Case Study:

In many Indian households, LPG cylinders are commonly used for cooking. One evening, Riya’s mother noticed a strong smell in the kitchen before lighting the stove. On checking, she realized that the LPG regulator was slightly loose, allowing gas to leak slowly.

LPG (Liquefied Petroleum Gas) mainly contains propane and butane. Although stored as a liquid under pressure, it turns into gas when released. LPG is heavier than air. When it leaks, it does not rise upward but spreads along the floor. The strong smell is due to a chemical called ethyl mercaptan added for safety.

Suppose 50 grams of LPG leaked into a closed kitchen of volume 30 m³ over 10 minutes. Because gas particles are in constant motion, they move randomly and mix with air. This spreading of gas particles from a region of higher concentration (near the cylinder) to a region of lower concentration (rest of the kitchen) is called diffusion.

If the kitchen windows are closed, diffusion still occurs but more slowly. When windows are opened and a fan is switched on, air circulation increases, speeding up the mixing and removal of gas.

Understanding diffusion, density differences, and safety precautions is essential to prevent accidents such as fire or explosion.

Section A – MCQs

1. Why does LPG spread throughout the kitchen even if the leak is at one corner?
A. Due to gravitational force
B. Due to diffusion caused by random motion of particles
C. Due to chemical reaction with air
D. Due to increase in temperature

2. LPG is heavier than air. Therefore, during leakage it:
A. Rises toward the ceiling
B. Remains only near the cylinder
C. Spreads along the floor first
D. Immediately disappears

3. Opening windows during LPG leakage helps because it:
A. Stops diffusion
B. Increases air circulation and reduces gas concentration
C. Makes LPG lighter
D. Changes LPG into liquid

4. If the kitchen volume is larger, the concentration of leaked gas (for same amount leaked) will be:
A. Higher
B. Lower
C. Same
D. Zero

Section B – Short Answer Questions

1. Explain why switching on an exhaust fan helps in reducing danger during LPG leakage. 

2. Why is a strong-smelling substance added to LPG even though diffusion can spread the gas?

3. If the kitchen were completely sealed, would diffusion stop? Justify your answer.

Section C – Long Answer Question

1. Suppose 80 grams of LPG leaked into a closed kitchen of volume 20 m³.
(a) Compare the gas concentration with the earlier case of 50 grams in 30 m³.
(b) Explain scientifically which situation is more dangerous and why.
(c) Suggest three scientifically justified safety measures.

Answer Key:

MCQs:

1. B
2. C
3. B
4. B

Section B Answers:

1. An exhaust fan increases air movement, which speeds up mixing of LPG with fresh air and reduces its concentration. Lower concentration reduces risk of ignition and explosion.
2. LPG is naturally odorless. Ethyl mercaptan is added so that leakage can be detected early. Early detection allows preventive action before dangerous concentration builds up.
3. No, diffusion will not stop. Gas particles are always in random motion. However, in a sealed kitchen, gas concentration will increase because there is no fresh air to dilute it.

Section C Answer:

(a) First case concentration ∝ 50 g / 30 m³
      Second case concentration ∝ 80 g / 20 m³

Second case has more gas in smaller volume -> higher concentration.

(b) Higher concentration increases chances of ignition and explosion. Therefore, second situation is more dangerous.

(c) Do not switch on electrical appliances (avoid spark).
     Open doors and windows for ventilation.
     Close regulator immediately.
     Avoid lighting matchsticks.