Why do acids change blue litmus to red?

Acids change blue litmus paper to red because of a fundamental property of acids known as their ability to donate protons (H⁺ ions). Litmus paper is an indicator that changes color depending on the pH (acidity or alkalinity) of a substance.

Litmus paper is made from a natural dye extracted from lichens. In its natural state, the litmus dye is purple. However, when the litmus paper is soaked in an acidic solution, the protons (H⁺ ions) from the acid react with the dye molecules, causing a shift in the electron distribution and resulting in a change in the color of the litmus paper from purple to red.

The litmus dye molecule is an example of a weak acid-base indicator. It can exist in two forms: the protonated form (HIn) and the deprotonated form (In⁻). In an acidic solution, there is an excess of H⁺ ions, and they react with the HIn form of the litmus dye, causing a shift in equilibrium towards the protonated form. This protonated form has a different color, which is red. Hence, when the litmus paper comes into contact with an acidic solution, it turns red.

It's worth noting that litmus paper is just one type of indicator, and there are other indicators available that change color in response to different pH ranges or in the presence of specific acids or bases.