Question

Each colour is associated with a particular pH in:
A. Litmus paper
B. Universal indicator
C. Methyl orange indicator
D. Phenolphthalein indicators

Answer 1

Hint:We have studied that the nature of a solution whether neutral, acidic or basic is expressed in terms of ${H_3}{O^ + }$. Another method for expressing the concentration of ${H_3}{O^ + }$is the pH of a solution which is defined as the negative logarithm (base 10) of its hydronium ion concentration.

Complete answer:
We know that pH of a solution is the negative logarithm (base 10) of its hydronium ion concentration or we can also define it as the logarithm of the reciprocal hydrogen ion concentration. pH scale is the range of expressing the nature of any solution. A solution whether acidic or basic can be measured using this pH scale and observing the change in colour. Let us discuss each option, So firstly the Litmus paper, it is basically a filter paper developed by the treating with a natural dye which is water soluble and obtained from Lichens. When the solution is acidic, a blue litmus paper will turn red in colour and when the solution is basic, the red litmus paper turns blue. The neutral litmus paper is generally purple. Thus, we came to know that litmus paper has these three primary colours.
Now comes the Methyl orange indicator, it is again a pH indicator and majorly used in titration experiments as it gives a clear and distinct colour at different values of pH. In the acidic medium, it turns red and in the basic medium it turns yellow. Hence, it also has a limited colour range.
Phenolphthalein is an organic compound belonging to the family phthalein which is an acid-base indicator. Below pH 8.5, it is colourless and when the pH is above 9.0 it shows a pink to deep red colour change.
Lastly, the Universal indicator, it is also a pH indicator and mainly composed of water, 1-propanol, phenolphthalein, sodium salt, NaOH, methyl red, thymol blue and bromothymol blue monosodium salt. When the pH range is less than 3, it shows red colour and indicates the medium as acidic, when the pH is between 3 to 6 it becomes orange or yellow showing a weak acid. When the colour is green, pH is 7 and medium is neutral, when medium is weakly basic then it shows blue colour and the pH range is 8 to 11 and when the pH is above 11, it represents the medium as a strong alkali and the colour shown is violet or indigo.

Thus, the correct answer is (B).

Note:
A universal indicator can be present in paper form where a strip of coloured paper is given which changes the colour according to the medium it is dipped in and the solution form as a mixture of components discussed above.