Question

Why do acids change blue litmus to red?

Answer 1

Litmus is a water-soluble mixture of different dyes extracted from lichens, especially Roccella tinctoria. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity.
Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic (i.e. alkaline) conditions.

The pigment in blue litmus reacts with `H^+` ions and changes chemically so the bonds are 'tuned' to reflect a longer wavelength of light to appear red to our eyes. The pigment is absorbing the blue to green wavelengths when it reacts with the free hydrogen ions in acids,

Otherwise the same pigment in base reacts differently with O`H^-` ions and becomes 'tuned' to reflect the short wavelength to appear blue. Now it absorbs the green and red wavelengths having reacted with the hydroxyl ions in the base.