Important Gases

Air is not a single substance but a mixture of several gases, dominated by just two of them.

• Nitrogen (N₂) is the most abundant gas, about 78% of air by volume.
• Oxygen (O₂) is the second most abundant, about 21% of air by volume.
• The remaining ~1% is argon (~0.9%), carbon dioxide (~0.03–0.04%), plus traces of neon, helium, krypton, xenon and water vapour.
• N₂ and O₂ together make up about 99% of the air.

Oxygen is the life-supporting gas, essential for both breathing and burning.

• Oxygen itself does not burn; it only helps other things burn (it is a supporter of combustion, not a fuel).
• It is colourless, odourless and tasteless, and slightly soluble in water (which lets fish breathe).
• Discovered by Carl Wilhelm Scheele and Joseph Priestley; named by Lavoisier.
• Liquid oxygen (LOX) is used as an oxidiser in rockets, and oxygen cylinders are used in hospitals and by divers and mountaineers.
• Ozone (O₃) is an allotrope of oxygen that shields the Earth from harmful ultraviolet rays.

• A glowing splinter (smouldering splint) rekindles and bursts into flame when put into oxygen — the classic test.

Hydrogen is the lightest of all gases and burns with an explosive ease.

• It is highly inflammable (combustible) and burns with a pale blue flame, forming water.
• It is colourless, odourless and tasteless, and almost insoluble in water.
• Discovered by Henry Cavendish.
• Used in making ammonia (Haber process), hydrogenation of oils into vanaspati ghee, and as a clean fuel (in fuel cells and rockets).
• Because it is so light, hydrogen was once used in balloons and airships, but its flammability made it dangerous.

• Hydrogen burns with a "pop" sound when a lighted splint is brought near it — the pop test.

Nitrogen is the quiet, inert majority of the air, prized precisely because it does not react.

• It is colourless, odourless, tasteless and slightly lighter than air.
• It dilutes oxygen in the air so that burning and breathing happen at a steady, safe rate.
• Used to make fertilisers (urea, ammonia) and nitric acid.
• Used in food packaging to keep packets fresh (it stops oxidation and spoilage), and to fill the space above wine and in sealed snack bags.
• Liquid nitrogen is a refrigerant used for freezing and preserving biological samples.
• Nitrogen fixation by bacteria and lightning converts atmospheric N₂ into usable compounds for plants.

Carbon dioxide is the gas we breathe out, the bubbles in soda, and a key player in global warming.

• It is heavier than air and does not support burning, which makes it useful for putting out fires.
• Used in fire extinguishers — it smothers flames by cutting off oxygen.
• Solid CO₂ is called "dry ice", used for refrigeration; it sublimes (turns straight from solid to gas) without melting.
• Used to make fizzy/carbonated soft drinks and soda water.
• CO₂ is a greenhouse gas — it traps heat and is a major cause of global warming.
• Plants use CO₂ in photosynthesis to make food and release oxygen.

• CO₂ turns lime water milky — the milkiness is due to the formation of insoluble calcium carbonate, the standard test for CO₂.

Carbon monoxide is the silent, deadly gas of incomplete burning.

• It is formed by the incomplete combustion of carbon fuels (when there is not enough oxygen) — e.g. from car exhaust, coal stoves and gas heaters.
• It is colourless and odourless, which makes it especially dangerous — you cannot smell or see it.
• CO binds to haemoglobin in the blood (forming carboxyhaemoglobin) far more strongly than oxygen does, so it stops the blood from carrying oxygen and can cause suffocation and death.
• CO is a reducing agent used in metallurgy to extract metals from their ores (e.g. iron in the blast furnace).

Ammonia is the sharp-smelling gas at the heart of the fertiliser industry.

• It is lighter than air and highly soluble in water, forming a weak alkali (ammonium hydroxide).
• Made industrially by the Haber process from nitrogen and hydrogen.
• Used to make nitrogenous fertilisers (urea, ammonium nitrate, ammonium sulphate) and nitric acid.
• Used as a refrigerant in ice plants and in cleaning agents.

• Ammonia turns moist red litmus paper blue (it is alkaline), and gives dense white fumes with hydrogen chloride.

Some gases released by burning fuels poison the air and fall back to earth as acid rain.

• Produced by burning coal and fossil fuels and from volcanoes and smelting.
• SO₂ dissolves in rain to form sulphurous and sulphuric acid, a major cause of acid rain.
• It is also used as a bleaching agent and a preservative/disinfectant.

• Nitrogen oxides are formed in vehicle engines and during lightning.
• NO₂ is a reddish-brown gas with a pungent smell.
• NOₓ gases dissolve in rain to form nitric acid, the other main cause of acid rain.

• Acid rain is caused mainly by SO₂ and oxides of nitrogen (NOₓ), which form sulphuric and nitric acids in the atmosphere and damage buildings, soil and aquatic life.

Chlorine is the greenish gas that bleaches cloth and keeps drinking water safe.

• It is poisonous and heavier than air; it was used as a poison gas in World War I.
• Used as a bleaching agent for cotton, paper and textiles (it bleaches in the presence of moisture).
• Used to disinfect and purify drinking water and swimming pools — it kills germs.
• Used to make bleaching powder, hydrochloric acid and PVC plastic.

• Chlorine turns moist blue litmus red and then bleaches it white — first acidic, then bleaching.

Nitrous oxide earned its nickname from the giddy effect it has on people.

• It has a faint sweet smell and taste.
• Used as a mild anaesthetic and pain reliever in dentistry and minor surgery.
• Used as a propellant (e.g. in whipped-cream cans) and as an oxidiser in rocket and racing engines.
• N₂O is also a greenhouse gas.

The noble gases sit on the far right of the periodic table and barely react with anything, which is exactly what makes them useful.

• They are chemically inert (unreactive) because their outer electron shells are complete.
• Helium is the second lightest gas (after hydrogen) and is used to fill balloons and airships because it is light and, unlike hydrogen, non-flammable; it is also used in deep-sea diving gas mixtures.
• Neon is used in glowing advertising signs and tube lights, giving a reddish-orange glow.
• Argon is used to fill electric (incandescent) bulbs and tube lights because it does not react with the hot filament, extending its life.
• Radon is radioactive.

Two gaseous fuels power most Indian homes and a growing share of vehicles.

• It is used as cooking gas in domestic cylinders and in some vehicles.
• LPG is naturally odourless, so a strong-smelling chemical (ethyl mercaptan) is added to detect leaks.
• Being heavier than air, leaked LPG settles near the floor.

• CNG is mainly methane (CH₄).
• It is used as a cleaner fuel for vehicles (buses, cars, autos), producing less pollution than petrol or diesel.
• Being lighter than air, leaked CNG disperses upwards quickly, making it relatively safer.

This table pulls the main exam gases together in one place.

These tests are the ones most often asked in exams.