Which one of the following is considered as a compound?

Correct — D, Calcium carbonate. A compound is a pure substance in which atoms of two or more different elements are chemically bonded in a fixed proportion, with properties of its own that are not the properties of its constituents, and which can be broken up only by a chemical change. Calcium carbonate, CaCO₃, meets every part of that. Its calcium, carbon and oxygen are always in the ratio one to one to three, whether the sample is limestone from a quarry, marble from a sculptor's yard, chalk, calcite or a seashell — and by mass that fixes it at forty per cent calcium. It behaves like neither a reactive metal nor a gas: it is a hard white solid that fizzes with dilute acid. Splitting it needs a chemical change, and a fierce one; heating it above about nine hundred degrees drives off carbon dioxide and leaves quicklime, which is what a lime kiln is for. The other three options fail on different grounds and it is worth being clear which. Silicon and tin are elements, so there is nothing in them to be combined. A sugar solution is the more interesting distractor, because sugar itself really is a compound — but the solution is not. You can make it as weak or as strong as you like, which means no fixed proportion, and boiling the water off recovers the sugar unchanged, which means no chemical bond between the two components. That is the definition of a mixture.

• (a)Silicon — An element, atomic number 14, and the second most abundant element in the earth's crust after oxygen. Silicon dioxide is the compound; silicon is not.
• (b)Tin — An element, symbol Sn, atomic number 50 — one of the earliest metals people worked, because alloying it with copper gives bronze. An alloy is a mixture, and the metal itself is neither.
• (c)Sugar solution — A mixture, and the trap of the question. Sugar is a compound, but a sugar solution has no fixed composition and evaporating the water returns the sugar unchanged, so nothing chemical has happened between the two.

Matter is sorted first into pure substances and mixtures. A pure substance is either an element, which cannot be broken into anything simpler by chemical means, or a compound, in which different elements are chemically combined in a fixed ratio by mass. A mixture is two or more substances physically together in any proportion, each keeping its own properties, and separable by physical means such as evaporation, filtration or distillation. Solutions, alloys and air are all mixtures, however uniform they look.

The item is a classification exercise, and the reliable way through it is to ask two questions of each option in turn: is it one element or more than one, and if more than one, is the proportion fixed. Silicon and tin fall at the first question. Sugar solution passes the first and fails the second. Only calcium carbonate passes both. The reason candidates lose this mark is that the word sugar dominates the reading of option (c) and the word solution slips past — a homogeneous solution looks like a single substance, which is exactly why it is the standard example used to show that looking uniform proves nothing.

• Calcium carbonate, CaCO₃, combines calcium, carbon and oxygen in the fixed ratio 1 : 1 : 3, which is forty per cent calcium by mass.
• Limestone, marble, chalk, calcite and seashells are all largely the same compound.
• Heating it above about 900 °C decomposes it into calcium oxide, quicklime, and carbon dioxide — the lime kiln reaction.
• Silicon is an element of atomic number 14 and the second most abundant element in the earth's crust by mass.
• Tin is an element, symbol Sn, atomic number 50; bronze is its alloy with copper, and an alloy is a mixture.
• A solution is a homogeneous mixture: its composition can be varied continuously and its components separate by physical means.

• Reading 'sugar solution' as 'sugar' — the sugar is a compound but the solution is a mixture.
• Assuming anything uniform in appearance must be a pure substance; solutions and alloys look uniform and are not.
• Calling an alloy such as bronze or steel a compound.

As a sorting item like this one, as a which-is-a-mixture question, or as a statements item on the properties that separate a compound from a mixture.

• practice — not a real PYQ

  Which one of the following is a mixture and not a compound?

  • (a)Common salt
  • (b)Brass
  • (c)Water
  • (d)Carbon dioxide

  Answer(b) Brass — an alloy of copper and zinc, whose proportions can be varied, which makes it a mixture.

• practice — not a real PYQ

  Strongly heating calcium carbonate produces quicklime and which gas?

  • (a)Oxygen
  • (b)Carbon monoxide
  • (c)Carbon dioxide
  • (d)Hydrogen

  Answer(c) Carbon dioxide — CaCO₃ decomposes to CaO and CO₂ above about 900 °C.