"Why is this purple?"
Toma held a test tube with the iodine-starch reaction.
"Colors have reasons," Rei answered. "The electronic structure of molecules."
"Electrons? Electrons again?" Kana laughed.
Milia began drawing a diagram. "Light is electromagnetic waves. When specific wavelengths of light are absorbed, color appears."
"Absorbed?"
"Yes. The molecule receives light energy and electrons become excited."
Rei supplemented. "From ground state to excited state. Electrons jump to higher energy levels."
Kana wrote in her notebook. "Absorb light → electrons excited → color appears"
"More precisely, the light that isn't absorbed appears," Milia corrected.
"What do you mean?"
"A molecule that appears purple is absorbing yellow light. The remaining blue and red mix to make purple."
Toma looked confused. "That's complicated."
Rei drew a color wheel. "Absorbed color and visible color are complementary. Absorb red and it looks green."
"I see!" Kana understood.
Milia showed another example. "Chlorophyll is green. It absorbs red and blue, reflects green."
"That's why leaves are green."
"Yes. The colors of light used for photosynthesis determine leaf color."
Rei continued. "Conjugated systems are important. When double bonds alternate, electrons become delocalized."
"Delocalized?"
"Electrons don't stay in one place but spread over a wide area."
Milia drew a molecule. "Beta-carotene. It has a long conjugated system."
"The color of carrots?" Toma asked.
"Yes. The longer the conjugated system, the longer the wavelength of light absorbed. That's why it looks orange."
Kana asked. "What if you make it longer?"
"It becomes red, and even longer turns blackish. It starts absorbing the entire visible spectrum."
Rei took out a pH indicator. "This also shows color change. Phenolphthalein."
Adding base, it changed from colorless to pink.
"Wow!" Kana was surprised.
"A proton came off, changing the molecular structure. The conjugated system extended, changing the absorption wavelength."
Milia showed another example. "Litmus paper works on the same principle."
"Red in acid, blue in base," Toma said.
"When molecular shape changes, electronic state changes too. That's why color changes."
Kana looked at the iodine-starch again. "So this purple is?"
Rei explained. "Iodine enters the helical structure of starch. Iodine molecules line up, electrons form a conjugated system."
"Without starch?"
"Brown. The color of iodine alone."
Milia added. "A good example of environment changing color. The same molecule can change color depending on surroundings."
Toma looked at another test tube. "What about blood red?"
"Iron in hemoglobin," Rei answered. "Electronic transitions involving iron's d orbitals."
"When it binds oxygen, color changes, right?" Kana remembered.
"Yes. Venous blood is dark red, arterial blood is bright red. Oxygen changes iron's electronic state."
Milia said with feeling. "Color is molecular language. It tells us structure and state."
Rei continued. "That's why chemists carefully observe color changes."
Kana looked around the experiment table. Yellow, blue, green, red, purple. Solutions of various colors.
"Each has a story."
"The story of electrons," Toma said.
Milia smiled. "Dialogue between light and matter."
Rei summarized. "Visible light is 380 to 780 nanometers. In that narrow range, we color the world."
Kana murmured. "But outside that range, there's light too."
"Ultraviolet, infrared, X-rays..." Milia enumerated.
"Invisible but existing."
Rei nodded. "Color is a window showing only part of reality."
Toma held the test tube up to the light. The purple liquid glowed.
"This purple is the language iodine and starch speak."
"And the story we receive," Kana continued.
Milia said quietly. "Color is the intersection of chemistry and perception."
The four stared at the colorful test tubes. Light is absorbed, electrons jump, colors are born. In each color, molecular secrets are hidden.
"Beautiful dialogue," Rei said.
The three nodded too. There's always a reason colors change.