EPFO EO/AO 2023 General Ability Test · Question 90 of 120

Correct — D, (d) Burning of magnesium in air. The test for a chemical change is whether a new substance with new properties has been produced, and burning magnesium is the clearest example in the school laboratory. A magnesium ribbon is a flexible silvery metal that conducts electricity. Held in a flame it catches and burns with a dazzling white light, and what is left is a light white powder that is brittle, does not conduct and cannot be bent or beaten. That powder is magnesium oxide, formed by the metal combining with oxygen from the air. Nothing about it resembles the ribbon: the composition has changed, not merely the shape or the state. The ash can be tested — dissolved in water it gives magnesium hydroxide, which is basic and turns red litmus blue, whereas the metal itself does no such thing. A new substance with new properties is exactly what defines a chemical change, and there is no way back: the powder cannot be cooled or squeezed into magnesium again. The change also shows the usual accompanying signs. Light and heat are given out in quantity — enough that the burning ribbon was once used for photographic flash and is still used in flares and fireworks — and the mass of the residue is greater than that of the ribbon, because oxygen has been taken up from the air. Neither the light nor the mass gain is the definition, but both are strong indications that a reaction has occurred. Set against that, each of the other three options changes only the physical state or the degree of division of a substance that remains chemically the same throughout, and each can be undone by reversing the condition that caused it. That is what makes them physical changes. Only option (d) produces a substance that was not there before, so it is the answer.

• (a)Melting of wax — Melting is a change of state and nothing more. Solid wax becomes liquid wax; the molecules are the same molecules, only now free to move past one another, and cooling the liquid returns solid wax with all its original properties. It is a physical change, and reversibility is the visible sign of that. The option is a good distractor because a burning candle is a favourite classroom example of both kinds of change happening at once — the wax near the wick melts, which is physical, while the vapour drawn up the wick burns, which is chemical. The stem here names only the melting.
• (b)Heating of iron — Heating a piece of iron makes it expand and, at a high enough temperature, glow and eventually melt; take the heat away and it contracts and returns to the metal it was. The identity of the substance has not altered, so the change described is physical. It is worth being precise about the boundary: iron left hot in air for long enough will develop a scale of oxide on its surface, and that oxidation would be a chemical change — but that is corrosion occurring alongside the heating rather than the heating itself, and it is not what the option describes. Option (d), by contrast, is a combustion whose product is a wholly different substance, and it is unambiguously the chemical change of the four.
• (c)Dissolution of sugar in water — Dissolving separates the sugar into individual molecules dispersed through the water, but it does not alter those molecules — the solution still tastes sweet, which is the plainest evidence that the sugar is still sugar. Evaporate the water and the sugar is recovered unchanged, so the change is reversible and physical. Candidates are sometimes misled by the fact that the sugar disappears from view, but disappearance is not the test; the test is whether a substance with new properties has been formed. Dissolution is a physical change even when, as with common salt, it involves the separation of ions.

A physical change alters the form, state or appearance of a substance while leaving its chemical composition intact, and it can usually be reversed by reversing the condition that caused it — melting and freezing, boiling and condensing, dissolving and evaporating, breaking, bending and magnetising. A chemical change produces one or more substances that were not there before, with properties different from the starting materials, and it is usually not reversible by simple physical means. Because the definition turns on composition rather than appearance, the reliable question to ask is not 'does it look different' but 'is what I have now the same substance'. Several signs commonly accompany a chemical change and are useful indicators without being the definition: evolution of a gas, formation of a precipitate, a change of colour, a change of smell, and the giving out or taking in of heat or light. The classic school examples are worth holding as a paired list — rusting of iron, burning of any fuel, souring of milk, cooking of food, digestion, photosynthesis, and the ripening of fruit are chemical; melting of ice, boiling of water, dissolving of salt or sugar, breaking of glass, and the drawing of a metal into wire are physical. The hardest cases are those in which both occur together, as in a burning candle, or those which look dramatic but are only physical, as when iodine sublimes to a violet vapour.

The classification of change is examined in almost every general science paper because it can be tested in one line and it exposes whether a candidate has learned a definition or only a list of examples. EO/AO papers use it in this shape — three physical changes and one chemical, or the reverse — and the discrimination is always the same: has a new substance appeared. The habit rewarded is applying the test explicitly to each option rather than choosing the one that seems most dramatic, since dramatic physical changes such as sublimation exist and quiet chemical changes such as rusting exist.

• A chemical change produces a new substance with different properties and is generally not reversible by physical means.
• Magnesium burns in air with a bright white light to form magnesium oxide, a white powder.
• Magnesium oxide dissolved in water gives magnesium hydroxide, which is basic and turns red litmus blue.
• The residue after burning weighs more than the ribbon, because oxygen from the air has been taken up.
• Melting, boiling, dissolving, breaking and bending are physical changes: composition is unaltered.
• Common signs accompanying a chemical change are gas evolution, precipitate formation, colour change, smell change and the release or absorption of heat or light.
• Rusting of iron, souring of milk, cooking, digestion and photosynthesis are standard chemical changes.
• A burning candle shows both kinds at once — the wax melts, which is physical, and the vapour burns, which is chemical.

• Using reversibility as the definition instead of as a clue; some chemical changes can be reversed and some physical changes are hard to undo.
• Treating the disappearance of a solid on dissolving as evidence that a new substance has formed.
• Reading 'heating of iron' as rusting; heating alone changes the state, not the substance.
• Choosing the most spectacular-looking option; sublimation of iodine is spectacular and purely physical.

This theme comes up in EO/AO papers as an odd-one-out like this one, as a direct 'which of the following is a physical change', or as a matching item pairing four processes with four types of reaction. Learn the standard examples in two columns and apply the new-substance test aloud to every option, and the family becomes reliable marks.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which one of the following is a physical change ?

  • (a)Rusting of iron
  • (b)Sublimation of iodine
  • (c)Souring of milk
  • (d)Burning of coal

  Answer(b) Sublimation of iodine

• practice — not a real PYQ

  When magnesium is burnt in air, the substance formed is :

  • (a)Magnesium nitride only
  • (b)Magnesium oxide
  • (c)Magnesium carbonate
  • (d)Magnesium hydroxide

  Answer(b) Magnesium oxide