"Can H⁺ exist alone?"
Kana stared at her notebook.
Rei answered. "Not in aqueous solution. It immediately bonds with water."
"Bonds?"
"Becomes H₃O⁺, hydronium ion."
Milia supplemented. "Protons are too small. Unstable when bare."
"So they travel?"
"Yes. From molecule to molecule, continuously moving."
Rei drew a diagram. "The essence of acid-base reactions is proton transfer."
"Transfer?"
"Acids give protons, bases receive them."
Kana understood. "The Brønsted-Lowry definition?"
"Correct. Acids are H⁺ donors, bases are H⁺ acceptors."
Milia gave an example. "HCl and water. HCl gives H⁺ to water."
"HCl is the acid, water is the base."
"In this reaction, HCl becomes Cl⁻, water becomes H₃O⁺."
Rei continued. "Cl⁻ and HCl are a conjugate acid-base pair."
"Conjugate?"
"Related by a difference of one proton. Cl⁻ is the conjugate base of HCl."
Kana wrote in her notebook. "Conversely, H₃O⁺ and H₂O are also a conjugate pair?"
"Exactly. When water receives H⁺ it becomes an acid, when it loses it, it becomes a base."
"Amphoteric?"
"Water is amphoteric. It can be an acid or base depending on the partner."
Milia proposed an experiment. "Let's dissolve acetic acid in water."
She dropped a few drops into clear liquid. Dipped pH paper.
"Acidic!" Kana saw the color change.
"CH₃COOH gave H⁺ to water," Rei explained.
"But not all dissociates?"
"Because it's a weak acid. Only part releases H⁺."
"Equilibrium state?"
"Yes. CH₃COOH ⇌ CH₃COO⁻ + H⁺"
Kana thought. "Then what about strong acids?"
"Like HCl. Almost completely dissociates."
Milia compared. "Strong acids have many H⁺ departing on journeys. Weak acids have few."
"That's why pH differs."
Rei supplemented. "pH is -log[H⁺]. The logarithm of H⁺ concentration."
"Smaller numbers are more acidic?"
"Yes. pH 7 is neutral, below is acidic, above is basic."
Kana asked. "How fast does H⁺ move?"
"Very fast," Rei emphasized.
"There's a special mechanism called the Grotthuss mechanism."
Milia drew a diagram. "H⁺ jumps to a neighboring water molecule. In a chain."
"Like a relay."
"Exactly. Called proton jumping."
Rei continued. "That's why H⁺ movement in aqueous solution is faster than diffusion."
Kana was moved. "Hydrogen is busy."
"Also important in living organisms," Milia added.
"Enzyme reactions, respiration, photosynthesis... all involve proton transfer."
"Proton gradient?"
"In mitochondria, the H⁺ concentration difference drives ATP synthesis."
Rei organized. "The journey of H⁺ carries energy."
Kana looked out the window. "Invisible particles are traveling."
"Even now, inside your body."
Milia said quietly. "Every second, trillions of H⁺ are moving."
Kana wrote in her notebook. "The day hydrogen goes on a journey."
"Every day is a journey day," Rei smiled.
"An endless journey."
The three fell silent. The journey of protons is the journey of life. The movement of tiny particles creates enormous phenomena. Hydrogen's journey continues today.