Question

Four samples of arhar dal were taken in four test tubes with some water in each and labelled ${\text{P}}$, \[{\text{Q}}\], ${\text{R}}$ and ${\text{S}}$. A few drops of the following were added to these test tubes, water to test tube ${\text{P}}$, ${\text{HCl}}$ to test tube \[{\text{Q}}\], ${\text{NaOH}}$ to test tube ${\text{R}}$ and alcohol to test tube ${\text{S}}$.
We would be able to confirm adulteration of the dal with metanil yellow in test tubes:
(A)- \[{\text{Q}}\]
(B)- \[{\text{Q}}\] and ${\text{R}}$
(C)- ${\text{R}}$ and ${\text{S}}$
(D)- ${\text{S}}$ and ${\text{P}}$

Answer 1

Hint: Adulteration means adding foreign substance to original or natural species, and we would be able to confirm adulteration only when the added substance shows any changes on adding other things like; water, acid, base, etc.

Complete step by step solution: Metanil yellow is also known as Acid yellow 36 which is a kind of dye that belongs from the azo family and used as an indicator in many reactions because it changes their color in an acidic environment. Its appearance is yellow in color that’s why it is used in the adulteration of arhar dal.
-When we take arhar dal on the test tube ${\text{P}}$ and add water on it then there is no change observed in the test tube.
-When we take arhar dal on test tube \[{\text{Q}}\] and add ${\text{HCl}}$on it then there we observe change in the test tube, because in the presence of acid solution gives pink color i.e. color of solution changes from yellow to pink.
-When we take arhar dal on the test tube ${\text{R}}$ and add ${\text{NaOH}}$ on it then there is no change observed in the test tube.
-When we take arhar dal on the test tube ${\text{S}}$ and add alcohol on it then there is no change observed in the test tube.
So, we would be able to confirm adulteration of the dal with metanil yellow in the test tube \[{\text{Q}}\] only.

Hence, option (A) is correct.

Note: Here you may think that why metanil yellow doesn’t show any changes on adding alcohol & ${\text{NaOH}}$on test tube, so the reason is that azo dye is already somewhat basic in nature that’s why they doesn’t show any changes with bases too.