In the metallurgy of aluminium,cryolite is mixed in the molten state because it

The least stable oxide at room temperature is

In the extraction of silver by the Mac-Arthur-Forrest cyanide process,a small amount of $KNO_3$ is also added as a flux. The function of $KNO_3$ is

During the extraction of sodium metal by electrolysis,a mixture of $40\% \ NaCl$ and $60\% \ CaCl_2$ is used because...

Which statement is correct regarding the slag formed during the extraction of copper or iron?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: In the Ellingham diagram,a sharp change in slope of the line is observed for $Mg \rightarrow MgO$ at $\sim 1120^{\circ} C$.
Reason $R$: There is a large change of entropy associated with the change of state.
In the light of the above statements,choose the correct answer from the options given below.