NDA (II) 2023 General Ability Test · Question 63 of 146

Correct — D, Burning of magnesium ribbon in air. Magnesium combines with the oxygen of the air to give magnesium oxide, 2Mg + O₂ → 2MgO. A brand-new substance appears — a white powder whose composition and properties are nothing like the shiny metal ribbon it came from — and you cannot get the ribbon back by cooling or by any simple physical step. That is the definition of a chemical reaction. The other three options rearrange or move molecules around without changing what those molecules are.

• (a)Evaporation of petrol — Evaporation is a change of state. The petrol molecules leave the liquid and enter the vapour, but they are still the same molecules — cool the vapour and you get liquid petrol back. No new substance is formed, so it is a physical change.
• (b)Dissolution of salt in water — Dissolving separates the sodium and chloride ions that were already present in the crystal; it does not build any new compound. Boil the water off and the same salt is recovered unchanged, which is why dissolving is treated as a physical change.
• (c)Storing of oxygen gas under pressure in a gas cylinder — Compressing a gas only pushes the same O₂ molecules closer together. Release the pressure and ordinary oxygen comes out. Nothing has reacted — this is a physical change of volume and pressure.

A chemical reaction is a change in which the atoms are regrouped into one or more substances that were not there before, so the products differ from the reactants in composition and in properties. A physical change alters only state, shape, size or the degree of mixing, and the substance itself survives intact. The usual field signs of a reaction are a new colour, a gas bubbling off, a precipitate settling, a smell appearing, or heat and light being given out.

Work through the four options asking a single question — after the change, is a different substance present? Petrol vapour is still petrol; salt solution still contains sodium chloride; compressed oxygen is still oxygen. Only the magnesium ribbon comes out as something else, magnesium oxide. Options (a) and (c) tempt candidates because they involve energy or force, and (b) tempts because the crystal visibly disappears — but disappearance is not the test, a new substance is.

• Burning magnesium in air forms magnesium oxide, 2Mg + O₂ → 2MgO.
• Magnesium oxide dissolved in water gives magnesium hydroxide, which is basic and turns red litmus blue.
• Evaporation, dissolution, melting, boiling and compression are physical changes — the substance is recoverable.
• Burning, rusting, souring of milk and digestion of food are chemical changes — a new substance is formed.

• Treating dissolution as a chemical change because the solid vanishes from sight.
• Treating any change that needs energy — heating, compressing — as chemical.
• Forgetting that reversibility by simple physical means is a strong hint of a physical change.

Asked as a straight 'which one is a chemical change' pick, or in statement form where crystallisation and melting are offered alongside one genuine reaction.

• practice — not a real PYQ

  Which one of the following is a physical change?

  • (a)Rusting of iron
  • (b)Sublimation of camphor
  • (c)Burning of coal
  • (d)Curdling of milk

  Answer(b) Sublimation of camphor — the solid turns straight to vapour, but it is still camphor, so no new substance is formed.

• practice — not a real PYQ

  The white powder formed when magnesium ribbon is burnt in air is

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

  Answer(c) magnesium oxide — magnesium combines with atmospheric oxygen to give MgO.