A sample of any of the following substances disappears after sometime when exposed to air like camphor, naphthalene or dry ice. This phenomenon is called

Question

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Correct — A, Sublimation. Sublimation is the direct change of a solid into vapour without any liquid stage in between. Camphor, naphthalene (the old mothball) and dry ice are the three textbook examples, and each disappears in open air without ever leaving a puddle behind. Camphor and naphthalene are molecular solids held together by weak van der Waals forces, so even at room temperature enough molecules escape from the surface for the block to shrink away. Dry ice is solid carbon dioxide, and its behaviour is decided by pressure: the triple point of CO2 lies at about 5 atmospheres, so at ordinary atmospheric pressure liquid carbon dioxide cannot exist at all — the solid must go straight to gas, which it does at about minus 78 degrees Celsius. The observation the question describes, a solid vanishing with no liquid ever forming, is the definition of sublimation.

Why the other options are wrong — (b) Evaporation is the escape of molecules from the surface of a LIQUID into vapour below its boiling point. It needs a liquid phase to start with — water in an open glass, petrol from a spill. Camphor, naphthalene and dry ice never melt into a liquid under ordinary room conditions, so what happens to them cannot be called evaporation.  (c) Diffusion is the spreading of one substance through another down a concentration gradient — it is why the smell of camphor reaches the far corner of the room. But diffusion only moves vapour that already exists; it does not convert the solid into vapour. It explains the smell, not the shrinking of the block.  (d) Radiation is the transfer of energy as electromagnetic waves, the way the Sun heats the Earth across empty space. It is a mode of heat transfer, not a change of state, and no material is lost from a body by radiating heat.

Concept: Matter changes state along a set of named routes: melting and freezing between solid and liquid, vaporisation and condensation between liquid and gas, and sublimation and deposition directly between solid and gas. Which route a substance takes depends on temperature and pressure together. Whether a liquid stage is even available is decided by the triple point — the unique combination of temperature and pressure at which all three phases coexist. If the surrounding pressure is below the triple-point pressure, as it is for carbon dioxide in open air, there is no liquid stage available and the solid must sublime. Substances such as camphor, naphthalene, iodine, ammonium chloride and anthracene sublime readily at ordinary pressure because their molecules are weakly held and their vapour pressure is high.

State PCS 2019 Q89 — Science & Technology PYQ with Answer

State PCS 2019 Q89 — Science & Technology PYQ with Answer