Which of the following came as alternatives to chlorofluorocarbons (CFCs) as…

Concept — A replacement refrigerant has to satisfy two conditions at once: it must be able to carry the same heat-transfer duty in a vapour-compression cycle, and it must cut the damage the original fluid did to the stratospheric ozone layer. Ozone destruction by a halocarbon is driven by the halogen the molecule releases high in the atmosphere — chlorine and bromine act as catalysts that break ozone apart, while fluorine held on carbon does not. Halocarbon family names are therefore read straight off the atoms present: a chlorofluorocarbon (CFC) carries chlorine and fluorine with no hydrogen; a hydrochlorofluorocarbon (HCFC) adds hydrogen, which lets the molecule be destroyed in the lower atmosphere so most of its chlorine never reaches the ozone layer; a hydrofluorocarbon (HFC) drops chlorine entirely; a perfluorocarbon (PFC) carries only carbon and fluorine; and a halon carries bromine. Under the Montreal Protocol, the fluids brought in to take over CFC duty in refrigeration and air-conditioning are exactly the families that keep the cooling behaviour while lowering the ozone-depleting halogen load.

Application — Test each substance named in the stem against both conditions — is it a working refrigerant fluid, and was it introduced to displace CFCs in that role?

• Hydrofluorocarbons (HFCs) contain carbon, hydrogen and fluorine and no chlorine at all, so their ozone-depletion potential is effectively zero. R-134a took over from CFC-12 in domestic refrigerators and car air-conditioners, and R-410A/R-32 serve room air-conditioners. These are the long-term ozone-safe successor fluids; their own drawback is a high global-warming potential, which the Kigali Amendment now addresses.

• Hydrochlorofluorocarbons (HCFCs) still carry chlorine, but the C–H bond makes them break down largely in the troposphere, so their ozone-depletion potential is roughly a tenth or less of a CFC’s. R-22 was the standard transitional replacement for CFCs in air-conditioning and commercial refrigeration, and is itself now being phased out in favour of the chlorine-free families.

• Perfluorocarbons (PFCs) are fully fluorinated carbon chains with no hydrogen and no chlorine. They are chemically inert and extremely long-lived, and their industrial role is semiconductor etching and chamber cleaning, specialty solvents and some fire-protection systems — they were not the fluid class introduced to run refrigeration cycles in place of CFCs.

• Halons are bromine-bearing halocarbons such as Halon 1301, used in fire extinguishing rather than cooling. Bromine is an even more efficient ozone-destroying catalyst than chlorine, so halons are themselves ozone-depleting substances and were among the first compounds the Montreal Protocol phased out — they are a target of the treaty, not a substitute produced by it.

Cross-check — Lining the four substances up by the atoms they carry and the job they actually do separates the refrigerant successors from the rest:

So the two families that came in as CFC refrigerant alternatives are the hydrofluorocarbons and the hydrochlorofluorocarbons — statements A and C — and the grouping that names exactly those two is “A and C only”. The NTA final answer key for UGC NET Paper 1, 2 September 2024 Shift 2 records the same choice.