Gases

PYQ-Based MCQs (Q1–10)

Which gas is known as laughing gas?
a) CO
b) NO₂
c) N₂O
d) SO₂
✅ c) N₂O
Explanation: Nitrous oxide (N₂O) causes mild anesthesia and laughter; used in dentistry.
(BPSC 2020)
Which gas turns lime water milky?
a) CO
b) CO₂
c) H₂S
d) SO₂
✅ b) CO₂
Explanation: CO₂ reacts with Ca(OH)₂ to form insoluble CaCO₃, turning lime water milky.
(BPSC 66th)
Which gas is responsible for Bhopal gas tragedy?
a) Phosgene
b) Methyl isocyanate
c) Carbon monoxide
d) Sulfur dioxide
✅ b) Methyl isocyanate
Explanation: MIC leak from Union Carbide plant caused the 1984 tragedy.
(BPSC 2017)
Which gas is used in fire extinguishers?
a) O₂
b) CO₂
c) N₂
d) H₂
✅ b) CO₂
Explanation: CO₂ displaces oxygen, suffocating the fire.
(BPSC 67th)
Which of the following gases is lighter than air?
a) CO₂
b) O₂
c) H₂
d) Cl₂
✅ c) H₂
Explanation: Hydrogen is the lightest gas and used in balloons.
(CDPO 2016)
Which gas is used in fluorescent tubes?
a) Chlorine
b) Argon
c) Neon
d) Xenon
✅ b) Argon
Explanation: Argon provides inert atmosphere and prevents filament oxidation.
(BPSC 2019)
Which gas is known as marsh gas?
a) CO
b) CH₄
c) C₂H₆
d) H₂
✅ b) CH₄
Explanation: Methane is produced by anaerobic decomposition in swamps.
(BPSC Assistant 2018)
The most abundant gas in Earth's atmosphere is:
a) O₂
b) CO₂
c) N₂
d) Ar
✅ c) N₂
Explanation: Nitrogen makes up 78% of Earth’s atmosphere.
(BPSC Teacher 2023)
Which gas is responsible for acid rain?
a) O₂
b) CO₂
c) SO₂
d) CH₄
✅ c) SO₂
Explanation: SO₂ and NO₂ form sulfuric/nitric acid in atmosphere causing acid rain.
(BPSC 65th)
Helium is preferred over hydrogen in balloons because:
a) It is heavier
b) It is cheaper
c) It is non-flammable
d) It glows
✅ c) It is non-flammable
Explanation: Unlike hydrogen, helium is inert and doesn’t explode.
(BPSC PYQ)

NCERT-Based MCQs (Q11–30)

