CDS (II) 2023 General Knowledge · Question 41 of 120

Correct — A, Sugar. A compound is a pure substance in which two or more elements are chemically combined in a fixed proportion by mass, so that the product has properties of its own and can be broken up only by chemical means. Table sugar is sucrose, whose formula is C₁₂H₂₂O₁₁ — carbon, hydrogen and oxygen locked together in that fixed ratio in every crystal. NCERT's own list of examples is explicit: 'Examples of some compounds are water, ammonia, carbon dioxide, sugar, etc.' The other three are mixtures, in which substances are merely put together in no fixed proportion and keep their own properties. NCERT names two of them in the same passage — 'sugar solution in water, air, tea, etc., are all mixtures' — which is worth reading carefully, because it is sugar *dissolved in water* that is the mixture there, not sugar itself. Air is a homogeneous mixture of nitrogen, oxygen, argon, carbon dioxide and water vapour in proportions that vary from place to place and can be separated by physical means. Milk is a colloid, and tea is a mixture of water with tea extract and usually milk and sugar as well.

• (b)Air — A mixture, and the one most often mistaken for a compound because it looks uniform. Its components are not chemically combined and their proportions vary — the oxygen fraction differs between a city street and a mountain top — and they can be separated physically, which is how industrial oxygen and nitrogen are obtained from liquefied air.
• (c)Milk — A colloid, not a compound. NCERT lists milk among its examples of colloids and describes milk as a common oil-in-water emulsion — fat droplets dispersed through water, along with dissolved sugars, proteins and salts. There is no fixed formula for it.
• (d)Tea — A mixture that NCERT names as such. Tea is water carrying dissolved extracts from the leaves, usually with milk and sugar added in whatever proportion the drinker chooses — the clearest sign of a mixture is that you can change the recipe.

Matter is first sorted into pure substances and mixtures. Pure substances divide again into elements, which cannot be broken into anything simpler by chemical means, and compounds, in which elements are chemically combined in a fixed ratio by mass. Mixtures contain two or more substances that are not chemically combined, in any proportion, and each keeps its own properties; they may be homogeneous, like air or a salt solution, or heterogeneous. Colloids such as milk are a special case in which particles are dispersed finely enough to look uniform but are large enough to scatter light.

Two questions settle any option of this kind. Is the composition fixed, and are the components chemically combined? Sugar passes both — every sample of sucrose has the same formula, and its elements cannot be separated by filtering, evaporating or distilling. Air, milk and tea fail both — you can change the proportions freely, and physical methods will separate them. The trap in this item is that air and milk both look uniform, and uniformity is easily mistaken for purity; that is exactly why NCERT teaches the homogeneous mixture as a separate category. Watch also for the difference between sugar and sugar solution: the crystal is a compound, the syrup is a mixture, and the textbook sentence quoted in support of 'mixture' is about the solution.

• NCERT: 'Examples of some compounds are water, ammonia, carbon dioxide, sugar, etc.'
• NCERT: 'sugar solution in water, air, tea, etc., are all mixtures.'
• Table sugar is sucrose, C₁₂H₂₂O₁₁ — carbon, hydrogen and oxygen combined in a fixed ratio by mass.
• NCERT lists milk among colloids and describes milk as a common oil-in-water emulsion.
• Air is a homogeneous mixture whose components can be separated physically, by liquefying it and distilling the liquid in fractions.

• Treating anything that looks uniform as a pure substance; air and a salt solution are uniform and are still mixtures.
• Confusing sugar with sugar solution. The crystal is a compound; dissolved in water it becomes part of a mixture.
• Thinking a compound must be an unfamiliar laboratory chemical. Water, common salt and table sugar are all compounds.

As a pick-the-compound or pick-the-mixture one-liner, or as a match-the-substance-to-its-category list mixing elements, compounds, solutions and colloids.

• practice — not a real PYQ

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

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

  Answer(b) Air — its gases are not chemically combined, their proportions vary, and they can be separated physically; the other three are compounds with fixed formulae.

• practice — not a real PYQ

  Milk is best classified as which one of the following?

  • (a)A compound
  • (b)An element
  • (c)A colloid
  • (d)A true solution

  Answer(c) A colloid — fat droplets are dispersed through water finely enough to look uniform but large enough to scatter light, which is why milk is opaque.