"Why does the color change?"
Kana stared at the phenolphthalein solution. Colorless in acid, pink in base.
Toma answered. "Because the indicator's mood changes."
"Mood?"
Rei corrected. "The protonation state changes. More precisely, the molecular structure changes."
Kana tilted her head. "Proton?"
"H⁺ ion. Abundant in acidic solutions."
Toma took out another indicator. "BTB changes from yellow to blue."
"This one has a green moment," Kana pointed out.
"Near neutral. Yellow and blue forms coexist," Rei explained.
"Coexist?"
"Equilibrium state. Both forms exist simultaneously."
Kana wrote in her notebook. "So what determines the color?"
Rei drew a diagram. "Conjugated system. Alternating double bond structure."
"Conjugated?"
"Electrons are delocalized. Spread across the molecule."
Toma supplemented. "And those electrons absorb light."
"What color light?"
"Determined by conjugated system length. Longer systems absorb longer wavelengths."
Kana thought. "So when structure changes, the conjugated system changes?"
"Correct," Rei acknowledged. "When protons attach or detach, conjugated system length changes."
Toma demonstrated. "Add acid... look, it turned yellow."
"Protonated form," Rei explained. "Conjugated system shortens, absorbs blue light. Appears yellow."
"The opposite?"
"Add base, proton detaches. Conjugated system lengthens, absorbs yellow light. Appears blue."
Kana was impressed. "Color is proof of structure."
"Foundation of spectroscopy," Rei said. "From wavelength of absorbed light, we infer structure."
Toma brought a pH meter. "So what's the pH of the color change point?"
"Determined by the indicator's acid dissociation constant," Rei answered.
"Acid dissociation constant?"
"pKa. Represents strength of proton release."
Kana asked. "What's phenolphthalein's pKa?"
"About 9.3. So it changes color in alkaline."
Toma tried another indicator. "Methyl orange changes on the acidic side."
"Because pKa is low. About 3.7."
Kana organized. "So color changes around pKa?"
"Precisely in the range pKa±1. Derived from Henderson-Hasselbalch equation."
Rei wrote the equation. "pH = pKa + log([A⁻]/[HA])"
"When [A⁻] and [HA] are equal, pH = pKa," Toma continued.
"At this point, the two forms are 1:1."
Kana asked curiously. "Why is there a range? ±1?"
"Human eye limitation. Can recognize color change in concentration ratio from about 10:1 to 1:10."
Toma experimented. "Adding base gradually... pink gradually intensifies."
"Equilibrium is shifting," Rei explained. "Le Chatelier's principle."
Kana summarized. "Indicators convert invisible pH into visible color."
"Good expression," Rei acknowledged.
Toma laughed. "From the indicator's mood, we know solution properties."
"More equilibrium state than mood," Rei corrected, but smiled.
Kana lined up multiple indicators. "Made a rainbow of colors."
"Universal indicator. Mix multiple indicators to detect wide pH range."
Toma was impressed. "Chemistry is colorful."
Rei said quietly. "Color is the molecule's voice. Tells stories of structure and electrons."
Kana wrote in her notebook. "The mood of acid-base indicators—colors drawn by equilibrium."
The three gazed at the colorful solutions. Visible colors reveal invisible chemistry.