Question 8
Why does the slag float on the molten iron inside a blast furnace?
The right answer is the one marked above.
Because slag is less dense than molten iron. That is what lets the two be drawn off separately — the slag from the upper outlet and the molten iron from the lower one.
Slag is calcium silicate and calcium aluminate — solids and liquids, not a gas. What makes it float is density, not temperature.