Important Gases and Their Uses: Notes and PYQs
Exam notes on important gases: composition of air, properties and uses of common gases, the noble gases, fuel gas mixtures and the gases that harm health.
By GK24 Editorial Team· Published · 4 min read
A large group of general science questions can be answered by one page of facts about gases: which gas does what, what each one smells like, how it is made and where it is used. Papers set by the Staff Selection Commission, the Railway Recruitment Boards and the state commissions ask these as single-line questions, usually in the form of a gas and its use, a gas and a disease, or a gas and its common name. This note collects the gases that appear in those papers, with the properties and uses that are tested.
What the air is made of
Dry air is a mixture, not a compound. By volume it is about 78 per cent nitrogen and about 21 per cent oxygen, with argon near 0.93 per cent as the commonest of the noble gases and carbon dioxide about 0.04 per cent, besides traces of neon, helium, methane and water vapour. Nitrogen is the most abundant gas in the atmosphere, while oxygen is the one that supports life and burning.
The gases asked most often
| Gas | Property to remember | Main uses |
|---|---|---|
| Hydrogen | Lightest gas; isotopes are protium, deuterium and radioactive tritium; made in the laboratory by the action of a dilute acid on a metal such as zinc | Hydrogenation of oils to make vanaspati, manufacture of ammonia, fuel cells, liquid hydrogen as rocket fuel |
| Oxygen | Supports combustion but does not itself burn; allotrope is ozone | Respiration and hospital use, oxy-acetylene welding, steel making, liquid oxygen as a rocket oxidiser |
| Nitrogen | Unreactive at room temperature; liquid nitrogen boils at about minus 196 degrees Celsius | Ammonia manufacture, packing of food to stop oxidation, cryogenic preservation of semen and cells |
| Carbon dioxide | Turns lime water milky; solid form is dry ice; a greenhouse gas | Fire extinguishers, aerated drinks, dry ice as refrigerant, raw material of photosynthesis |
| Carbon monoxide | Colourless and odourless but highly poisonous; forms carboxyhaemoglobin with blood; product of incomplete burning | Reducing agent in metallurgy; a constituent of water gas and producer gas |
| Ammonia | Pungent smell, turns moist red litmus blue; made by the Haber process from nitrogen and hydrogen over an iron catalyst | Fertilisers such as urea, refrigeration, cleaning liquids, nitric acid manufacture |
| Chlorine | Greenish-yellow, pungent and poisonous | Disinfecting drinking water, bleaching powder, making plastics such as PVC |
| Sulphur dioxide | Pungent; dissolves to give an acid, so it contributes to acid rain | Preservative, bleaching of wool and silk, manufacture of sulphuric acid by the contact process |
| Hydrogen sulphide | Smell of rotten eggs; poisonous | Laboratory reagent in qualitative analysis |
| Methane | Simplest hydrocarbon, also called marsh gas; a greenhouse gas | Chief constituent of natural gas, compressed natural gas and biogas; fuel and feedstock |
| Nitrous oxide | Known as laughing gas | Mild anaesthetic in dentistry and surgery, propellant in whipped cream |
| Ozone | Allotrope of oxygen, pungent; absorbs ultraviolet radiation in the stratosphere | Disinfecting water and air, bleaching; the ozone layer shields life from ultraviolet rays |
The noble gases and their uses
- Helium: second lightest gas, non-inflammable, used in balloons and airships in place of hydrogen, in the oxygen-helium mixture breathed by deep-sea divers, and as liquid helium to cool the magnets of magnetic resonance imaging machines.
- Neon: gives a reddish-orange glow in a discharge tube, so it is used in advertising signs and in warning lights.
- Argon: the commonest noble gas in air, used to fill electric bulbs so that the filament does not burn away, and as an inert shield in arc welding.
- Krypton: used in flash lamps for high-speed photography and in airport runway lights.
- Xenon: used in high-intensity lamps and flash tubes, and is sometimes called the gas of the bactericidal lamp.
- Radon: radioactive and used formerly in radiotherapy; it seeps from rocks and is a hazard in mines and closed basements.
Fuel gases and gas mixtures
- Liquefied petroleum gas is mainly butane with some propane; the strong smell is added on purpose, with ethyl mercaptan, so that leaks can be noticed.
- Compressed natural gas is mainly methane and burns more cleanly than petrol or diesel.
