In brown ring test, the bown colour of the ring is due to 

Distinguish between the following pair of compounds using a reagent as a chemical test:

• Magnesium chloride and magnesium nitrate solution.

Represent the union of two sets by Venn diagram for each of the following.

$X=\{x | x$ is a prime number between $80$ and $100\}$

$Y=\{y | y$ is an odd number between $90$ and $100\}$

Select the correct answer from A, B, C and D. 

Compound responsible for brown ring.

Nitrite $\left({\mathrm{NO}}_2^{-}\right)$ interferes in the ring-test of nitrate $\left({\mathrm{NO}}_3^{-}\right)$. Some following reagents can be used for the removal of nitrite:

I : Potassium dichromate (s)
II : $\mathrm{CO}{\left({\mathrm{NH}}_2\right)}_2$ (urea)
III : ${\mathrm{NH}}_2{\mathrm{SO}}_3\mathrm{H}$ (sulphamic acid)

IV: Zinc/sodium hydroxide

Salt $\mathrm{C}$ undergoes $\left(\mathrm{c}\right)$ reaction. Identify the anion present in this salt on the basis of this reaction.

When a freshly prepared solution of ferrous sulphate is added to a solution of $\mathrm{C}$ and concentrated sulphuric acid is gently poured from the side of the test-tube, a brown ring is formed.