Matter exists in three primary states — solid, liquid, and gas. Of these, gases are the most behaviourally distinct. In a gas, molecules move freely at high speeds, have negligible intermolecular attraction, and spread to fill any container completely. This is why a single drop of perfume fills an entire room within minutes.
Here's a simple way to lock in the three states:
For the Bihar Police Constable exam, the questions on gases almost never test deep thermodynamics. They test which gas does what — which gas is used in welding, which is toxic, which smells like rotten eggs, which is used in balloons. Think of this topic as a vocabulary test about gases, with a thin layer of gas-law logic on top.
A useful mental model: think of gases as personalities. Oxygen is the life-giver. Nitrogen is the silent majority (78% of air, mostly inert). Hydrogen is the lightest daredevil. Carbon monoxide is the silent killer. Noble gases are the loners who refuse to react with anyone. Once you attach personality traits to each gas, the properties stick naturally.
The laws governing gas behaviour — Boyle's Law and Charles's Law — are foundational, but the exam rarely asks you to solve numerical problems from them at this level. You need to understand what the laws say, not derive them mathematically.
When matter changes state, it does so through specific processes:
Sublimation is a favourite exam trap. Dry ice (solid CO₂) sublimates — it never becomes liquid at normal atmospheric pressure. Iodine (आयोडीन) also sublimates.
Boyle's Law (Robert Boyle, 1662): At constant temperature, the pressure of a gas is inversely proportional to its volume.
P ∝ 1/V → PV = constant
Real-world example: when you compress a syringe with your thumb, the pressure inside increases as volume decreases.
Charles's Law (Jacques Charles): At constant pressure, the volume of a gas is directly proportional to its absolute temperature (in Kelvin).
V ∝ T → V/T = constant
Real-world example: a balloon shrinks when taken into a cold room, and expands in heat.
Combined Gas Law:
(P₁V₁)/T₁ = (P₂V₂)/T₂
At the exam level, remember: pressure and volume are inversely related (Boyle), volume and temperature are directly related (Charles).
Air is a mixture, not a compound. Its approximate composition by volume:
| Gas | Percentage | |---|---| | Nitrogen (N₂) | ~78% | | Oxygen (O₂) | ~21% | | Argon (Ar) | ~0.93% | | Carbon Dioxide (CO₂) | ~0.04% | | Other gases | trace |
Lock this in: Nitrogen dominates air (78%), Oxygen is second (21%). This pair alone makes up ~99% of dry air.
Oxygen (O₂)
Nitrogen (N₂)
N₂ + 3H₂ → 2NH₃Hydrogen (H₂)
Carbon Dioxide (CO₂)
Carbon Monoxide (CO)
Ammonia (NH₃)
Chlorine (Cl₂)
Sulphur Dioxide (SO₂)
SO₂ + H₂O → H₂SO₃Ozone (O₃)
Noble gases are chemically inert — they have completely filled outermost electron shells, so they do not react with other elements under normal conditions.
| Gas | Symbol | Key Use | |---|---|---| | Helium | He | Balloons, airships (non-flammable) | | Neon | Ne | Neon signs (glows red-orange) | | Argon | Ar | Filling electric bulbs (prevents filament oxidation) | | Krypton | Kr | High-intensity lighting | | Xenon | Xe | Flash lamps, anaesthesia | | Radon | Rn | Radioactive — found in soil |
Memory device: He Ne Ar Kr Xe Rn — "He Never Argues; Krooks Xerox Readily" (crude but functional).
Nitrogen is 78%, Oxygen is 21% of air. Subtract: 78 - 21 = 57 (irrelevant number, but the gap helps you remember the order). A cleaner memory hook: "Nitrogen is Nearly 4 times more than Oxygen" (78/21 ≈ 3.7). If a question gives you 78% nitrogen and asks what percentage is oxygen, you know it's ~21% without calculating. Standard recall: 10s. Without this: forces guessing.
CO (one oxygen) = Carbon Monoxide = Killer gas (odourless, toxic, binds haemoglobin). CO₂ (two oxygens) = Carbon Dioxide = Fizz gas (drinks, fire extinguisher, dry ice). The exam frequently swaps these two in options. Lock it: one oxygen = danger, two oxygens = safe but greenhouse. Eliminates wrong answer in 5s versus rereading definitions in 30s.
Whenever a question says "this gas does not react with anything under normal conditions" or "used to fill electric bulbs," you eliminate all non-noble-gas options immediately. Noble gases: He, Ne, Ar, Kr, Xe, Rn. If the option is N₂, O₂, H₂, Cl₂ — eliminate. This cuts 4-option questions to 1-2 options in under 5s versus reading each option's properties separately (30+ seconds).
