Which ore is concentrated by the froth flotation process?

What type of reaction occurs when sulfur dioxide $(SO_2)$ reacts with an acidic solution of potassium dichromate $(K_2Cr_2O_7)$?

$(a)$ $A$ substance $X$,an oxide of a metal,is used extensively in the cement industry. This element is found in our bones also. On treatment with water,it forms a solution which turns red litmus blue. Identify $X$ and also write the chemical reaction involved.
$(b)$ Choose a metal from the following metals which reacts only with hot water: Sodium,magnesium,iron. Mention the products formed during the reaction.

Heating of carbonate ores in the absence of air to convert them into oxides is known as:

Stainless steel is a very useful material in our daily life. In stainless steel,iron is mixed with:

In the $Bayer$ process,what is obtained when concentrated $NaOH$ is added to bauxite?