CDS (II) 2023 General Knowledge · Question 39 of 120

Correct — A, Burning of candle. A candle does two different things at once, and that is the whole point of the item. The heat of the flame melts the solid wax near the wick, and the liquid wax drawn up the wick is vaporised. Melting and vaporising produce no new substance — the wax is still wax, and it solidifies again when the candle is put out — so these are physical changes. The wax vapour is then what actually burns, combining with oxygen in the air to give carbon dioxide and water vapour, along with heat and light. New substances have been formed and the wax cannot be recovered from them, so that is a chemical change. NCERT's Class 7 chapter on physical and chemical changes treats this as settled and asks students to work it out for themselves: 'When a candle burns, both physical and chemical changes take place. Identify these changes.' The other three options are one-sided. Freezing of water is purely physical, rusting of iron is purely chemical, and cooking of food is conventionally classified as a chemical change.

• (b)Freezing of water — A physical change and nothing more. Liquid water becomes solid ice, its molecules are unchanged, no new substance appears, and the change reverses completely on warming — NCERT's own tests for a physical change.
• (c)Cooking of food — Classified as a chemical change, because heating alters the food chemically and the raw ingredients cannot be recovered. It is the closest of the three to an argument — water does evaporate and fats do melt while food cooks — but NCERT names the candle, not cooking, as the example in which both kinds of change occur together, and in a candle it is one substance, the wax, that undergoes both.
• (d)Rusting of iron — Purely chemical. Iron reacts with oxygen and water to form a new substance, hydrated iron oxide, which has different properties from iron and cannot be turned back into iron by any physical means.

A physical change alters the state, shape, size or appearance of a substance while leaving its chemical identity intact; no new substance is formed and the change is generally reversible. A chemical change produces one or more new substances with new properties, usually cannot be reversed by physical means, and is often accompanied by heat, light, gas, a colour change or a precipitate. Most everyday processes are one or the other, but a few involve both at the same time — a burning candle is the standard classroom example.

Test each option twice, once for each kind of change, instead of trying to label it in a single word. Ask first whether anything melted, dissolved, evaporated or merely changed shape, and second whether any new substance appeared. Only the candle answers yes to both. A useful way to see it is that the wax has to become a vapour before it can burn at all — the physical change is the step that makes the chemical change possible, which is why the two are inseparable here. Note honestly that cooking of food is not an absurd choice; water is driven off and fats melt while food cooks, so a physical component exists there too. What settles the item for the examiner is that the standard textbook names the burning candle by name as the case in which both changes take place, and asks students to supply another such example themselves.

• NCERT, Class 7, exercise on physical and chemical changes: 'When a candle burns, both physical and chemical changes take place.'
• In a candle, wax melting and vaporising are physical changes; the wax vapour burning in air to give carbon dioxide, water vapour, heat and light is a chemical change.
• A physical change forms no new substance and is generally reversible — freezing of water is the textbook case.
• Rusting is written as iron plus oxygen plus water giving rust, an iron oxide — a new substance, so a chemical change.
• The usual signs of a chemical change are a new substance, evolution of a gas, a change of colour, a change in temperature and the giving off of light.

• Labelling a process from its overall name instead of examining the steps; the candle looks like a single event but contains two different kinds of change.
• Assuming every change involving heat is chemical. Melting, boiling and freezing all involve heat and are physical.
• Treating reversibility as an infallible test. Most physical changes reverse and most chemical ones do not, but the formation of a new substance is the surer criterion.

As an identify-the-change one-liner, or as a statements item asking which of several everyday processes are chemical, physical or both.

• practice — not a real PYQ

  Which one of the following is a purely physical change?

  • (a)Burning of magnesium ribbon
  • (b)Souring of milk
  • (c)Melting of wax
  • (d)Rusting of iron

  Answer(c) Melting of wax — the wax changes state but remains wax, no new substance forms, and it solidifies again on cooling; the other three all produce new substances.

• practice — not a real PYQ

  The formation of rust on an iron object requires the presence of which of the following?

  • (a)Oxygen alone
  • (b)Water alone
  • (c)Both oxygen and water
  • (d)Neither oxygen nor water

  Answer(c) Both oxygen and water — rusting is written as iron reacting with oxygen in the presence of water to give hydrated iron oxide, which is why iron rusts far faster in damp air than in dry.