Physical and Chemical Changes
Ice melting into water and a matchstick burning into ash both look like transformations — something clearly happened — but only one of them creates an entirely new substance. Melt the ice and you still have HO; burn the match and the wood is gone, replaced by ash and gases with completely different properties. Chemistry needs a sharp, reliable way to tell these two kinds of change apart, because it decides whether a process can be reversed, whether mass appears to change, and whether a new compound has come into existence at all. This article builds that test from everyday examples — melting wax, dissolving salt, burning magnesium, rusting iron, curdling milk — and shows how the same single question sorts every one of them correctly, tying back to why purifying a mixture is a physical change while forming a compound is a chemical one.
A change is physical if no new substance forms (only appearance, state, or shape changes, and it is usually reversible) and chemical if one or more new substances with different properties form (usually irreversible), and this single test — not how dramatic or heat-involving the change looks — is what decides the classification.
The core idea
Physical change. Only the physical properties of a substance — its state, shape, size, or appearance — change, while its chemical composition stays exactly the same. Physical changes are generally temporary and reversible: given the right conditions, the substance can usually be brought back to its original form. Melting wax, dissolving sugar in water, tearing paper, and boiling water into steam are all physical changes — cool the steam and you get water back, evaporate the sugar solution and you recover sugar.
Chemical change. One or more new substances are formed, with properties different from the starting substance(s). Chemical changes are generally irreversible under ordinary conditions, because the new substance cannot simply be turned back into the original by a physical step like cooling or evaporating. Burning magnesium ribbon (forming magnesium oxide, a white powder utterly unlike the shiny grey metal), rusting of iron, digestion of food, and curdling of milk are all chemical changes.
The single test. Whatever the change looks like — however much heat, light, or fizzing is involved — one question decides the category:
where: "new substance" means matter with a different chemical composition and different properties (colour, texture, conductivity, reactivity, and so on) from what you started with — not merely a different shape, state, or appearance of the same substance.
Mass is conserved either way. A chemical change can look like it destroys or creates matter, but it never does. If magnesium combines with oxygen from the air to form magnesium oxide, the mass of magnesium oxide produced equals the combined mass of the magnesium and the oxygen that reacted — nothing is created or destroyed, only rearranged into a new substance. This is a preview of the Law of Conservation of Mass, studied formally in "Atoms and Molecules"; balancing the chemical equation itself is not required at this stage.
Why it works
This topic is descriptive rather than a formula to derive, but the reasoning behind the test is still worth spelling out carefully, because it is exactly the reasoning examiners expect in a "justify your answer" question. Properties belong to substances, not to their shape or state. A given substance — say, water — has a fixed set of chemical properties: it is HO, it reacts with sodium in a particular way, it has a particular boiling point. Freezing it, melting it, or boiling it changes how it looks and behaves physically (solid, liquid, gas), but every molecule is still HO. Nothing about its chemical identity has changed, so no new substance exists, and the change is physical — and because the identity never changed, reversing the state (cooling steam back to water, freezing water back to ice) restores the original in every respect. A chemical change is different in kind, not just degree. When magnesium burns, the magnesium atoms bond with oxygen atoms to form a new compound, magnesium oxide, whose particles are arranged and bonded completely differently from either magnesium metal or oxygen gas. The new substance has its own distinct properties — colour, texture, conductivity — that belong to it and not to the starting materials. Because the bonding itself has changed, there is no simple physical step (cooling, evaporating, cutting) that undoes it; you cannot get magnesium metal back from magnesium oxide by melting or grinding it. That is why chemical changes are treated as irreversible under ordinary conditions, while physical changes are treated as reversible. The conservation-of-mass idea explains why "losing" or "gaining" matter during a chemical change is only apparent. Atoms are neither created nor destroyed — they are only rearranged into new combinations. So even though magnesium ash looks nothing like magnesium metal, weighing the ash together with the oxygen consumed from the air (in a closed system) shows the total mass is unchanged. The "new substance" is new in identity and properties, not in the amount of matter involved.