Gases have:
a) Fixed volume, no shape
b) Fixed shape, no volume
c) No fixed shape or volume
d) Fixed shape and volume
✅ c) No fixed shape or volume
Explanation: Gases take shape and volume of the container due to weak intermolecular force.
The SI unit of pressure is:
a) Newton
b) Pascal
c) Bar
d) Atmosphere
✅ b) Pascal
Explanation: 1 Pa = 1 N/m², used in gas laws.
Which of these expands the most on heating?
a) Solid
b) Liquid
c) Gas
d) Plasma
✅ c) Gas
Explanation: Gases are most compressible and expand rapidly with temperature rise.
Boyle’s law relates pressure with:
a) Volume
b) Mass
c) Density
d) Temperature
✅ a) Volume
Explanation: At constant temperature, pressure is inversely proportional to volume.
Charles’ law relates volume with:
a) Pressure
b) Temperature
c) Mass
d) Area
✅ b) Temperature
Explanation: Volume of gas is directly proportional to absolute temperature.
Which gas is used in soda water?
a) Oxygen
b) Nitrogen
c) CO₂
d) Hydrogen
✅ c) CO₂
Explanation: Carbon dioxide is dissolved in soda water under pressure.
Which gas is filled in electric bulbs?
a) Nitrogen
b) Oxygen
c) Helium
d) Hydrogen
✅ a) Nitrogen
Explanation: Inert nitrogen prevents oxidation of filament.
A balloon filled with which gas will rise the fastest?
a) Helium
b) Hydrogen
c) Oxygen
d) CO₂
✅ b) Hydrogen
Explanation: Being lightest, hydrogen gives the most lift, though it is flammable.
Gases can be liquefied under:
a) Low pressure, low temp
b) High pressure, low temp
c) High pressure, high temp
d) Low pressure, high temp
✅ b) High pressure, low temp
Explanation: This brings molecules closer to condense into liquid.
Which gas is a greenhouse gas?
a) O₂
b) N₂
c) CO₂
d) He
✅ c) CO₂
Explanation: Carbon dioxide traps infrared radiation, contributing to warming.
Which of the following is not a noble gas?
a) Neon
b) Argon
c) Oxygen
d) Xenon
✅ c) Oxygen
Explanation: Noble gases are chemically inert and include He, Ne, Ar, Kr, Xe, Rn.
At STP, 1 mole of any gas occupies:
a) 22.4 L
b) 44.8 L
c) 1 L
d) 1 cm³
✅ a) 22.4 L
Explanation: This is a standard value used in ideal gas calculations.
Gases diffuse faster than solids because:
a) They are heavier
b) They are cooler
c) They have weaker bonds
d) Their particles move freely
✅ d) Their particles move freely
Explanation: Gas molecules are in constant random motion.
Which gas is used in neon signs?
a) Helium
b) Argon
c) Neon
d) Oxygen
✅ c) Neon
Explanation: Neon glows reddish-orange in discharge tubes.
Which gas is used in cryogenics?
a) Oxygen
b) Nitrogen
c) Hydrogen
d) Argon
✅ b) Nitrogen
Explanation: Liquid nitrogen is used for low-temperature experiments and storage.
Ideal gas shows deviation at:
a) Low temp and high pressure
b) High temp and low pressure
c) STP
d) Room temperature
✅ a) Low temp and high pressure
Explanation: Real gases deviate due to intermolecular forces under such conditions.
Which gas is used in welding?
a) Oxygen
b) Methane
c) Acetylene
d) CO₂
✅ c) Acetylene
Explanation: Oxy-acetylene flame is used for metal welding.
Which gas supports combustion?
a) Hydrogen
b) Oxygen
c) Nitrogen
d) Helium
✅ b) Oxygen
Explanation: Oxygen is essential for burning of fuels.
Which gas is responsible for ozone layer depletion?
a) CO₂
b) CH₄
c) CFCs
d) SO₂
✅ c) CFCs
Explanation: Chlorofluorocarbons release chlorine that destroys ozone.
Gases exert pressure because:
a) They have volume
b) They are hot
c) They collide with walls of container
d) They are compressible
✅ c) They collide with walls of container
Explanation: Constant molecular collisions result in pressure.
Which gas is most soluble in water?
a) Oxygen
b) Carbon dioxide
c) Nitrogen
d) Helium
✅ b) Carbon dioxide
Explanation: CO₂ dissolves readily in water and forms carbonic acid.
What is the gas law combining Boyle’s and Charles’ laws?
a) Ideal gas law
b) Combined gas law
c) Dalton’s law
d) Avogadro’s law
✅ b) Combined gas law
Explanation: It links pressure, volume, and temperature for a fixed gas sample.
Avogadro's Law states that:
a) Equal masses contain equal atoms
b) Equal volumes of gases under same conditions contain equal molecules
c) Gases expand equally on heating
d) Gases liquefy at 0°C
✅ b) Equal volumes of gases under same conditions contain equal molecules
Explanation: It holds at constant temperature and pressure.
Which gas is used to fill tyres in airplanes?
a) Oxygen
b) Nitrogen
c) Carbon dioxide
d) Hydrogen
✅ b) Nitrogen
Explanation: Nitrogen is inert and resists expansion with temperature change.
The density of gas increases with:
a) Increase in temperature
b) Increase in volume
c) Increase in pressure
d) Increase in entropy
✅ c) Increase in pressure
Explanation: Higher pressure compresses gas into less volume, raising density.
Which gas is stored under high pressure in cylinders?
a) Oxygen
b) Hydrogen
c) Nitrogen
d) All of the above
✅ d) All of the above
Explanation: All industrial gases are compressed in steel cylinders for storage.
Which gas is used in weather balloons?
a) Hydrogen
b) Helium
c) Oxygen
d) Methane
✅ b) Helium
Explanation: Helium is light and non-flammable, making it safe for meteorological use.
Which of these gases is monoatomic?
a) Oxygen
b) Nitrogen
c) Helium
d) Hydrogen
✅ c) Helium
Explanation: Noble gases like helium consist of single atoms.
Graham’s law relates to:
a) Rate of effusion
b) Pressure and temperature
c) Solubility of gas
d) Critical point
✅ a) Rate of effusion
Explanation: It states that lighter gases effuse faster than heavier ones.
Which gas helps in ripening of fruits?
a) Methane
b) Ethylene
c) Carbon dioxide
d) Propane
✅ b) Ethylene
Explanation: Ethylene is a plant hormone used commercially to ripen fruits.
Which gas is toxic in closed rooms with burning coal?
a) CO₂
b) CO
c) CH₄
d) H₂
✅ b) CO
Explanation: Carbon monoxide binds with hemoglobin, causing suffocation.
Which gas is used in lasers and barcode scanners?
a) Helium
b) Argon
c) Helium-Neon
d) Xenon
✅ c) Helium-Neon
Explanation: He-Ne lasers are used in barcode readers and optical devices.
Which gas is essential in photosynthesis?
a) CO
b) O₂
c) CO₂
d) N₂
✅ c) CO₂
Explanation: Plants use carbon dioxide and sunlight to make food.
Which gas causes choking and burning of eyes in smog?
a) Ozone
b) Carbon monoxide
c) Sulfur dioxide
d) Oxygen
✅ c) Sulfur dioxide
Explanation: SO₂ is a major air pollutant with irritating effects on eyes.
Which of the following is not a greenhouse gas?
a) CO₂
b) CH₄
c) N₂
d) O₃
✅ c) N₂
Explanation: Nitrogen does not absorb infrared radiation, unlike greenhouse gases.
Which gas is released from decomposing garbage?
a) Methane
b) Nitrous oxide
c) Oxygen
d) Ethanol vapor
✅ a) Methane
Explanation: Anaerobic decomposition of organic waste produces CH₄.
Van der Waals equation modifies which law?
a) Boyle’s law
b) Charles’s law
c) Ideal gas law
d) Graham’s law
✅ c) Ideal gas law
Explanation: It accounts for non-ideal behavior of real gases.
Which gas is detected using a lime water test?
a) CO
b) CO₂
c) NO
d) N₂
✅ b) CO₂
Explanation: Lime water turns milky due to the formation of CaCO₃ with CO₂.
Which gas is used in MRI machines as coolant?
a) Nitrogen
b) Oxygen
c) Helium
d) Argon
✅ c) Helium
Explanation: Liquid helium cools superconducting magnets in MRI scanners.
Which gas is responsible for suffocation in closed coal mines?
a) O₂
b) CO
c) CH₄
d) N₂
✅ b) CO
Explanation: Carbon monoxide is lethal, odorless, and replaces oxygen in blood.