Question

Which of the following is an indicator that does not change color to determine the nature of acid or base ?
(A). Olfactory
(B). Natural
(C). universal
(D). methyl orange

Answer 1

Indicators are the substance that indicate the change in color or size due to the change in the pH of the solution. These are called acid base indicators. The indicator which does not show change in color with change in pH is our required answer.

Complete step by step answer:
Now, there are various categories of indicators:
1. Natural indicators
2. Universal indicators
3. Olfactory indicators
$\to$ Natural Indicators :- These are the indicators that are obtained from the plants that are from nature itself.
Example : turmeric, china rose, red cabbage.
They change color when they come in contact with an acid or base.
$\to$Universal Indicator :- universal indicators are the mixture of always that change color gradually or a variation in the pH of the solution.
For example : Phenolphthalein, methyl orange, thymol blue then , pH paper it is also a universal indicator.
$\to$ Olfactory Indicators :- Olfactory indicators are the substances whose smell varies depending upon whether it is mixed with acid or mixed with a base.
For example : onion, clove oil and vanilla extract.
For these indicators , the smell change plays an important role in identification of an acid or base not the color.
So, option A that is, olfactory indicators is the correct one.

Note: The best known pH indicator is known to be litmus paper or litmus solution. It is the universal indicator for identifying acid-base solutions. Phenol red, methyl orange these can also be used as acid- base indicators commonly. Most of the indicators be it natural, universal all of them change color when they are subjected to a change in their pH. But, olfactory indicators are the one that change smell when comes in contact with acid or base. Onions are the natural olfactory indicator, it becomes odorless when it comes in contact with the strongly basic solutions.