In the blast furnace during iron extraction, limestone acts as a flux, reacting with impurities like silicon dioxide (sand) to form a molten slag that can be easily separated from the molten iron. 

A flux is a substance added to a smelting process to react with and remove impurities from the molten metal. 

In the blast furnace, limestone (calcium carbonate, CaCO\(_3\)) decomposes at high temperatures into calcium oxide (CaO) and carbon dioxide (CO\(_2\)). The calcium oxide (CaO) then reacts with the silicon dioxide (SiO\(_2\)), which is a common impurity in iron ore, to form calcium silicate (CaSiO\(_3\)), which is the molten slag.