"Which is stronger, acids or bases?"
Toma asked provocatively.
"What are you saying?" Rei looked exasperated. "They're not comparable. It depends on the system."
"But hydrochloric acid is strong, right?"
"It's a strong acid, but loses to superbases."
Kana was confused. "What's the standard for strong or weak?"
Rei wrote on the whiteboard. "Acids donate protons H⁺. Bases accept them."
"Brønsted-Lowry definition," Toma said.
"Yes. Acids and bases are determined by proton transfer."
Kana raised her hand. "What about water?"
"Good question," Rei smiled. "Water is amphoteric. It can be either acid or base."
"H₂O → H⁺ + OH⁻, this is acidic."
"H₂O + H⁺ → H₃O⁺, this is basic."
Toma was surprised. "Water is versatile."
"Its role changes depending on the proton transfer partner."
Rei continued. "Strong acids easily release protons. Weak acids release them reluctantly."
"Strong bases easily accept protons. Weak bases accept them reluctantly."
Kana wrote in her notebook. "So what's the order of strength?"
"Expressed as pKa. The negative logarithm of the acid dissociation constant Ka."
Toma interjected. "Same notation as pH?"
"Yes. The smaller the pKa, the stronger the acid."
"Hydrochloric acid's pKa is around -7. Acetic acid is 4.76."
Kana calculated. "If pKa differs by 1, the strength differs by 10 times?"
"Precisely, 10 to the first power. It's a logarithmic scale."
Rei gave an example. "Stomach acid pH is around 2. Lemon juice is 2.5."
"They look almost the same, but actually differ by about 3 times."
Toma prepared experimental equipment. "Let's talk about buffers."
"Buffers?" Kana asked back.
"Solutions that maintain constant pH. Even if you add acid or base, pH hardly changes."
Rei explained the mechanism. "Mixing a weak acid with its conjugate base creates buffering action."
"Like acetic acid and sodium acetate."
Toma drew a diagram. "When you add acid, acetate ions neutralize it."
"When you add base, acetic acid neutralizes it."
"Can handle both, so pH stays stable."
Kana got excited. "Blood is also a buffer!"
"Correct," Rei acknowledged. "The carbonic acid-bicarbonate system. Maintains pH around 7.4."
"If pH becomes below 7.0 or above 7.8, life is endangered."
Toma supplemented. "That's why breathing regulates carbon dioxide."
"When CO₂ increases, carbonic acid increases and pH decreases."
"Breathing faster to expel CO₂ raises pH."
Kana took notes. "Living systems constantly adjust pH..."
Rei gave another example. "Intracellular pH is around 7.2. But lysosomes are below 5."
"Optimal pH differs by location."
"Enzyme activity also depends on pH. Pepsin in acidic, trypsin in alkaline."
Toma asked. "Why does activity change with pH?"
"Because protein amino acid side chains become protonated or deprotonated."
"Structure and charge change, altering active site shape."
Kana said with a serious face. "Acid-base isn't just simple chemical reactions, it's life's control system."
"Protons are the smallest and fastest control signals," Rei said.
"That's why acid-base equilibrium is the foundation of biochemistry."
Toma said finally. "The debate between acids and bases continues forever. As long as protons keep moving."
Outside the window, rain began falling. Atmospheric carbon dioxide dissolves in water, becoming weakly acidic rain. The acid-base debate continues on a global scale too.