Question

The following table lists the pH values of some acids and bases.

Solutions	pH
Hydrochloric acid	1
Milk	6
Pure water	7
Baking soda	9
Sodium hydroxide	14

What would happen to the pH of an acid and a base when each is diluted (pure distilled water is added to it)?
A.On dilution, the pH of an acid and base will both decrease
B.On dilution, the pH of an acid will increase and that of a base will decrease
C.On dilution, the pH of an acid and base will both increase
D.On dilution, the pH of an acid will decrease and that of a base will increase

Answer 1

We know that pH of a solution is the measure of the hydrogen or hydronium ion concentration of the solution. The pH is the negative logarithm of the concentration of hydrogen ion, that is, ${\rm{pH}} = - \log \left[ {{{\rm{H}}^ + }} \right]$. It means more the hydrogen ions in the solutions, less the value of pH is.

Complete step by step answer: We know that,

1. Acidic solution has higher concentration of hydrogen ion than hydroxide ion
2. Basic solution has concentration of hydroxide ions than hydrogen ion
3. Neutral solution has equal concentration of hydrogen and hydroxide ions.

Now, when an acidic solution is diluted by distilled water, the hydrogen ion concentration decreases in the solution. And we know, the pH is the negative logarithm of the concentration of hydrogen ions. So, due to decrease of concentration of hydrogen, pH of the solution increases. So, for acidic solutions like hydrochloric acid (pH=1), milk (pH=7), pH increases.

Next we have to find out if there is an increase or decrease of pH of a basic solution on dilution. When we dilute a basic solution by distilled water the hydroxide ion concentration decreases. This results, pH of the basic solution falling towards 7 and making it less alkaline. So, we can say that dilution decreases the pH of the basic solution. In case of basic solutions like baking soda (pH=9) and sodium hydroxide (pH=11), Ph decreases.

So, we get to know that on dilution pH of acidic solution increases and pH of basic solution decreases.

Hence, the correct answer is option B.

Note: Always remember that an acid loses one proton to form the conjugate base and the base accepts the proton and forms the conjugate acid. There is the difference of one proton in a conjugate acid-base pair. The strength of acid-base can be determined from the conjugate acid base pair. The more the base is strong, its conjugate acid is weak and stronger the acid, weaker its conjugate base.