"Acids are sour things?"
Kana read the test tube label.
"That's a taste description," Rei corrected. "Chemically, a substance that donates protons."
"Protons?"
"Hydrogen ions, H⁺."
Milia supplemented. "Bases are opposite. They accept protons."
Kana wrote in her notebook. "Acid = donate, Base = accept."
"But," Rei continued, "the same substance can act as acid or base depending on situation."
"Capricious?" Kana laughed.
"Called amphoteric. Water is a prime example."
Milia drew a diagram. "Water acts as a base when donating protons to an acid."
"Conversely, acts as an acid when accepting protons from a base," Rei continued.
Kana was confused. "So which is water?"
"Both. Role changes depending on partner."
"Like relationships," Kana murmured.
Rei laughed. "Interesting metaphor."
Milia took out a pH meter. "pH is an indicator of hydrogen ion concentration."
"7 is neutral?"
"Precisely, pure water at 25 degrees is pH 7. But it changes with temperature."
Rei explained. "pH is -log[H⁺]. Logarithmic scale."
"Logarithmic?" Kana asked.
"pH 6 is 10 times more acidic than pH 7. pH 5 is 100 times."
"Big difference..."
Milia started an experiment. "When I put pH meter in acetic acid solution..."
"3.2," Rei read.
"Because it's a weak acid, doesn't completely dissociate."
Kana asked, "Weak acid?"
"Acid that releases only part of its protons. Reaches equilibrium state."
Rei wrote an equation on the whiteboard. "HA ⇌ H⁺ + A⁻"
"Double-headed arrow?"
"Because reaction is reversible. Always, acid form and base form coexist."
Milia continued. "Equilibrium constant Ka determines acid strength."
"Larger means stronger acid?"
"Yes. Strong acids dissociate almost completely."
Kana took notes. "What's biological pH?"
"Blood is 7.4. Even slight change is dangerous," Rei answered.
"How is it maintained?"
"Buffer solution. Mixture of weak acid and its conjugate base."
Milia showed an experiment. "Even adding a little acid to this solution..."
The pH meter reading barely changed.
"Why?" Kana was surprised.
"Weak acid's conjugate base accepts the protons from added acid," Rei explained.
"Conversely, if we add base?"
"Weak acid donates protons."
Kana understood. "Maintaining balance."
"Yes. Buffer solution minimizes pH changes."
Milia added. "Blood buffer system is complex. Carbonic acid, phosphoric acid, proteins... multiple systems cooperate."
"Can't do with one?"
"To expand range. Different buffers work at different pH regions."
Rei organized. "Acid-base equilibrium is dynamic balance. Constantly changing, but stable overall."
Kana pondered. "Capricious but orderly?"
"Beautiful expression," Milia smiled.
"Enzymes are also pH-sensitive," Rei continued. "Activity drops outside optimal pH."
"Why?"
"Because protein charge state changes. Shape changes too."
Kana added to her notes. "pH, shape, function—all connected."
"Biochemistry is all connected," Milia said.
Rei put away the pH meter. "Relationship of acids and bases is simple but deep."
"Capricious but predictable?" Kana asked.
"With equilibrium constant, predictable. But biological systems are too complex."
Milia said quietly. "That's why it's interesting."
The three imagined invisible proton movements.
"Acids and bases are dancing," Kana murmured.
"To the music of equilibrium," Rei responded.
Milia opened the window. "Beauty of chemistry lies in the invisible."
Kana nodded. Capricious relationships support life.