Why can an indicator change color with pH?

An indicator liquid can change color because its molecules shift between related chemical forms as their surroundings change. An acid form can release a proton, a positively charged hydrogen nucleus, to become a base form. In this fictional indicator, HIn names the red acid form and In− the blue base form.

pH indicates how acidic or basic a solution is. Lower pH corresponds to greater hydrogen-ion activity, meaning more availability for proton-transfer interactions. An indicator's forms continually interconvert. In more acidic surroundings the proton-bearing acid form is favored; in less acidic surroundings the form that has lost that proton becomes more prominent. The indicator therefore samples its chemical surroundings rather than merely receiving a new colored label.

The initial pH is 7, where the toy has equal proportions of the two forms and a purple appearance. Its stated key associates values below 6 with red and values above 8 with blue. The enlarged circles and squares represent the form balance rather than actual visible particles.

Gaining or losing a proton changes the forms' electronic structures and their interactions with visible light. Their proportions determine the absorbed light and the color reaching the eye. Each indicator has its own transition range: this fictional red–purple–blue key makes purple a balance near its midpoint under the stated conditions, rather than an identification of the liquid or an exact pH measurement.