Question

Water is a neutral substance. What colour will you get when you add a few drops of universal indicator to a test tube containing distilled water?

Answer 1

Universal indicator is a pH indicator that exhibits smooth colour change over a wide range of values of pH to indicate the nature of the solution (acidic or alkaline). Distilled water is purified water free from impurities and ions.

Complete step by step answer:
A universal indicator is made up of a variety of indicators that work over a small pH range. As a result, it can indicate pH over a wide range. Generally the components of a universal indicator are water, sodium salt of phenolphthalein, propanol, sodium hydroxide, methyl red, monosodium salt of thymol blue and mono sodium salt of bromothymol blue.
The colors shown by universal indicator by the combined effect of all of its constituent indicators is as follows,
For a pH < 3, that is a strong acid, the colour indicated is red
For pH range of 3 to 6, that is mild acid, the colour is orange or yellow
For pH =7, that is a neutral solution, the colour indicated is green
For pH range of 8 to 11 that is mild alkali, the colour is blue
And for pH > 11, that is strong alkali, the colour is violet.
We know that distilled water is free from impurities and is completely neutral. Thus in addition to universal indicators to it, green colour is obtained.

Note:
Thymol blue changes its colour from red to yellow in the pH range of $1.2{\text{ to }}2.8$
Methyl orange changes its colour from orange to yellow in the pH range of $3.2{\text{ to }}4.4$
Methyl red changes its colour from red to yellow in the pH range of $4.8{\text{ to }}6.0$
Bromothymol blue changes its colour from yellow to blue in the pH range of $6.0{\text{ to }}7.6$
Thymol blue changes its colour from yellow to blue in the pH range of $8.0{\text{ to }}9.6$
Phenolphthalein changes from colorless to pink in the pH range of $8.3{\text{ to }}10$