Question

Any substance that does not change the colour of an indicator is called _____substance.
(A) Acidic
(B) Neutral
(C) Basic
(D) Dilute

Answer 1

Indicator is substances that change colour when they are added to acidic or alkaline solutions. pH indicators exist as liquid dyes and dye infused paper strips which are added to the various solutions to determine the pH value of those solutions.

Complete step by step solution:
A pH value is determined from the negative logarithm of this concentration and is used to indicate the acidic, basic, or neutral character of the substance you are testing.
Recall that the value of pH is related to the concentration of ${H^ + }$ ( ${H_3}{O^ + }$ ) of a substance. pH itself is approximated as the co logarithmic or negative logarithm of the H+ ion concentration.
pH $\approx$ -log [ ${H_3}{O^ + }$ ]
A pH of 7 indicates a neutral solution like water. A pH of less than 7 indicates an acidic solution and a pH greater than 7 indicates a basic solution. Ultimately the pH value indicates how much H+ has disassociated from molecules, certain a solution.
Litmus indicates solution turns red in acidic solution, blue in alkaline solutions and purple in neutral solution.
The colour change of pH indicator is caused by the dissociation of the H+ ion from the indicator itself. The equation for the dissociation of the H+ ion of the pH indicator is shown below-
$HIn + {H_2}O \rightleftharpoons {H_3}{O^ + } + I{n^ - }$
$HIn \to$ Acidic pH indicator.
$I{n^ - }$ $\to$ Conjugate base of the pH indicator.
Hence, the correct option is (B) that is neutral.

Note:
Equation expressed above is in equilibrium meaning Le Chateliar’s principle applies to it. Thus as the concentration of ${H_3}{O^ + }({H^ + })$ increases or decreases, the equilibrium shifts to the left or right accordingly. An inc in the $HIn$ acid concentration causes the equilibrium to shift to the right.