Three gases with distinctive, pungent smells: NH₃ (ammonia — sharp, like cleaning agents), SO₂ (sulphur dioxide — burnt matchsticks), Cl₂ (chlorine — bleach smell). Two gases with NO smell that are dangerous: CO (carbon monoxide — silent killer) and CO₂ (carbon dioxide). If a question says "colourless and odourless toxic gas," the answer is CO, not CO₂ (CO₂ is not acutely toxic in normal concentrations). 3-step pattern recognition vs 30s reading: saves ~25s per question.
Photosynthesis equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Input = CO₂. Output = O₂. Any question about "gas released during photosynthesis" = Oxygen. If you see CO₂ as an option, it is the input, not the output — eliminate it immediately. This is a 5-second recognition versus a 20-second formula recall.
In the exam hall, when you see a gases question, run this decision tree in 10 seconds:
Is it asking about a specific gas's use? — Match the gas name to its personality (O₂ = life/combustion, N₂ = inert/food packing, H₂ = lightest/flammable, CO = odourless killer, CO₂ = fizz/extinguisher, NH₃ = fertiliser/pungent, Cl₂ = disinfectant/tear gas, noble gases = inert/bulbs).
Is it asking which gas has no smell? — CO and CO₂ have no smell. CO is toxic; CO₂ is not acutely toxic. Pick accordingly.
Is it asking about air composition? — N₂ first (78%), O₂ second (21%). Always.
Is it a photosynthesis gas question? — Answer is always Oxygen (O₂).
Is it asking about the lightest gas? — Hydrogen (H₂) is lightest element and gas. Helium is second lightest but lightest noble gas.
Two options seem correct? — Use the smell test or the CO vs CO₂ distinction to eliminate one within 5 seconds.
Why this question: Tear gas identification tests your gas-properties vocabulary. A common confusion arises because multiple gases are irritants — the exam picks the historically correct answer from the given options.
Solving path: Look at the options — Cl₂ (chlorine), H₂ (hydrogen), SO₂ (sulphur dioxide), NH₃ (ammonia). Eliminate H₂ immediately — hydrogen is flammable, not an irritant used as tear gas. SO₂ is a pollutant and irritant but not historically classified as tear gas. NH₃ is pungent but not tear gas. Cl₂ — greenish-yellow, toxic, causes severe eye and throat irritation, was used as a chemical weapon and lachrymatory agent. Answer: Cl₂. Time taken with this elimination: 15s.
Why this question: Photosynthesis gas output is tested repeatedly across Bihar and UP Police exams. Students sometimes confuse the input gas (CO₂) with the output gas (O₂).
Solving path: Photosynthesis takes in carbon dioxide and water, and releases oxygen as a by-product. The equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The question asks what is released — that is Oxygen. If CO₂ appears as an option, remember it is the input, not output. Answer: Oxygen (ऑक्सीजन). Time with framework: 8s.
Why this question: The "colourless and odourless toxic gas" is a classic trap. CO and CO₂ are both colourless and odourless, but CO is acutely toxic at low concentrations while CO₂ is dangerous only in very high concentrations.
Solving path: Options include CO, NH₃, SO₂, Cl₂. Eliminate NH₃ (pungent smell), SO₂ (sharp smell), Cl₂ (sharp greenish-yellow, visible). CO is colourless and odourless, produced by incomplete combustion. It binds to haemoglobin far more readily than oxygen, causing internal suffocation without any warning smell. This makes it uniquely dangerous among the options. Answer: Carbon Monoxide (CO). Time: 12s.
Confusing CO and CO₂: Carbon monoxide (CO) is the odourless killer; carbon dioxide (CO₂) is what you exhale and what fizzy drinks contain. The exam exploits this confusion constantly. Never swap them.
Thinking Oxygen burns: Oxygen supports combustion (दहन-पोषक) but does not itself burn. Hydrogen burns; oxygen helps other things burn. If a question says "which gas is flammable," Oxygen is not the answer.
Mixing up nitrogen percentage in air: Some students write 72% or 80%. It is 78%. Oxygen is 21%. These two numbers together must be memorised exactly.
Assuming noble gases are used because they are colourful: Noble gases glow particular colours when electricity passes through them (neon glows red-orange), but they are used in bulbs and signs primarily because they are inert, not because of their colour. The inertness is the reason, the colour is the byproduct.
Calling CO₂ a tear gas or toxic gas: CO₂ is not a tear gas. Among common gases, CO is the odourless toxic one; Cl₂ is the tear/irritant gas. CO₂ simply suffocates by displacing oxygen in very high concentrations but is not classified as a toxic gas in exam contexts.
Forgetting that Hydrogen is the lightest — not Helium: Helium is lighter than air and used in balloons, but Hydrogen (H₂) is the lightest gas overall. Helium is used in balloons instead of hydrogen specifically because hydrogen is flammable (Hindenburg). Do not let the balloon association make you write Helium as "lightest gas."