Question

A solution has a pH of 7. Explain how would you:
Increase its pH
Decrease its pH

Answer 1

A solution has a pH of 7 which means it is neutral. To decrease its pH value we need to add an acidic substance to it for example lemon juice. To increase its pH value we need to add an alkaline substance to it for example baking powder.

Complete step by step answer:
Any solution with a pH value 7 is neutral because we have equal concentration of ${{H}^{+}}$ ions and $O{{H}^{-}}$ ions. We can increase its pH value by adding small amount of base like sodium hydroxide (NaOH) that is the concentration contains more amount of $O{{H}^{-}}$ ions then ${{H}^{+}}$ ions. Basic solutions have a pH more than 7. Similarly, we can decrease pH value by adding small amount of acid like hydrochloric acid (HCl) that is the solution contains more amount of ${{H}^{+}}$ ions than $O{{H}^{-}}$ ions. Acidic solutions have pH values less than 7.
We can test whether the solution is acidic or basic with the help of litmus solution or litmus paper. The change in colour of litmus paper from red to blue shows that the solution is of basic nature with pH value more than 7. The change in colour of litmus paper from Blue to Red shows that the solution has a pH value less than 7 which indicates that the solution is acidic in nature.

Note: pH can be defined as the negative logarithm to the base 10 of the H+ ion concentration present in the solution. Mathematically, $pH\text{ }=-\text{ }log{{\left[ H \right]}^{+}}$. Also, rain water is never neutral, in fact it is slightly acidic. Rain water generally has a pH of 5.0 to 5.5.