Question

Five solutions A, B, C, D and E when tested with universal indicators showed pH as 4,1,11,7 and 9 respectively. Which solution is:
(A)- strongly alkaline?
(B)- strongly acidic?
(C)- weakly acidic?
(D)- weakly alkaline?

Answer 1

pH is a measure of how acidic or basic any solution is. The range of pH goes from 0 to 14, with 7 being neutral i.e. neither acidic nor basic. pH of water can be calculated by measuring the relative amount of free hydrogen and hydroxyl ions in the water.

Complete step by step answer:
As we can see in the question that the pH was tested by a universal indicator. So, before finding the answer, first we have to understand what a universal indicator is and how it works.
A universal indicator is an indicator used to measure pH of a solution. It is made up of a solution of several compounds that exhibits several colors which change over a wide range of pH values to indicate the acidity or alkalinity of any given solution. A common mixture includes methyl red, bromothymol blue, thymol blue and phenolphthalein.
Different colors show different pH ranges. The colors that indicate the pH of a given solution, after adding a universal indicator are:
Red 0 ≥ pH ≥ 3
Yellow 3 ≥ pH ≥ 6
Green pH = 7
Blue 8 ≥ pH ≥ 11
Purple 11 ≥ pH ≥ 14
Now, since we know about the universal indicator, let’s get back to our question.
As we know, if the pH ranges from 0 to 7, it indicates that the solution is acidic whereas if the pH ranges from 7 to 14 then it is alkaline or basic. Also, pH is related to concentration of [H+] by the relation:
$pH{\text{ }} = {\text{ }} - {\text{ }}log{\text{ }}\left( {\left[ {{H^ + }} \right)} \right]$
This shows that higher the value of pH, lower its acidity and higher its alkalinity will be.
So, by the help of the above discussion, we can easily find the answers of each part so let’s begin.
(A)- strongly alkaline: C
(B)- strongly acidic: B
(C)- weakly acidic: A
(D)- weakly alkaline: E

Note: Remember, no matter how weakly acidic something is, it’s value can never exceed 7. Similarly, no matter how weakly alkaline something is, it’s value can never be decreased from 7.