In reaction :
\(FeO + SiO_2 \longrightarrow FeSiO_3\)
What is slag among three compounds involved in metallurgy of copper?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(FeSiO_3\)
In metallurgy, a flux is added to combine with the unwanted impurity (gangue) and form an easily fusible substance called slag, which floats on the molten metal or matte and is removed.
Copper pyrites (\( \ce{CuFeS2} \)) contains iron. When the roasted ore is heated with silica in a reverberatory furnace, iron oxide combines with the silica (an acidic flux) and is removed as iron silicate, while copper stays in the matte (\( \ce{Cu2S} \) + FeS):
\( \displaystyle \ce{FeO + SiO2} \longrightarrow \underset{\text{slag}}{\ce{FeSiO3}} \)
So the slag is FeSiO3.
Why the other options are incorrect
Option (1) FeO — Incorrect. The impurity (gangue) to be removed; it is the substance the flux acts on.
Option (2) SiO2 — Incorrect. The flux that is added to remove FeO.
Option (4) FeO and SiO2 both — Incorrect. These are the reactants; the slag is the product they form together.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Extraction of Copper from Cuprous Oxide [Copper(I) Oxide] – “… In actual process, the ore is heated in a reverberatory furnace after mixing with silica. In the furnace, iron oxide ‘slags of’ as iron silicate and copper is produced in the form of copper matte. This contains Cu2S and FeS.”
\( \displaystyle \ce{FeO + SiO2} \longrightarrow \underset{\text{Slag}}{\ce{FeSiO3}} \)