Question

A cloth strip dipped in onion juice is used for testing a liquid ‘X’. The liquid ‘X’ changes its odour. Which type of an indicator is onion juice? The liquid ‘X’ turns blue litmus red. List the observations the liquid ‘X’ will show on reacting with the following:
(a) Zinc granules
(b) Solid sodium carbonate
Write the chemical equations for the reaction involved.

Answer 1

The answer to this question is based on the general concepts of acidity and basicity where the onion juice solution turns blue litmus into red and this fact indicates that it is an acid. Write the reactions of acid with the zinc granules and salt.

Complete answer:
We have studied in our lower classes about the litmus paper and the experiment to find out about the acidity and basicity of the solutions and also are familiar with the reactions of acid with metal and also salt.
Let us now see what the reaction taking place is and also what type of indicator is the lemon juice.
Since in the above question, they have provided the data that the liquid ‘X’ changes its odour when a cloth strip is dipped in onion juice, this means that the change in the odour represents the acidic nature of the substance and the indicator it acts as will be called an olfactory indicator.
- Thus, onion juice is called an olfactory indicator.
- Also, since the liquid ‘X’ turns blue litmus to red, the liquid is nothing but an acid because it changes the blue colour of litmus to red.
(a) Now, when this acid reacts with metal, it produces salt with the evolution of hydrogen gas. Here, the zinc granules (metal) react with an acid producing salt and hydrogen gas. As we do not know which acid, we write it in the general form and the reaction is as shown below,
$Acid+Zn\to Salt+{{H}_{2}}(g)$
(b) When the liquid ‘X’ which is an acid when treated with solid sodium carbonate then the products formed will be salt, water along with the release of carbon dioxide gas. The reaction in general is,
$Acid+N{{a}_{2}}C{{O}_{3}}(s)\to Salt+{{H}_{2}}O(l)+C{{O}_{2}}(g)$

Note:
Note that the onions are acidic in nature and the pH of these solutions range from 5.6 to 6.0 which depends upon the type of onions available in nature that is red, white, ripe etc.