Why does thymol blue change color at the pH that is does? I know it goes through transition one and transition two but I don't know why it changes at the pH that it does.

Indicators exist in a basic form and an acid form. They have different colors in the two forms.

Thymol blue is an indicator dye commonly used in chemistry to determine the pH of a solution by observing its color change. It belongs to a class of indicators known as thymol sulfonephthalein dyes.

Thymol blue undergoes two major color transitions, which are generally referred to as "transition one" and "transition two." Transition one occurs around a pH of 1.2–2.8, where the indicator shifts from red (acidic) to yellow (slightly acidic or neutral). Transition two takes place around a pH of 8.0–9.6, where the indicator changes from yellow (slightly acidic or neutral) to blue (basic).

The reason thymol blue changes color at specific pH ranges is due to its chemical structure and the behavior of different forms of the indicator molecule. Thymol blue exists in two main forms: the acidic form (HIn) and the basic form (In-). The ratio of these two forms is strongly dependent on the solution's pH.

In an acidic solution with a lower pH, the concentration of H+ ions is relatively high. These H+ ions interact with the indicator molecule, causing it to predominantly exist in the acidic form (HIn). The acidic form appears red.

As the pH of the solution increases, the concentration of H+ ions decreases, allowing the indicator molecule to shift towards the basic form (In-). This shift leads to the yellow color, which is characteristic of the indicator in a slightly acidic or neutral solution.

At a pH around 8.0–9.6, the solution becomes more basic, leading to a higher concentration of OH- ions. These OH- ions react with the indicator molecule, causing it to predominantly exist in the basic form (In-), resulting in a blue color.

The precise pH ranges for thymol blue's color transitions are determined by the specific chemical properties and equilibrium constants of its acidic and basic forms. These properties are influenced by factors such as the presence of sulfonate groups in the molecule and its ability to accept or release protons (H+ ions) depending on the pH of the environment.

Therefore, by observing the color change of thymol blue in a solution, one can estimate the pH value of that particular solution based on the known pH ranges of these color transitions.