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UPPCS Chemistry Gap Sheet: Gases: Identity, Uses and Pollution.
UPPCS has asked all 23 of these. Read this page as a full revision list: these are the facts that come back.
First, the concept
Gas use follows its properties. An inert gas can protect materials from oxidation; a reactive gas can disinfect or bleach. Liquefied gases also provide very low temperatures.
Formula and location matter. Carbon monoxide is a toxic product of incomplete burning. Ozone protects against ultraviolet rays high in the atmosphere, but near the ground it is an air pollutant.
✅ Already asked (23)
Useful gases
- Hydrogenation gas · Hydrogen, H₂; hardens vegetable oils, commonly with a nickel catalyst.
- Aircraft tyre gas · Nitrogen, N₂; low chemical reactivity and low moisture when dry.
- Cryogenic nitrogen · Liquid N₂ boils near −196°C; used in cryobanks.
- Oxygen · O₂ supports combustion but is not itself a fuel.
- Helium in balloons · He is light and non-flammable.
- Heliox · Helium–oxygen mixture used for some deep dives.
- Neon signs · Ne gives orange-red light in discharge tubes.
- Argon shielding · Ar protects hot metals from air during welding.
- Laughing gas · Nitrous oxide, N₂O; used with oxygen in anaesthesia.
- Ethylene · C₂H₄; a plant hormone that promotes fruit ripening.
- Acetylene · C₂H₂; used with oxygen in welding flames.
Reactive and harmful gases
- Ammonia · NH₃; pungent, very soluble in water and alkaline in solution.
- Carbon dioxide · CO₂; used in carbonated drinks and some fire extinguishers.
- Carbon monoxide · CO binds haemoglobin strongly and reduces oxygen transport.
- Hydrogen sulphide · H₂S; toxic gas with a rotten-egg smell; forms during decay.
- Sulphur dioxide · SO₂; contributes to acid rain and irritates airways.
- Nitrogen dioxide · NO₂; a brown gas involved in photochemical smog and acid rain.
- Ozone · O₃; a strong oxidant used in water disinfection.
- Chlorine · Cl₂; greenish-yellow gas used in water disinfection.
- PAN · Peroxyacetyl nitrate is an eye irritant in photochemical smog.
- Dry ice · Solid CO₂; sublimes at ordinary atmospheric pressure.
- Sulphur hexafluoride · SF₆; electrical insulating gas and a potent greenhouse gas.
- Phosphine · PH₃; highly toxic gas released when aluminium phosphide contacts moisture.
⚠️ Laughing gas is N₂O, not NO; helium is non-flammable but hydrogen is flammable.
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