What is the use of potassium dichromate $(K_2Cr_2O_7)$?

Describe the oxidising action of potassium dichromate and write the ionic equations for its reaction with:
$(i)$ iodide
$(ii)$ iron $(II)$ solution and
$(iii)$ $H_{2}S$

Which of the following is a product of the first step and is used as a reactant in the next step for the manufacture of $K_{2}Cr_{2}O_{7}$ from concentrated chromite ore?

When an oxide of manganese $(A)$ is fused with $KOH$ in the presence of an oxidising agent and dissolved in water,it gives a dark green solution of compound $(B)$. Compound $(B)$ disproportionates in neutral or acidic solution to give purple compound $(C)$. An alkaline solution of compound $(C)$ oxidises potassium iodide solution to a compound $(D)$ and compound $(A)$ is also formed. Identify compounds $A$ to $D$ and also explain the reactions involved.

An aqueous solution of $FeSO_4$,$Al_2(SO_4)_3$ and chrome alum is heated with excess of $Na_2O_2$ and filtered. The materials obtained are

The most basic hydroxide among the following is