Question

If a solution changes the color of the litmus from red to blue. What can you say about its pH?

Answer 1

The pH scale

• The pH scale is a simple way to represent the acidity and alkalinity of aqueous solutions.
• The hydrogen ion $\left[{\mathrm{H}}^{+}\right]$ index is referred to as pH. As a result, acid strength is determined by the concentration of hydrogen ions.
• pH is nothing but the negative logarithm (to the base $10$) of the hydrogen ion concentration, given in moles per liter.

$\mathrm{pH}=-{\log }_{10}\left[{\mathrm{H}}^{+}\right]$

• The condition of pure water in alkalinity, i.e., the condition of exact neutrality, is represented by the number $7$ on the pH scale.
• The number of hydrogen ion $\left({\mathrm{H}}^{+}\right)$ and hydroxide ion $\left({\mathrm{OH}}^{-}\right)$ in pure water is equal.
• Here, the temperature condition is $25°\mathrm{C}$.

${\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left[\mathrm{at}25°\mathrm{C}\right]$

• The pH levels below $7$ indicate acidity, whereas pH values over $7$ indicate basicity.

Litmus changes red to blue:

• Litmus is used to check the chemical's nature, whether it is acidic or basic.
• The change in the litmus paper color from red to blue indicates that the solution is basic in nature.

The pH of the solution:

• The pH range for the alkaline solution is $7-14$ (at $25°\mathrm{C}$).
• The pH value of the solution is greater than $7$ (at $25°\mathrm{C}$).