The combustion of candle under limited supply of air forms

Analysis

The question asks for the products formed when a candle (which is primarily a hydrocarbon, like paraffin wax) undergoes combustion in a limited supply of air.

Concept Explanation

Combustion is a chemical reaction between a fuel and oxygen. There are two main types:

1. Complete Combustion: Occurs when there is an excess supply of oxygen. Hydrocarbons burn completely to produce carbon(IV) oxide ($CO_2$) and water ($H_2O$).
2. Incomplete Combustion: Occurs when there is a limited supply of oxygen. Because there isn't enough oxygen to convert all the carbon into $CO_2$, the reaction produces carbon(II) oxide ($CO$) and/or carbon particles (soot).

Step-by-Step Solution

1. Identify the fuel: A candle is a hydrocarbon ($C_nH_{2n+2}$).
2. Analyze the condition: The phrase "limited supply of air" indicates that oxygen is insufficient for the complete oxidation of the carbon atoms in the hydrocarbon.
3. Determine the products: • In limited oxygen, the hydrogen still tends to form water ($H_2O$). • The carbon, however, cannot fully oxidize to $CO_2$. Instead, it forms carbon(II) oxide ($CO$) and/or solid carbon (soot).
4. Evaluate the options: • Option A ($CO_2$ and water) is the result of complete combustion. • Option B ($CO$ and water) is a classic result of incomplete combustion. • Option C ($soot$ and $CO$) lacks the water component, which is a standard product of hydrocarbon combustion. • Option D ($soot$ and $CO_2$) suggests a mix of incomplete and complete combustion, but $CO$ is the primary indicator of limited oxygen.

Correct Answer

The correct option is B.

Exam Tip

Remember the "Oxygen Rule":

• Plenty of Oxygen: $CO_2$ (The "clean" burn).
• Little Oxygen: $CO$ (The "poisonous" burn).
• Very little Oxygen: Soot (The "black/smoky" burn).