CDS (I) 2023 General Knowledge · Question 14 of 120

Correct — B, Opening of soda bottle. Nothing is oxidised when a fizzy drink is opened. Carbon dioxide has been forced into the liquid under pressure and stays dissolved only while that pressure holds; lift the cap and the pressure drops, the gas becomes far less soluble and comes out of solution as bubbles. No bonds are broken or made, no new substance appears, and the dissolved gas and the escaping gas are the same carbon dioxide — a physical change. Each of the other three is an oxidation. Iron rusts by combining with oxygen and water to give hydrated iron oxide; fats and oils turn rancid when atmospheric oxygen attacks them and produces the stale smell and taste; and combustion is a substance burning in oxygen.

• (a)Rusting of iron — A textbook oxidation. Iron in damp air combines with oxygen and water to form the reddish-brown flaky hydrated oxide, and the metal is genuinely consumed in the process.
• (c)Rancidity — Also an oxidation, of the fats and oils in stored food. It is precisely why packets are flushed with nitrogen, sealed against air, or dosed with an antioxidant, and why oils are kept in dark, tightly closed containers.
• (d)Combustion — Burning is oxidation by definition — the substance combines with oxygen and releases heat, and often light as well.

Oxidation in school chemistry has two equivalent definitions: the gain of oxygen by a substance, or the loss of electrons from it. Rusting, rancidity and combustion all satisfy the first at once. A physical change, by contrast, alters only the state or the arrangement of a substance and makes nothing new — melting, dissolving, and gas escaping from a solution are all of this kind. The dissolving of a gas in a liquid follows Henry's law, under which the amount dissolved is proportional to the pressure of that gas above the liquid; releasing the pressure therefore drives the gas back out.

Three of the four options are chosen so that the odd one out is not the least familiar but the least chemical, and that is the discrimination being tested — a change can be dramatic to watch and still involve no reaction at all. The useful test to apply is whether a new substance with new properties has appeared. Rust is not iron, and rancid butter is not butter; the gas fizzing out of a cold drink is the same carbon dioxide that was dissolved in it a second earlier. It is worth adding the one nuance: carbon dioxide dissolved in water does form a little carbonic acid, so a soda is mildly acidic, but that equilibrium exists in the sealed bottle as much as in the open one, and opening the cap is not what creates it.

• Rusting is the oxidation of iron in the presence of oxygen and water to hydrated iron oxide.
• Rancidity is the oxidation of fats and oils in food, countered by antioxidants, airtight packing or flushing packets with nitrogen.
• Combustion is oxidation accompanied by the release of heat, and usually light.
• The fizz on opening a bottle is dissolved carbon dioxide escaping as the pressure falls — a physical change, with no new substance formed.
• Under Henry's law the amount of a gas dissolved in a liquid is proportional to the pressure of that gas above the liquid.

• Reading dramatic effervescence as evidence of a chemical reaction.
• Forgetting that rancidity is a chemical change and not merely food drying out.
• Assuming that anything involving a gas must be a reaction involving that gas.

As an odd-one-out among oxidations, as a physical-versus-chemical-change item, or as a question on how rancidity is prevented.

• practice — not a real PYQ

  Packets of potato chips are commonly flushed with nitrogen gas before sealing mainly in order to

  • (a)keep the chips crisp by absorbing moisture
  • (b)prevent the oxidation of the oil in the chips
  • (c)increase the weight of the packet
  • (d)kill bacteria inside the packet

  Answer(b) prevent the oxidation of the oil in the chips — nitrogen is unreactive and keeps oxygen away, so rancidity is delayed.

• practice — not a real PYQ

  Which one of the following is a physical change?

  • (a)Souring of milk
  • (b)Burning of a candle wick
  • (c)Melting of wax
  • (d)Rusting of an iron nail

  Answer(c) Melting of wax — only the state changes and the wax can be solidified again; the other three produce new substances.