What makes acids change the colour of litmus paper?
The litmus paper is an acid-base indicator, and the change in color is attributed to the formation of a compound that is dependent on the number of H+ (which predicts the equilibrium shift in Bronsted acid-base reactions). For example, HA and A- are the compounds in your litmus paper existing in equilibrium (HA is red, and A- is blue) HA <---------> [H+] + [A-] Once you put acid, the acid will introduce an excess of H+(since it donates a proton, once it is in a water solution), bringing an excess of H+ and will shift the reaction to the left, producing more HA and turning the color red.