system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do pH indicators work?

How do pH indicators, for example hydrogencarbonate indicator, work? What determines the colour they change to?
gpt
PH is one measure of acidity based on hydrogen ion concentration in a solution.

Some kinds of dye molecules gain a hydrogen ion when the acidity increases, and this changes the shape of the molecule. 

When the shape changes, the color of the dye molecules changes. 

The color is determined by the shape of the molecule.