"It turned blue!"
Toma held up the test tube.
Kana approached in surprise. "It was red just now."
"BTB solution. Yellow in acid, green in neutral, blue in alkaline," Rei explained.
"Why does the color change?" Kana asked the fundamental question.
Rei drew molecular structures on the whiteboard. "Because the molecular shape changes."
"Shape?"
"More precisely, electron arrangement changes."
Toma shook another test tube. "What about this?"
"Phenolphthalein. Colorless in acid, pink in alkaline."
Kana took notes. "Does the shape change too?"
"Presence or absence of protons changes structure. And color changes."
"Proton?"
"Hydrogen ion. H⁺," Rei supplemented.
Toma continued the experiment. "What if I add acid?"
One drop of hydrochloric acid in pink solution. Color disappeared.
"It disappeared!" Kana was surprised.
"Protons bonded and returned to the original colorless structure."
Kana had a question. "But why does color appear in the first place?"
Rei started explaining. "Color is related to light absorption."
"Light?"
"Absorbing visible light makes the complementary color visible."
Toma asked. "Complementary color?"
"Absorb red, appears cyan. Absorb blue, appears red."
Kana tried to understand. "So what absorbs light?"
"Conjugated system," Rei drew a new diagram. "Alternating double bonds."
"Double bonds?"
"Bonds between carbons. Single, double, triple."
Toma questioned. "Why do double bonds make color?"
"Pi electrons move easily. Can absorb low-energy photons."
Rei continued. "Longer conjugated system absorbs longer wavelengths."
Kana wrote in her notebook. "Longer wavelength?"
"Red and orange. Conversely, short conjugated systems absorb short wavelength UV."
Toma asked for examples. "Familiar examples?"
"Beta-carotene. Carrot pigment. Long conjugated system, so absorbs blue light and appears orange."
Kana was moved. "Color is determined by molecular shape."
"More precisely, electronic structure," Rei corrected.
Toma started another experiment. "Let me reduce potassium permanganate."
Added reducing agent to purple solution. Color disappeared.
"Disappeared again!" Kana shouted.
Rei explained. "Mn⁷⁺ reduced to Mn²⁺. Color changes when oxidation state changes."
"Oxidation state?"
"Number of electrons. When electrons increase or decrease, energy levels change."
Toma understood. "So wavelength of absorbed light changes."
"Correct. This is the principle of redox indicators."
Kana remembered another example. "Blood color?"
"Hemoglobin. Color changes with iron's oxidation state," Rei answered.
"When it binds oxygen?"
"Bright red. Dark red when releases oxygen."
Toma became interested. "Iron changes color too."
"Transition metals have d orbitals. Color comes from d-d transitions."
Kana was confused. "d orbital?"
Rei explained simply. "Like rooms where electrons enter. Transition metals have special rooms."
"That makes color?"
"Ligand influence splits energy levels. When that difference corresponds to visible light energy, color appears."
Toma asked for concrete examples. "Copper?"
"Copper ion is blue-green. Copper plate is reddish-brown. Changes with oxidation state and coordination environment."
Kana admired. "Gem colors too?"
"Yes. Ruby is chromium, sapphire is iron or titanium. Impurities create color."
Toma pointed at tea. "This too?"
"Tannin. Polyphenol conjugated system," Rei answered.
Kana squeezed in lemon. Color lightened.
"It changed!"
"pH change. Tannin structure changes, and color changes too."
Toma thought of another example. "Autumn leaves?"
"Anthocyanin. Color changes with pH and metal ions."
Rei continued. "Red, purple, blue... all from molecular structure differences."
Kana looked out the window. "The world overflows with color."
"All chemistry," Rei said.
Toma lined up test tubes. "The moment color changes, molecules change."
"And we have eyes to see that change."
Kana smiled. "Learning chemistry makes the world more colorful."
Rei nodded. "Behind colors, there are stories."
The three continued experimenting. Each color change brought new discoveries.