Question

When a solution is added to a cloth strip treated with onion extract, then the smell of onion cannot be detected. State whether the given solution contains an acid or a base.

Answer 1

Hint: To understand the question, we must have the understanding about the uses and characteristics, and components of onion extract. The main chemical of onion extract which is responsible for the smell is propanethiol. Hence, we need to understand the reactions of propanethiol with acid and base, to check in which reaction the smell goes away.

Complete answer:
Here, we are given a cloth which is dipped in onion extract. We know that onions has an inherent unique smell. Now, when the cloth was dipped in a solution, due to some reaction the smell of onion extract was nullified. Now, we know that onion possesses a chemical compound known as the propanethiol with the chemical formula $(CH_3CH_2CH_2SH)$ which is responsible for the inherent smell of the onion. Now, we will check the reaction of propanethiol with an acid and a base. Let us consider the reaction of propanethiol with sodium hydroxide $(NaOH)$ .

The reaction is to be observed and we get the products as
$C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}SH+NaOH\to C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}SNa+{{H}_{2}}O \\ $
From the above reaction, the product obtained $(C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}SNa)$ is known as mercaptan, which will be an odourless compound. Hence, we can say that by the reaction of propanethiol, which is responsible for the smell of onion with sodium hydroxide, which acts as a base, the smell of onion gets nullified. When we consider the reaction of propanethiol and any acid, no such results will be obtained.

Hence, as we are given that the cloth treated in onion extract is dipped in solution, the smell goes away. Hence, we can say that the solution will act as a base.

Additional Information: Due to this discussed property, onion extract is used as an olfactory indicator, to check whether an unknown solution acts as a base or acid. If the solution is base, the smell of onion will go away, while if the solution is acidic, the smell will remain as it is. This experiment can be used for people with visual disabilities.

Note: We can note here that onions have a $pH\;$ in the range of $5.6\;$ to $\;6.0$ depending upon the type of the onion. Hence, we can infer from this that as onions are acidic, they will mostly react with base. Hence, the given solution should be base. We should remember this test as a fact that if the smell of onion extract goes away on reaction with some solution, then that solution should be base.