The main function of limestone in the blast furnace is to

Analyze the question: The question asks about the role of limestone (CaCO₃) in the blast furnace used to extract iron from its ore (haematite, Fe₂O₃).

Concept: Iron ore contains impurities, mainly silica (SiO₂, sand). If these are not removed, they would contaminate the iron. Limestone is added to the furnace as a flux. A flux is a substance that combines with impurities to form an easily removed waste product called slag.

Step-by-step solution:

1. Limestone decomposes in the heat of the furnace: CaCO₃ → CaO + CO₂

2. The calcium oxide (a basic oxide) reacts with silica (an acidic oxide) in the ore: CaO + SiO₂ → CaSiO₃ (calcium silicate, or slag)

3. The molten slag is less dense than molten iron, so it floats on top and is tapped off separately from the iron.

Checking the options:

• A. Catalyst: No. Limestone is consumed in the reaction, so it is not a catalyst.
• B. Reducing agent: No. The reducing agents are coke (carbon) and carbon (II) oxide (CO).
• C. Remove impurity: Yes. It forms slag with silica and other impurities.
• D. Supply carbon (IV) oxide: Limestone does produce CO₂, but this is not its main function. The CO₂ is a by-product that reacts with coke to form CO.

**Answer: C. Remove impurity.

Exam tip: Remember "Limestone = Flux = Slag." Coke is the fuel and the source of the reducing agent, while limestone is the cleaner that removes impurities.