Worked example
Question. A student takes a small piece of magnesium ribbon, cleans its surface with sandpaper, and holds it in a flame with tongs. The ribbon burns with a dazzling white light and turns into a white, powdery ash. Separately, the student also dissolves a spoon of common salt in water and later evaporates the water to recover salt crystals. For each process, state whether it is a physical or a chemical change, and justify using properties before and after.
Step 1. Look at magnesium before burning. Magnesium ribbon is a shiny, grey, malleable metallic solid, a good conductor of electricity — properties characteristic of a metal.
Step 2. Look at the product after burning. The white powdery ash (magnesium oxide) is dull, brittle, and does not conduct electricity — properties completely different from magnesium metal, and there is no way to melt or grind the ash back into shiny magnesium.
Step 3. Apply the test to the burning. Since a new substance (magnesium oxide) has formed with properties entirely different from magnesium metal, and the process cannot be reversed by any simple physical step, burning magnesium is a chemical change. If weighed carefully in a closed container, accounting for the oxygen consumed from the air, the ash would weigh more than the original ribbon — the mass of magnesium plus the oxygen that reacted equals the mass of magnesium oxide formed, so mass is conserved overall even though a new substance has appeared.
Step 4. Apply the test to dissolving and recovering salt. Salt disperses as individual particles in water, but when the water is evaporated, the same salt crystals reappear with exactly the same properties as before — taste, crystal shape, chemical identity. No new substance was ever formed at any stage, so dissolving (and recovering) salt is a physical change, and it is fully reversible by evaporation.
Final answer: burning magnesium is a chemical change (new substance, irreversible); dissolving and recovering salt is a physical change (same substance throughout, reversible).
Common traps
- Assuming any change involving heat or a dramatic visual effect must be chemical. Melting and boiling involve heat and a visible state change but are physical changes — the test is whether a new substance forms, not how dramatic the change looks.
- Believing all chemical changes are strictly, absolutely irreversible. "Generally irreversible" describes typical behaviour, not an unbreakable law, but for Class 9 purposes chemical changes are treated as irreversible.
- Thinking dissolving is always physical with no exceptions. Simple dissolving (sugar or salt in water) is physical because the solute can be recovered unchanged, but if a solid reacts with the liquid to form a new substance (as when a metal reacts with an acid), that is chemical.
- Confusing "change in state" with "change in substance." Ice, water, and steam are three physical states of the same HO; no new substance forms between them, so all these transitions are physical changes, however different they look.
Ask the tutor: Not sure whether a specific everyday process, like baking a cake or a candle burning, is physical, chemical, or a bit of both — ask the AI tutor to break it down step by step with you.
Is curdling of milk physical or chemical, and why does it feel different from just cooling milk?
Curdling is a chemical change. Bacterial action alters the milk proteins themselves, producing curd with a different taste, texture, and composition from milk — a genuinely new substance. This is different from simply cooling or heating milk, which changes its temperature and perhaps its texture slightly but does not alter what the milk is made of. Because curd cannot be turned back into milk by any simple physical step, the change is treated as irreversible, matching the usual chemical-change pattern.
If rusting happens slowly over days, how can it still be a chemical change like fast, dramatic burning?
Speed has nothing to do with the classification — only whether a new substance forms does. Rusting is iron reacting slowly with oxygen and moisture in the air to form rust (hydrated iron oxide), a reddish-brown, flaky substance with completely different properties from shiny metallic iron. Burning magnesium does the same kind of thing, combining a metal with oxygen to form a new oxide, just very fast and with visible light. Both are chemical changes because both produce a new substance that cannot be reversed back into the original metal by any simple physical step.
Why is melting wax a physical change but burning a candle's wick a chemical change, when both happen at the same time?
A burning candle actually involves two changes happening together, and it helps to separate them. The wax near the flame melts and later re-solidifies as it cools — that part is a physical change, since it is the same wax substance throughout, only in a different state. But the wick and the wax vapour that actually burn in the flame combine with oxygen and are converted into carbon dioxide, water vapour, soot, and light and heat energy — genuinely new substances with properties nothing like wax, and this cannot be reversed. So a candle is a good example of a process that is part physical (melting) and part chemical (burning), and a full answer should identify both parts separately rather than calling the whole candle one single type of change.