CDS (II) 2023 General Knowledge · Question 8 of 120

Correct — A, Electrochemical method. The word doing the work in the stem is compound. In a mixture the components keep their own identity and are held together only physically, so a physical process — filtering, evaporating, sublimating, distilling — will pull them apart. In a compound the elements are joined by chemical bonds in a fixed proportion, and no amount of sieving, boiling or spinning will undo a bond. Breaking a compound back into what it is made of therefore needs a chemical change, and the standard, generally applicable way of forcing one is to pass electricity through the substance in solution or in the molten state. Electrolysis of acidified water yields hydrogen at the cathode and oxygen at the anode in a two-to-one ratio by volume; electrolysis of molten sodium chloride yields sodium metal and chlorine gas; aluminium is obtained from molten alumina the same way. That is the sense in which an electrochemical method separates the constituents of a compound.

• (b)Heating method — Heat alone is not a general answer. It does break some compounds — calcium carbonate gives calcium oxide and carbon dioxide, and mercuric oxide gives mercury and oxygen — but for most compounds, water and common salt among them, heating simply melts or boils the substance without touching the bonds.
• (c)Fractional distillation — A technique for mixtures, not compounds. It separates liquids whose boiling points differ, which is how petroleum is split into its fractions and how oxygen and nitrogen are obtained from liquefied air — in every case the components were already there, unbonded.
• (d)Decomposition followed by ion exchange — Reads like a laboratory procedure but is not a recognised route to the elements of a compound. Ion exchange swaps one ion for another already in solution — it is how hard water is softened and how resins purify water — and it presupposes that the compound has already been broken up.

Matter divides 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 two or more elements are chemically combined in a fixed mass ratio and the product's properties differ from those of its parts. A mixture keeps its components' identities and can be separated physically. The dividing test that examiners use is exactly this: physical methods separate mixtures, chemical or electrochemical methods separate compounds.

The four options are a deliberate sorting exercise. Two of them — fractional distillation and, in ordinary use, heating — are separation techniques for mixtures, and a candidate who does not register the word compound will pick one of them. Note honestly that option (b) is not absurd: thermal decomposition is a genuine chemical route, and heating mercuric oxide really does give the two elements back. The key nevertheless takes (a), and the reason is generality — an electrochemical route works across compounds, ionic and covalent alike, while heat works only on the small set that happen to decompose before they vaporise. Anyone who argued to (b) from the calcium-carbonate reaction has spotted something real; the key is (a) and that is what the exam counted. The textbook sentence behind the item is the one that says a compound's constituents can be separated only by chemical or electrochemical reactions.

• In a compound the elements are chemically bonded in a fixed proportion by mass; in a mixture they are not bonded at all.
• Physical methods — filtration, evaporation, sublimation, chromatography, distillation and fractional distillation — separate mixtures, not compounds.
• Electrolysis of acidified water gives hydrogen at the cathode and oxygen at the anode in a two-to-one ratio by volume.
• Electrolysis of molten sodium chloride yields sodium metal and chlorine gas; the same electrochemical route obtains aluminium from molten alumina.
• Thermal decomposition works for a limited set of compounds, such as calcium carbonate into calcium oxide and carbon dioxide, or mercuric oxide into mercury and oxygen.

• Missing the word 'compound' in the stem and answering as though a mixture were being separated.
• Assuming that because heating decomposes some compounds it is the general method for all of them.
• Treating ion exchange as a way of obtaining elements; it only swaps ions already in solution.

Either as this compound-against-mixture sorting item, or as a match between a named mixture and the technique that separates it.

• practice — not a real PYQ

  On passing electricity through acidified water, the volumes of gases collected at the cathode and the anode are in the ratio

  • (a)1 : 1
  • (b)1 : 2
  • (c)2 : 1
  • (d)4 : 1

  Answer(c) 2 : 1 — hydrogen collects at the cathode and oxygen at the anode, and water's formula H₂O fixes the ratio of their volumes at two to one.

• practice — not a real PYQ

  Which one of the following can be separated into simpler substances by a physical method?

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

  Answer(c) Air — it is a mixture, and its components are obtained by liquefying it and then distilling it fractionally. The other three are compounds and need a chemical change.