Final answer:
The waste that forms the top layer on molten iron is called dross. Dross is less dense than the molten iron and floats on top, creating a barrier that prevents exposure to oxygen.
Step-by-step explanation:
The uppermost layer on molten iron, known as dross, is a byproduct of the decomposition of limestone (calcium carbonate).
During this process, the liberated calcium oxide reacts with silica and silicates present in the iron, resulting in the formation of calcium silicate.
Notably, dross exhibits lower density compared to molten iron, causing it to float atop the iron surface.
This floating layer functions as a protective barrier, crucially preventing the molten iron from exposure to oxygen.
Such exposure could lead to the undesirable oxidation of iron back to iron oxide.
The formation and positioning of dross in the iron production process serve a critical role in maintaining the integrity of the molten iron, ensuring its protection against oxygen-induced reactions that might compromise its quality.
Hence, the waste that forms the top layer on molten iron is called dross.