Question

Substances that are used to determine whether a particular substance is an acid or a base are called _______.

Answer 1

Hint: Acid are those substances which release hydrogen ion in presence of water while bases are those substances which release hydroxide ion in a water mixture. Acids turn blue litmus red while bases turn red litmus into red to indicate their existence.

Complete answer:
Initially it was believed that acids are those substances that release hydrogen positive ions when they are put in water solution. Bases are those substances that release hydroxide ion when they are put in some water solution.
But later on, experimentation there were many solutions which were acidic but had no hydrogen ion in it, so how were they supposed to be categorised as acids. So, acids were categorised as those substances that on being added in water released hydrogen ion irrespective of where it came from acid or water.
In bases also, there were many bases that did not have any hydroxide ion in it. So, bases were defined as those things that dissolve in water released hydroxide ions, irrespective of where it came from, base or water.
To determine whether a solution is acidic or basic what we can do is have a solution which is a weak base or acid which doesn’t release its hydrogen and hydroxide ions so readily.
These weak acid or weak bases are called indicators.
Substances that are used to determine whether a particular substance is an acid or a base are called indicators.

Note:
Indicators are weak acid and weak bases. An indicator works as a dye. There are few indicators that give the indication at the middle point while some at the end point only. Some of the examples of indicators are methyl orange, bromothymol blue.