Question

For making good quality mirrors, plates of float glass are used. These are obtained by floating molten glass over a liquid metal which does not solidify before glass. The metal used can be
(A) Tin
(B) Sodium
(C) Magnesium
(D) Mercury

Answer 1

The answer can be identified using the melting point of the metals. The metal with the lowest melting point would not freeze before the metal with higher melting point.

Complete step by step answer:
Plates of float glass are obtained by floating molten glass over a mercury metal which does not solidify before glass. This is because mercury has the lowest melting point than all the given metals. That means mercury has the lowest freezing point. Mercury is the only element which remains in the liquid state at room temperature. So while the glass alloy is floated over mercury, the glass solidifies first as mercury has a freezing temperature of $- 38.83\,^\circ C$ much lower than the glass.

So, the correct answer is option (D).

Additional information:
Melting point: It is the temperature at which the solid changes its state from solid to liquid. The melting point and freezing point is the same temperature and the state of liquid changes to solid at that temperature.

Note: We can also answer this question by an alternative approach. That is if we get the melting point of all the metals along with the melting point of glass we can easily arrive at the solution. The metal with the lowest melting point is used. The metal with the lowest melting point has the lowest freezing point.