Physical and Chemical Changes
Physical and Chemical Changes
This topic gives 1 to 2 questions and looks simpler than it is, because several everyday changes sit close to the boundary. The examiner picks exactly those borderline cases, so learn the test rather than memorising a list.
The Distinction
| Point | Physical change | Chemical change |
|---|---|---|
| New substance | Not formed | Formed |
| Reversibility | Usually reversible | Usually irreversible |
| Composition | Unchanged | Changed |
| Energy change | Small | Often large, with heat or light |
| Examples | Melting ice, dissolving sugar | Burning paper, rusting iron |
The single question to ask is whether a new substance with new properties has been formed. Reversibility is a useful guide but not the definition, since some chemical changes can be reversed and some physical changes cannot easily be.
✗ Dissolving sugar in water is a chemical change because the sugar disappears | ✓ It is physical — the sugar can be recovered by evaporation and is chemically unchanged
The Borderline Cases
- Melting of wax is physical; burning of wax is chemical. The same substance, two different changes.
- Cutting a vegetable is physical; cooking it is chemical.
- Dissolving salt is physical; curdling milk is chemical.
- Boiling water is physical; electrolysis of water is chemical.
- Photosynthesis, digestion, respiration and ripening of fruit are all chemical.
The wax pair is the one examiners like most, because it shows that the change, not the material, decides the answer.
Rusting and Its Prevention
Iron + Oxygen + Water → Rust (hydrated iron oxide)
Rusting is the standard chemical change, and it needs both oxygen and moisture. Iron does not rust in dry air, nor in water that has no dissolved air. Salt in the air speeds it up, which is why rusting is faster in coastal regions.
| Prevention method | How it works |
|---|---|
| Painting or oiling | Keeps air and moisture away |
| Galvanisation | Coating with zinc |
| Electroplating with chromium | Protective and decorative layer |
| Making stainless steel | Alloying iron with chromium and nickel |
Galvanisation is coating iron with zinc specifically, and the word is asked with the metal named. Answering tin instead is a common slip, since tin is used for food cans.
Crystallisation
Crystallisation is a physical change used to obtain large pure crystals from a solution, as with copper sulphate. It is the standard method for purifying a solid, and it appears as a question asking whether it is physical or chemical — it is physical, because the substance itself is unchanged.
Note the pattern in this topic: almost every question is a sorting exercise. Building the habit of asking whether a new substance appeared, rather than whether the change looks dramatic, answers all of them.
नया पदार्थ बना या नहीं — यही एकमात्र कसौटी है।
TRE pointer: Papers set this topic through borderline cases, so the melting wax versus burning wax pair is the model to hold in mind. Rusting is the most examined chemical change, and the requirement of both oxygen and moisture is asked as a reasoning question. Remember that galvanisation uses zinc, not tin. Since every question here is a sorting decision, applying the new-substance test is faster and safer than recalling a memorised list.
60-Second Recap
- A chemical change forms a new substance with new properties; a physical one does not.
- Reversibility is a guide, not the definition.
- Melting wax is physical, burning wax is chemical; cutting is physical, cooking is chemical.
- Rusting needs both oxygen and moisture, and is faster in coastal air.
- Galvanisation coats iron with zinc; tin is used for food cans.
- Crystallisation is a physical change used to purify solids.
Frequently Asked Questions
Is dissolving sugar in water a physical or chemical change?
Physical. The sugar is still sugar and can be recovered unchanged by evaporating the water, so no new substance has formed. Only the physical state and appearance have changed. Students often assume that because the sugar vanishes from view, a chemical change has occurred.
What conditions are needed for iron to rust?
Both oxygen and moisture must be present. Iron kept in completely dry air does not rust, and neither does iron in water from which all dissolved air has been removed. Salt in the atmosphere accelerates the process, which is why rusting is faster near the sea.
What is galvanisation?
It is the coating of iron or steel with a layer of zinc to prevent rusting. The zinc keeps oxygen and moisture away from the iron and also corrodes in preference to it. Tin, by contrast, is used to coat food cans, and confusing the two metals is a common error.
Is melting of wax a chemical change?
No, melting wax is a physical change, because the wax remains wax and can be solidified again. Burning the wax is a chemical change, since new substances such as carbon dioxide and water are formed and cannot be turned back into wax.