Question

(a) What is a universal indicator? For what purpose is it used?
(b) How does a universal indicator work?
(c) Water is a neutral substance. What colour will you get when you add a few drops of universal indicator to a test-tube containing water?

Answer 1

A substance will be known as an acid if it gives out hydrogen ions that is ${H^ + }$ ions when dissolved in water. A substance will be known as a base if it gives out hydroxide ions that is $O{H^ - }$ ions when dissolved in water. In order to find whether a solution is acidic or basic, indicators are used. Universal refers to the fact that it can predict the pH value qualitatively to a certain extent.

Complete answer:
Indicator is a substance which changes its colour when it comes in contact with an acid or a base. An acidic substance is a substance which includes acid within itself. A basic substance is a substance which includes base within itself. An indicator is used to differentiate between an acidic substance and a basic substance. There is a range of different indicators. Among all, the common indicators are as follows; litmus, china rose, turmeric and phenolphthalein.
(a) A mixture of various indicators that changes its colour at different pH values of the whole scale, is known as a universal indicator. Usually, the common indicators do not have the ability to tell us the strength of the solution that is how weak or strong the acid is. For this purpose, a universal indicator is used that is to measure the strength of an acidic or basic solution.
(b) The pH scale starts from 0 and ends at 14, 0 indicating most acidic and 14 indicating most basic. The pH value of 7 indicates neutrality. The universal indicator can be used as universal indicator paper or solution. So, when the universal indicator comes in contact with a solution, it changes its colour according to the pH value of the solution. There is a particular colour for each pH value in the pH chart. We can tell whether the solution is a strong or weak acid or whether the solution is a strong or weak base by checking the pH value of the appeared colour in the pH chart.
(c) As water is a neutral substance, the solution of universal indicator and water will get a green colour. At pH value 7, universal indicator forms a green coloured solution.

Note:
Indicators are subdivided into two types which are as follows; natural indicators and synthetic indicators. The examples of natural indicators are litmus, china rose and turmeric. The example of a synthetic indicator is phenolphthalein.
At pH value 0, universal indicator solution shows dark red colour and at pH value 14, it shows violet colour.