Question

Among the following, the total number of compounds whose aqueous solution turns red litmus paper blue is :
$KCN,{{K}_{2}}S{{O}_{4}},{{(N{{H}_{4}})}_{2}}{{C}_{2}}{{O}_{4}},NaCl,Zn{{(N{{O}_{3}})}_{2}},FeC{{l}_{3}},{{K}_{2}}C{{O}_{3}},N{{H}_{4}}N{{O}_{3}},LiCl$
(A)- 0
(B)- 1
(C)- 2
(D)- 3
(E)- 4
(F)- 5
(G)- 6
(H)- 7
(I)- 8
(J)- 9

Answer 1

Blue litmus paper under acidic solutions turns blue indicating the solution as acidic. Red litmus paper turns blue under basic solutions indicating the solution as basic. In neutral solutions, both red litmus and blue litmus remain unchanged indicating the solution as neutral.

Complete step by step answer:
-Litmus is a water-soluble mixture which is a mixture of different dyes extracted from lichens and after being absorbed onto filter paper is used as a pH indicator.
-Litmus paper is used to test whether a solution is acidic, basic or neutral. There are two types of litmus paper available- blue litmus and red litmus.
-Litmus paper sometimes causes colour change other than acid-base chemical reactions. For example, chlorine gas when subjected to blue litmus paper turns white. White litmus paper conversion indicates that litmus dye is bleached and this happens due to the irreversible reaction producing hypochlorite ions.

-Let us now understand the mechanism of how litmus paper works.
-Red litmus paper contains a weak diprotic acid which when exposed to a basic compound reacts with hydrogen ions present in the added base. A conjugate base is formed from such a reaction which has a blue colour, hence the wet red litmus paper turns blue in alkaline solutions.
-According to the question we are asked to select the compound which will turn red litmus paper blue. We already know that red litmus turns blue in two conditions, first when the solution is basic and second when the solution is a combination of salt of a weak acid and strong base.
-So we need to isolate the compounds from the list which falls in the given category. We will get,
$KCN,{{K}_{2}}S{{O}_{4}},LiCN$
So, the correct answer is “Option D”.

Note: Despite various chemical solutions which are used as pH indicators, several natural pH indicators are also available. Some of the natural pH indicators are given below-
(i) Beets- A solution of high pH will change the colour of beets or beet juice from red to purple.
(ii) Blueberries- In acidic conditions and basic conditions, blueberries, black currants and black raspberries change from red to blue or violet simultaneously.
(iii) Cherries- In an acidic solution, cherries and their juices are red and turn blue or purple in a basic solution.
(iv) Curry powder- Curcumin pigment present in curry leaves will change from yellow at pH 7.4 and pH 8.6.
(v) Onion- Onions are olfactory indicators, that is you will not smell onions in strongly basic solutions. Also, in acidic conditions red onion changes from pale red and basic conditions, red onion changes to a basic solution.
(vi) Baking soda – It will freeze in an acidic solution and will not freeze in alkaline conditions, hence differentiating alkaline and acidic solutions.