General Science MCQ Practice

• Correct option: A — Frequency remains unchanged when a sound wave passes from one medium to another. The source fixes frequency, and boundary conditions preserve it.
• Key concept: for a wave, speed = frequency × wavelength (v = fλ). When speed changes with the medium, wavelength changes but frequency stays the same.
• Incorrect options:
  • Amplitude may change due to impedance differences.
  • Wavelength changes with speed (λ = v/f).
  • Speed changes with the medium (v changes).

• Why correct: In space with no gravity, there is no medium for conduction or natural convection. Heat moves mainly by electromagnetic waves (radiation).
• Key concept: Heat transfer by radiation follows the Stefan–Boltzmann law: P = εσA(T^4 − T_env^4).
• Exam tip: If there is no medium, assume radiation dominates.
• Why others are wrong:
  • Conduction requires contact between substances.
  • Convection requires a fluid and gravity to drive buoyancy (natural convection) or an external force.
  • “Cannot be heated” is incorrect; heating occurs by radiation.

• Why: The Sun is seen before actual sunrise due to atmospheric refraction. Light from the Sun bends as it passes through Earth's atmosphere, bending toward the normal, making the Sun appear higher than it really is.
• Key concept: Atmospheric refraction and Snell’s Law cause bending of light at air–water/air–air boundaries, lifting the image of the Sun before sunrise.
• Tip: If the Sun is just below the horizon, refraction can make it visible about 2–3 minutes earlier.

• Correct answer: D. silicon dioxide of glass
• Other options are incorrect because HF does not react significantly with visible light, sodium oxide, or aluminium oxide under ordinary storage conditions.

• Reason: Sodium stearate is a soap; it is formed by the saponification of fats/oils with a strong base. The reaction yields glycerol and soap (sodium stearate).
• Key concept: Saponification—fat/oil + NaOH → glycerol + fatty acid salt (soap).
• Simple formula: Fat + NaOH → Glycerol + Sodium stearate (soap).
• Why other options are wrong: Gunpowder, paint, and fertilizer do not require sodium stearate as a primary component.
• Exam tip: Recognize salts of fatty acids as soaps; look for “stearate” or similar names.

• Reason: Phosphor bronze (Cu-Sn-P) has high tensile strength, good elasticity, and excellent fatigue resistance, making it ideal for springs and suspension filaments in electrical instruments.
• Key concept: Phosphorus improves wear resistance and elasticity in copper alloys, enhancing springiness.
• Other options:
  • Bronze (Cu-Sn) is softer for springs.
  • Aluminum bronze is hard but not chosen for springs.
  • German silver is nickel-silver, not optimized for spring elasticity.
• Exam tip: Remember: use phosphor bronze for high fatigue resistance and good elasticity in springs.

• Correct option: B – Sodium azide
• Reason: Airbags use a chemical gas generator that rapidly decomposes sodium azide to produce nitrogen gas, inflating the bag in milliseconds.
• Key concept: Thermal decomposition produces a large volume of N2 gas quickly: 2 NaN3 → 2 Na + 3 N2.
• Why others are incorrect:
  • A. Sodium bicarbonate and C. Sodium nitrite do not rapidly produce enough gas on impact.
  • D. Sodium peroxide is not used for airbags and has different reactions.
• Exam tip: Remember “rapid gas generator” in airbags, historically sodium azide was used.

• Reason: Carbon monoxide (CO) can combine with the oxygen (O2) inside the body to form carbon dioxide (CO2), reducing available oxygen for tissues.
• Key concept: Oxygen transport impairment and respiratory poisoning by CO binding/convertible oxidation reduces oxygen delivery.
• Formula note: Conceptually: CO + O2 → CO2 (oxygen used up in forming CO2, less O2 for tissues).
• Why others are incorrect: A) Not about reducing organic matter; B) Not about drying blood; D) Real danger is CO binding to haemoglobin, but the statement focuses on CO2 formation.

Exam tip: Remember CO binds hemoglobin with high affinity, but this question highlights its reaction with O2 to form CO2.

• Why it’s correct: Domestic cooking gas (natural gas) is a hydrocarbon mix mainly of light alkanes, chiefly methane (CH4) and some ethane (C2H6).
• Key concept: Composition of natural gas vs LPG; light hydrocarbons dominate the fuel gas.
• Brief formula: CH4 (methane) and C2H6 (ethane).
• Exam tip: If the question asks specifically about LPG, remember LPG is propane (C3H8) and butane (C4H10), not methane/ethane.
• Why others are incorrect: B lists LPG components (butane/isobutane) not the main; C includes unsuitable gases (ethylene/CO); D gas pair is not used as cooking fuel.

• Why: Helium is inert and non-flammable, so it does not form an explosive mixture with air.
• Key concept: Safety of gases in balloons; flammable gases can ignite in air.
• Other options:
  • A: Helium is not cheaper than hydrogen.
  • B: Helium is less dense than air, but hydrogen is even lighter, so lift isn’t the reason here.
  • C: Hydrogen has greater lifting power, not helium.