yet nothing new formed — it is physical. "Can be undone" fails as a formal definition because some chemical changes can, in principle, be reversed by further chemical reactions, and because ease of reversal is a consequence of the identity test, not an independent rule. Only "has a new substance, with different properties, been formed?" survives every counter-example, which is why it is the test the syllabus asks you to state and apply.

Question. A shiny iron nail is left outdoors in moist air for several weeks and develops a reddish-brown flaky coating. A second, identical nail is heated in a flame until it glows red, then allowed to cool back to room temperature, and returns to its original shiny grey appearance. Classify each change and justify your answer.

Step 1 — Nail 1: identify what happened. The shiny grey iron surface has been replaced by a reddish-brown, flaky coating called rust. This is the everyday process of rusting, which needs both oxygen and moisture from the air.

Step 2 — Nail 1: apply the test. Is rust the same substance as iron? No — rust is soft and flaky rather than hard and shiny, a different colour, and chemically distinct. A new substance has clearly formed, and it cannot be turned back into iron by drying, polishing, or any other simple physical method.

Step 3 — Nail 1: conclusion. This is a chemical change — a new substance (rust), with different properties from iron, has formed, and it cannot be undone by simple physical means.

Step 4 — Nail 2: identify and apply the test. Heating the second nail red-hot and letting it cool changes only its temperature and, briefly, its glow. Once cooled, it is shiny grey iron again, with the exact same properties it started with — no new substance has appeared at any stage, only a temporary change in how hot the iron is.

Step 5 — Nail 2: conclusion. This is a physical change — heating and cooling altered only the temperature (and briefly the glow) of the same iron; nothing chemically new was formed, and the nail returned exactly to its starting condition.

Result: nail 1 undergoes a chemical change (rusting); nail 2 undergoes a physical change (heating and cooling), even though both nails experienced striking visual changes.

Competitive stretch

Question. A student argues: "Nail 1's change is chemical because it cannot be reversed, and nail 2's change is physical because it can be reversed — reversibility is the real test." Is this acceptable for full marks?

Step 1. The student has swapped "has a new substance formed?" for "can it be reversed?" These usually agree — which is why the mix-up is tempting — but they are not the same statement, and reversibility alone is not the definition examiners want.

Step 2 — verdict. Full marks require stating the identity test as the definition and mentioning reversibility only as a secondary clue. The student loses a mark for defining the test by reversibility instead of by new-substance formation.

• Thinking "a big visual change always means chemical change." Melting, freezing, dissolving sugar, and cutting paper all look dramatic but form no new substance — they stay physical.
• Assuming all physical changes are reversible and all chemical changes are irreversible. This is often true but is not the official test; always fall back on "has a new substance formed?"
• Confusing dissolving with reacting. Salt dissolved in water forms no new substance — evaporating the water recovers the same salt, unchanged in mass and identity.
• Forgetting that every change of state (melting, freezing, boiling, condensing, sublimation) is physical, because the chemical formula and properties of the substance never change — only its state does.

Ask the tutor: Not sure whether a change you have thought of — like whipping cream, a fizzy tablet dissolving, or ice cream melting and refreezing — counts as physical or chemical? Ask your AI tutor to walk through the "new substance formed?" test with your example, step by step.