Question

Four students were given colourless liquids A, B, C of water, lemon juice and mixture of water and lemon juice respectively. After testing these liquids with pH paper, following sequences in colour change of pH paper were reported.
I. Blue, red and green
II. Orange green and green
III. Purple, red and green
IV. Green, red and red

- The correct sequence observed is:
A) I
B) III
C) II
D) IV

Answer 1

Hint: The answer to this question is based on the pH scale wherein for a particular number of pH, a particular colour is obtained on pH paper and this concept leads you to the correct answer.

Complete Solution :
We have studied in our previous classes of chemistry which includes the concept of pH and its variation.
We shall now see in detail about the pH and its changes in colour with different solutions.
- pH is the one we all know and is defined as the negative logarithm of the hydrogen ion concentration in the solution.
- pH is different for different solutions. For acidic solutions the pH value is low and for basic solutions, the pH value is high.
- pH for acidic solutions is lesser than 7 and for basic solutions it is more than 7. pH = 7 indicates that the solution is neutral.
In the above question it is given that 4 students were given three solutions : A = water, B = lemon juice and C = mixture of water and lemon juice.

Now, since water is neutral, the pH colour for the neutral solution in pH paper is green and lemon juice is acidic in nature and thus the pH paper colour for this acidic solution is depending on the strength of acids where for the strong acids it is red in colour.
Among the options given, the first one should be green and that is in series number IV.
Also a solution of mixture of water and acid that is lemon juice, also produces an acidic solution.
So, the correct answer is “Option D”.

Note: Note that pH is an important quantity that reflects the conditions of the chemical solution and this can control the availability of the nutrients, microbial activity, some biological functions and also the behaviour of chemicals.