MCQs with Answers and Explanations
1. When a gas changes directly into a solid, the process is called:
a) Evaporation
b) Condensation
c) Sublimation
d) Deposition ✅
Explanation:
Deposition is the direct conversion of a gas into a solid without passing through the liquid state (e.g., frost formation).
Tip:
Deposition is the reverse of sublimation. “Depo” = “down” to solid.
2. Which factor does NOT affect the rate of evaporation?
a) Surface area
b) Temperature
c) Humidity of air
d) Color of the liquid ✅
Explanation:
Evaporation depends on surface area, temperature, and air movement/humidity. The color of the liquid doesn’t affect evaporation.
Tip:
Focus on physical and environmental factors for evaporation—not aesthetic properties like color.
3. Which law explains that volume of a gas is inversely proportional to pressure at constant temperature?
a) Boyle’s Law ✅
b) Charles’s Law
c) Avogadro’s Law
d) Dalton’s Law
Explanation:
Boyle’s Law: P∝1/V when T is constant.
Tip:
Remember: Boyle = Pressure & Volume, and “Boil” compresses volume.
4. Which of the following occurs only at a fixed temperature?
a) Evaporation
b) Boiling ✅
c) Sublimation
d) Condensation
Explanation:
Boiling happens at the boiling point (fixed temperature). Evaporation occurs over a range of temperatures.
Tip:
Evaporation = surface phenomenon, Boiling = bulk and fixed temp.
5. When temperature of a gas increases at constant volume, its pressure:
a) Decreases
b) Increases ✅
c) Remains constant
d) Becomes zero
Explanation:
By Gay-Lussac’s Law, pressure is directly proportional to temperature at constant volume.
Tip:
Think of a pressure cooker: heat it, pressure rises!
6. Which of the following has the fastest rate of diffusion?
a) Oxygen
b) Carbon dioxide
c) Hydrogen ✅
d) Nitrogen
Explanation:
Rate of diffusion ∝ 1/Molar Mass. Hydrogen has the lowest molar mass → highest diffusion rate.
Tip:
Use Graham’s Law: lighter gas = faster diffusion.
Bonus Quick Trick Table
| Process | Description | Trick to Remember |
|---|---|---|
| Sublimation | Solid → Gas | “Sub” up in smoke |
| Deposition | Gas → Solid | Gas “deposits” as solid |
| Boiling | Liquid → Gas (bulk) | Happens at boiling point |
| Evaporation | Liquid → Gas (surface) | Below boiling point |
| Condensation | Gas → Liquid | Think: cloud formation |
MCQ 7: Kinetic Particle Theory and Pressure
Q7. According to kinetic molecular theory, why does the pressure of a gas increase when temperature increases (at constant volume)?
a) The volume increases
b) The particles move faster and hit the walls more frequently ✅
c) The number of particles increases
d) The particles get bigger
Answer: ✅ b) The particles move faster and hit the walls more frequently
Explanation:
As temperature increases, kinetic energy of gas particles increases. They move faster and collide more frequently and forcefully with the container walls, thus increasing pressure.
Tip:
Remember:
- T↑ ⇒ KE↑ ⇒ Collisions↑ ⇒ Pressure↑
- Think of a balloon in the sun—it expands due to pressure increase.
MCQ 8: Diffusion of Gases
Q8. Which factor affects the rate of diffusion of gases most significantly?
a) Molar mass of gas ✅
b) Color of gas
c) Amount of gas
d) Size of the container
Answer: ✅ a) Molar mass of gas
Explanation:
According to Graham’s Law, lighter gases diffuse faster than heavier ones. Rate ∝ 1/Molar mass
Tip:
Hydrogen diffuses faster than oxygen because it is lighter.
Use:
- Rate1/Rate2=M2/M1
MCQ 9: Sublimation and Phase Change
Q9. Which everyday product uses sublimation?
a) Salt in water
b) Dry ice used in smoke machines ✅
c) Cooking oil
d) Ice cubes in freezer
Answer: ✅ b) Dry ice used in smoke machines
Explanation:
Dry ice (solid CO₂) skips the liquid phase and turns directly into gas—a classic example of sublimation.
Tip:
Sublimation = Substance skips liquid stage
🔍 Summary of Tips (7 to 9)
| Concept | Tip / Mnemonic |
|---|---|
| Pressure ↑ with Temp ↑ | Hotter gas = faster particles = more pressure |
| Diffusion | Lighter gas = faster diffusion (Graham’s Law) |
| Sublimation | Solid → Gas = “Sub” up in the air |
