Question

Which of the following will turn blue litmus solution red?
[A] $C{{H}_{3}}OH$
[B] $C{{H}_{3}}COOH$
[C] $C{{H}_{3}}COOC{{H}_{3}}$
[D] $C{{H}_{4}}$

Answer 1

HINT: Blue litmus will turn red in presence of an acidic solution.Here, thus, the compound which is acidic in nature and has pH less than 7 will give this test.. To check if it's acid or not, see the ions it will dissociate in a solution.


COMPLETE STEP BY STEP SOLUTION: Litmus is a water soluble mixture of different dyes that we extract from lichens. This is absorbed in the filter paper and we use it as a pH indicator.
This is the main function of litmus i.e. to determine whether the solution is acidic or basic.
A blue litmus paper will turn red in an acidic solution but remain blue in a basic solution.
A red litmus paper changes colour in a basic solution and becomes blue whereas remains unchanged in an acidic solution.
In a neutral solution, the paper is purple.
We know that for a basic solution pH is greater than 7 and for an acidic solution the pH is less than 7. Thus, using litmus paper we can estimate the pH of a solution as it changes colour in acidic medium.
We can also make a litmus solution which is red in acidic condition and blue in basic condition.
In the question it is asked which of the following will turn blue litmus solution red. We can understand from the above discussion that an acid will turn blue litmus red.
So let us identify which among the given options is an acid.
In the first option we have $C{{H}_{3}}OH$. It will not release ${{H}^{+}}$ ions in solution. Therefore, this is not an acid.
In the next option we have $C{{H}_{3}}COOH$. We know that this is acetic acid which is a weak acid but it will dissociate ${{H}^{+}}$ ions in solution which means it is an acid and thus will turn blue litmus red.
Then we have $C{{H}_{3}}COOC{{H}_{3}}$which is methyl acetate, a carboxylate ester. It will not release ${{H}^{+}}$ ions as it is covalently bonded. Therefore, it will not turn blue litmus red.
And lastly, we have $C{{H}_{4}}$. Here also the hydrogen atoms are covalently bonded thus will not dissociate in solution.
We can see from the above discussion that $C{{H}_{3}}COOH$ is an acid and thus will turn blue litmus red.


Therefore, the correct answer is option [B] $C{{H}_{3}}COOH$


NOTE: Generally a litmus paper is used as an indicator for chemical reactions. However, it also causes a colour change even though the chemical reaction is not an acid-base reaction. Chlorine gas turns a filter paper white as it bleaches the paper due to presence of hypochlorite ions. In this case, litmus paper is not acting as an indicator.