A gas stove flame that was previously blue has turned yellow and the bottom of the cooking vessel is getting…

Question

A gas stove flame that was previously blue has turned yellow and the bottom of the cooking vessel is getting covered with a black deposit. Identify the cause of this change and explain what the black deposit is.

Accepted Answer

## Model Answer **Cause:** The air holes of the gas stove are blocked, causing insufficient oxygen supply. This leads to **incomplete combustion** of the fuel (LPG). **Blue → Yellow flame:** When the air supply is adequate, complete combustion occurs giving a clean blue flame. With blocked air holes, incomplete combustion produces a yellow, sooty flame. **Black deposit:** The black substance deposited on the bottom of the vessel is **carbon (soot)**. During incomplete combustion, the carbon in the fuel is not fully oxidised to CO₂, and unburnt carbon particles are deposited as soot. Source: Chapter 4, Section 4.3.1 Combustion --- ## Explanation - Examiners expect three clear points: (1) cause — blocked air holes/insufficient air, (2) incomplete combustion producing yellow flame, (3) black deposit = unburnt carbon/soot. - The textbook explicitly states: *"If you observe the bottoms of cooking vessels getting blackened, it means that the air holes are blocked and fuel is getting wasted.…

Science (086) — AI-generated practice question