Question

The pH value of a sample of hydrochloric acid is 2. pH value of this sample when diluted by adding water will be:
(A) Less than 2 but more than 0
(B) More than 2 but less than 7
(C) More than 7
(D) No change in pH.

Answer 1

Hint: Water is less acidic than hydrochloric acid. Water consists of hydrogen ions and hydroxide ions in the ratio of 1:1. pH is the quantitative measurement of these ions in the solution.

Complete step by step answer:
pH is the measure of acidity and basicity of an aqueous solution. pH is used to measure the number of free hydrogen ions in the water and pOH is the measure of free number of hydroxide ions in water. Water having more hydrogen ions are acidic while water having more hydroxide ions are basic.
Acid when dissolved in water releases H+ ions.
$HCl + {H_2}O \to {H_3}{O^ + } + C{l^ - }$
pH is the negative logarithm of concentration of protons in the solution.
\[{\text{pH = - lo}}{{\text{g}}_{10}}{\text{[}}{{\text{H}}^{\text{ + }}}{\text{]}}\]

The pH value ranges from 0 to 14. So accordingly, pH + pOH = 14. pH value equal to 7 is considered as neutral. While pH value less than 7 indicates acidic solution and pH value more than 7 indicates basic solutions. pH can be in the laboratory by using universal indicators which show different colors for different pH ranges. To measure the precise value of pH one can use a pH meter.

When water is added to acid or base the pH of the solution gets changed. When an acidic solution is diluted with water, the number of ${{\text{H}}^{\text{ + }}}$ ions decreases. The pH of strong acid then increases towards 7. Similarly, when water is added to an alkali, the number of ${\text{O}}{{\text{H}}^{\text{ - }}}$ ions decreases. In this case, the pH of the alkali decreases towards 7.
The pH of the hydrochloric acid is 2 according to the given data which indicates that it is acidic in nature. On dilution that is when we add water to the solution the pH will increase till 7. The pH will be less than 7 as it contains hydrogen ions more as compared to hydroxide ions.
So, the correct answer is “Option B”.

Additional Information:
Rain water has the pH value of 5.6 which indicates slightly acidic due to the presence of ${\text{C}}{{\text{O}}_{\text{2}}}$ in the atmosphere.
Pollution can change pH of water which affects the ecosystem.

Note: pH is inversely related to acidity of the solution. Low value of pH indicates more acidic solution. Dilution of solution indicates addition of water to the solution. Acidity and basicity can be checked in the laboratory by using litmus paper. Litmus paper turns red indicating that the solution is acidic and when it turns blue it indicates that the solution is basic. With this method we get the rough idea whether the solution is acidic or basic.