Final answer:
An acid-base indicator represented by HIn ⇌ H+(aq) + In-(aq) changes color to indicate pH levels, with the transition guided by the Henderson-Hasselbalch equation and the indicator's pH titration characteristics.
Step-by-step explanation:
The substance represented by the chemical equilibrium HIn ⇌ H+(aq) + In-(aq) is an acid-base indicator, which is a type of weak acid or weak base used to determine the pH of a solution by changing color. Acid-base indicators undergo dissociation in water resulting in a protonated form (HIn) and a conjugate base (In-). The color change of the indicator solution occurs over a pH range and is dependent on the Henderson-Hasselbalch equation, which relates the pH of the solution to the ratio of the concentrations of the protonated form [HIn] and the deprotonated form [In-]. For example, methyl orange transitions from red in acidic environments to yellow in basic environments, indicating the pH level where this change occurs.
An indicator's pH titration range typically spans about two pH units, during which the color transition occurs gradually as the ratio of the indicator's forms changes. The hydronium ion concentration is key to this transition, which leads to a perceptible color shift in response to the surrounding pH level.