- Water gas is a mixture of carbon monoxide and hydrogen; producer gas is carbon monoxide with nitrogen; coal gas contains hydrogen, methane and carbon monoxide.
- Biogas from a gobar gas plant is mostly methane with carbon dioxide, and is produced by the anaerobic decay of organic matter.
- Acetylene, also called ethyne, burns with oxygen to give the very hot flame used in welding, while ethylene, or ethene, is the gas that ripens fruit and the raw material of polythene.
Gases and health
- Carbon monoxide poisoning acts by taking the place of oxygen on haemoglobin, so the blood cannot carry oxygen even when breathing continues.
- Methyl isocyanate, the gas released at Bhopal in 1984, is remembered as the worst industrial gas disaster.
- Sulphur dioxide and oxides of nitrogen produce acid rain, which damages crops, buildings and the limestone of monuments.
- Chlorofluorocarbons release chlorine high in the atmosphere, which destroys ozone; the loss is deepest over the polar regions, where the cold clouds of the stratosphere speed the reaction, and the Montreal Protocol of 1987 was agreed to phase these substances out.
Exam Point of View
Questions come as one-line pairs: a gas and its use, a gas and its common name, a gas and the test that identifies it, or a gas and the harm it does. Learn argon for bulbs, neon for signs, helium for balloons and divers, nitrous oxide for laughing gas, methane for CNG and biogas, carbon monoxide for carboxyhaemoglobin, and lime water for carbon dioxide. The common traps are mixing nitric oxide, nitrogen dioxide and nitrous oxide, choosing carbon dioxide instead of carbon monoxide for blood poisoning, and naming hydrogen rather than helium for modern balloons.
Important Facts
| Most abundant gas in air | Nitrogen, about 78 per cent by volume |
|---|---|
| Lightest gas | Hydrogen; helium is the second lightest and is non-inflammable |
| Isotopes of hydrogen | Protium, deuterium and tritium; tritium is radioactive |
| Laboratory preparation of hydrogen | Zinc with dilute sulphuric acid |
| Test for carbon dioxide | Turns lime water milky, forming calcium carbonate |
| Dry ice | Solid carbon dioxide, used as a refrigerant |
| Carbon monoxide poisoning | Forms carboxyhaemoglobin with haemoglobin |
| Haber process | Manufacture of ammonia from nitrogen and hydrogen, iron catalyst |
| Laughing gas | Nitrous oxide, a mild anaesthetic |
| Gas in electric bulbs | Argon, sometimes with nitrogen |
| Gas in advertising signs | Neon, which glows reddish orange |
| Chief constituent of CNG and biogas | Methane, also called marsh gas |
| LPG | Mainly butane with propane; ethyl mercaptan added for smell |
| Bhopal gas tragedy gas | Methyl isocyanate, 1984 |
| Montreal Protocol | 1987, for phasing out ozone-depleting substances |
Practice MCQs on this topic
Which gas is typically produced when a metal reacts with an acid?
- A.Carbon dioxide
- B.Oxygen
- C.Hydrogen
- D.Nitrogen
Show answer
A metal more reactive than hydrogen displaces it from a dilute acid, so zinc with dilute sulphuric acid gives zinc sulphate and hydrogen gas, and this is the standard laboratory preparation of the gas; the test is that it burns with a pop.
Read the full explanation →Which of the following is a radioisotope of hydrogen?
- A.Xenon
- B.Technetium
- C.Thorium
- D.Tritium
Show answer
Hydrogen has three isotopes: protium with one proton and no neutron, deuterium with one neutron, which gives heavy water, and tritium with two neutrons, which is radioactive and is used in fusion research and in luminous dials.
Read the full explanation →Ozone depletion is greatest near
- A.tropics
- B.equator
- C.poles
- D.oceans
Show answer
Chlorine released from chlorofluorocarbons destroys ozone fastest where polar stratospheric clouds form in the extreme cold of winter, because those ice clouds hold the chlorine compounds that sunlight then breaks up; the thinning is therefore deepest over Antarctica, where it is called the ozone hole, and is seen over the…
Read the full explanation →Which gas is commonly known as laughing gas?
- A.Nitric oxide
- B.Nitrogen dioxide
- C.Nitrous oxide
- D.Ammonia
Show answer
It is called laughing gas because it produces a feeling of elation when inhaled in small quantities, and it is used as a mild anaesthetic in dentistry and minor surgery and as the propellant in whipped cream; it is also a greenhouse gas.
