Key facts

  • A physical change alters state, size, shape or solubility without forming a new substance.
  • A chemical change forms one or more new substances with properties different from the starting materials.
  • Melting, freezing, evaporation, condensation, dissolving and crystallisation are usually physical changes.
  • Burning, rusting, curd formation, digestion, cooking and souring are chemical changes.
  • Common indicators of chemical change include colour change, gas evolution, precipitate formation, heat, light, smell or taste change.

Key Points at a Glance

  1. 1

    A physical change alters state, size, shape or solubility without forming a new substance.

  2. 2

    A chemical change forms one or more new substances with properties different from the starting materials.

  3. 3

    Melting, freezing, evaporation, condensation, dissolving and crystallisation are usually physical changes.

  4. 4

    Burning, rusting, curd formation, digestion, cooking and souring are chemical changes.

  5. 5

    Common indicators of chemical change include colour change, gas evolution, precipitate formation, heat, light, smell or taste change.

  6. 6

    Reversibility is only a clue: the decisive test is whether a new substance has formed.

  7. 7

    Oxidation means addition of oxygen, removal of hydrogen or loss of electrons.

  8. 8

    Reduction means removal of oxygen, addition of hydrogen or gain of electrons.

  9. 9

    An oxidising agent oxidises another substance and gets reduced itself.

  10. 10

    A reducing agent reduces another substance and gets oxidised itself.

  11. 11

    Rusting, combustion, rancidity, respiration, bleaching and metal displacement are common redox-linked examples.

  12. 12

    A catalyst changes the rate of a reaction and is not consumed in the overall reaction.

  13. 13

    Positive catalysts speed up reactions; negative catalysts or inhibitors slow reactions down.

  14. 14

    Enzymes are biological catalysts that help reactions such as digestion occur under mild conditions.

  15. 15

    A catalytic converter uses catalyst metals to convert harmful vehicle exhaust gases into less harmful gases.

How do you distinguish physical and chemical changes in matter?

A change in matter is physical when no new substance is formed and chemical when the original substance forms one or more new substances. A change in matter means that a substance has shown a visible or measurable difference from its earlier condition. In school science and in LDC objective questions, the first task is not to use a difficult equation; it is to ask one clear question: has a new substance been formed? If the answer is no, the change is usually physical. If the answer is yes, the change is chemical. This single test explains most examples such as melting ice, tearing paper, burning paper, rusting iron, cooking food and forming curd. The RSMSSB LDC 2018 syllabus lists Physical and Chemical Changes, Oxidation and Reduction Reactions, and Catalysts as item 1 under Everyday Science.

Matter can change in state, shape, size, colour, temperature, smell, taste, solubility, hardness or chemical composition. Physical changes affect mainly physical properties. A physical property is a property that can be observed without changing the identity of the substance, such as state, shape, size, mass, volume, colour, density or solubility. When ice melts, solid water becomes liquid water. When water freezes, liquid water becomes ice. The substance remains water in both cases, so the change is physical. When sugar dissolves in water, the sugar spreads through the water as a solution; it has not become a new chemical substance. If the water is evaporated carefully, sugar can be recovered. This recovery is why dissolving sugar is treated as a physical change.

Chemical changes are different because the original substances are converted into one or more new substances. The new substance has different properties from the starting material. When iron rusts, brown rust forms on the surface. Iron and rust are not the same substance. When food is digested, complex food materials are broken into simpler substances that the body can absorb. When milk turns into curd, its taste, smell and texture change because new substances form during the action of microorganisms. These are chemical changes.

Reversibility is a useful clue but not an absolute rule. Many physical changes are reversible: melting can be reversed by freezing, evaporation by condensation, and dissolving by crystallisation or evaporation of the solvent. Many chemical changes are difficult to reverse by simple physical methods: burnt paper cannot be turned back into paper, curd cannot be changed back into milk by cooling, and rust does not become clean iron merely by wiping. However, exam answers should not rely only on reversibility. The deciding feature is new substance formation. Some physical changes may be hard to reverse in practice, such as grinding a grain into flour, but no new substance has been formed merely by grinding. Some chemical changes can be reversed by another chemical reaction in a laboratory, but they are still chemical changes because new substances are involved.

For quick classification, use a two-step test. First, identify what changed: state, size, shape, solubility, colour, smell, gas, heat, light or precipitate. Second, ask whether the material's chemical identity changed. Melting, freezing, evaporation, condensation, sublimation, crushing, cutting, bending, stretching, dissolving and crystallisation are usually physical unless accompanied by a reaction. Burning, rusting, digestion, cooking, curd formation, souring of food, fermentation and corrosion are chemical because new substances form.