How do pH indicators, for example hydrogencarbonate indicator, work? What determines the colour they change to?
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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.
Certain chemicals, like bromothymol blue, are one color when protonated (acidified) and a different color in basic solution.
You are seeing yellow when all molecules are acidifed, blue when alkylated, or a mixture of yellow and blue, which appears green, at neutral pH.
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
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