Read the full explanation →Which gas is filled in electric bulbs so that the filament lasts longer?
- A.Oxygen
- B.Argon
- C.Chlorine
- D.Carbon dioxide
Show answer
Argon is chemically inert, so the hot tungsten filament does not react with it, and its presence also slows the evaporation of tungsten, which is why bulbs are filled with argon, sometimes mixed with nitrogen, instead of being left as a vacuum.
Read the full explanation →Which gas is used in advertising signs and gives a reddish-orange glow in a discharge tube?
- A.Helium
- B.Neon
- C.Krypton
- D.Radon
Show answer
When an electric discharge is passed through neon at low pressure it glows a bright reddish orange, which is why the tubes of advertising signs and warning lights are filled with it, and the name neon sign has stuck to such tubes whatever gas they now contain.
Read the full explanation →Lime water turns milky when which gas is passed through it?
- A.Hydrogen
- B.Oxygen
- C.Carbon dioxide
- D.Nitrogen
Show answer
Carbon dioxide reacts with lime water, a solution of calcium hydroxide, to form insoluble calcium carbonate, whose fine white particles make the liquid look milky; if more of the gas is passed the milkiness disappears as soluble calcium bicarbonate is formed, and that second step is often the harder half…
Read the full explanation →The Haber process is used for the manufacture of which gas?
- A.Chlorine
- B.Ammonia
- C.Sulphur dioxide
- D.Ethylene
Show answer
In the Haber process nitrogen from the air and hydrogen are combined under high pressure at a few hundred degrees Celsius over a catalyst of finely divided iron, and the ammonia so made is the starting point of nitrogenous fertilisers such as urea and of nitric acid.
Read the full explanation →Which gas is the chief constituent of compressed natural gas and of biogas?
- A.Butane
- B.Propane
- C.Methane
- D.Ethylene
Show answer
Compressed natural gas is mainly methane, the simplest hydrocarbon, and biogas from a gobar gas plant is also mostly methane along with carbon dioxide, produced by the anaerobic decay of organic matter; methane is also called marsh gas because it bubbles up from waterlogged ground.
Read the full explanation →Which gas combines with haemoglobin to form carboxyhaemoglobin, making it unable to carry oxygen?
- A.Carbon dioxide
- B.Carbon monoxide
- C.Sulphur dioxide
- D.Nitrous oxide
Show answer
Carbon monoxide binds to haemoglobin far more strongly than oxygen does and forms carboxyhaemoglobin, so the blood cannot pick up oxygen even though breathing goes on; the gas is colourless and odourless, which makes it the more dangerous, and it is produced by the incomplete burning of fuel in closed…
Read the full explanation →Frequently Asked Questions
Why is helium used in balloons instead of hydrogen?
Hydrogen is lighter but highly inflammable, while helium is almost as light and does not burn, so it is safe for balloons and airships. Helium is also used in the oxygen-helium mixture breathed by deep-sea divers.
Why is carbon monoxide more dangerous than carbon dioxide?
Carbon monoxide is colourless and odourless, so it gives no warning, and it binds haemoglobin far more strongly than oxygen to form carboxyhaemoglobin, so the blood cannot carry oxygen. Carbon dioxide is the normal waste gas of respiration and harms only in very high concentration.
Which gas is used to ripen fruit?
Ethylene, also called ethene, is the natural ripening hormone used for the purpose; acetylene from calcium carbide is sometimes misused for it, which is unsafe and not permitted.
What is the difference between water gas and producer gas?
Water gas is a mixture of carbon monoxide and hydrogen, made by passing steam over hot coke, while producer gas is a mixture of carbon monoxide and nitrogen, made by passing air and steam over coke, and so has a lower heating value.
Why is a strong smell added to cooking gas?
Liquefied petroleum gas is itself nearly odourless, so ethyl mercaptan is added to give it a sharp smell by which a leak can be detected before the gas reaches a dangerous concentration.
Why does ozone depletion show up most over Antarctica?
Because the extreme cold of the polar winter forms stratospheric clouds that hold chlorine compounds released from chlorofluorocarbons, and sunlight in spring then sets off the reactions that destroy ozone, producing what is called the ozone hole.
