How do pH indicators work?

Question

How do pH indicators, for example hydrogencarbonate indicator, work? What determines the colour they change to?

In: 21

Answer 1

For simplicity (not scientifically accurate, but easy to understand), a molecules color depends on its size.

Now if you add an acid (or H3O+ Ions), it can take something away from the molecules and make them smaller, therefore changing the color.

Or if you add a base (or OH- ions), it can add some of itself to the molecules, making them larger, also changing the color but in a different way