"This water is pH 4.5."
Kana was measuring a rainwater sample.
"Close to lemon juice," Rei said quietly. "This is acid rain."
"Rain is sour?" Toma was surprised.
"Normal rain is around pH 5.6. CO2 from the atmosphere dissolves, making it weakly acidic."
Milia showed her notebook. "CO2 + H2O ⇌ H2CO3"
"Carbonic acid," Rei supplemented. "But when sulfur oxides and nitrogen oxides mix in, it becomes more acidic."
Kana thought. "From factories and car exhaust?"
"Yes. When SO2 dissolves in water, it becomes sulfuric acid H2SO4. NOx also changes to nitric acid HNO3."
Toma looked worried. "This is bad for plants, right?"
"Of course. When soil pH drops, aluminum dissolves out. It damages roots."
Milia wrote another equation. "Al3+ + 3H2O ⇌ Al(OH)3 + 3H+"
"Metal ion solubility is pH-dependent," Rei explained. "In acidic conditions, toxic metals dissolve."
Kana suddenly thought. "But what about pH inside the human body?"
"Blood is pH 7.4. Nearly neutral."
"Very precise."
"Yes. Even ±0.1 variation is dangerous. That's why there's a buffer system."
Toma became interested. "Buffer system?"
Rei drew on the whiteboard. "A mechanism that suppresses pH changes. A combination of weak acid and its conjugate base."
"In blood, bicarbonate ion and CO2 form a buffer system."
Milia wrote. "H2CO3 ⇌ HCO3- + H+"
"When acid increases, HCO3- neutralizes it. When base increases, H2CO3 responds."
Kana understood. "That's why pH stays stable."
"What if there was no buffer system?" Toma asked.
"A little acid or base would cause pH to fluctuate wildly. Enzymes couldn't work."
Rei continued. "Protein tertiary structure is pH-dependent. Just a slight deviation causes loss of function."
Kana looked at the rainwater sample. "So what about organisms in acid rain?"
"When lake pH falls below 5, fish die. It exceeds buffering capacity."
Milia nodded sadly.
"But limestone areas are relatively safe," Rei added. "CaCO3 works as a buffer."
Toma wrote the equation. "CaCO3 + H+ → Ca2+ + HCO3-"
"Correct. Calcium carbonate neutralizes acid. So impact varies by geology."
Kana pondered. "Acid rain's tears, it's like Earth is crying."
"Poetic but accurate," Rei acknowledged. "Environmental pH balance is disrupted."
Milia showed a note. "Ocean acidification is the same principle."
"Atmospheric CO2 is increasing, lowering ocean pH. Organisms that make coral and shells are suffering."
Toma looked serious. "It's not a chemistry problem, it's a life problem."
"Exactly," Rei nodded. "pH is the foundation of life. Small deviations change entire ecosystems."
Kana stared at the pH meter. "7.4. Human blood. Such delicate balance."
"To maintain that balance, the body works ceaselessly."
Milia opened another notebook. "Lungs regulate pH by expelling CO2. Kidneys excrete H+."
"Two systems cooperating," Rei explained. "Lungs provide fast regulation, kidneys are slower but more powerful."
Toma was impressed. "The body is amazing."
"But there are limits. Like acid rain, when buffering capacity is exceeded, it collapses."
Kana looked outside the window. Rain was starting again.
"What's the pH of this rain?"
Rei said quietly. "Let's measure it."
The four took sample bottles and went outside. Raindrops fell on their palms. Transparent, cold, and slightly sour tears.
"Earth is crying," Kana murmured.
"But understanding means we can protect it," Rei answered. "Chemistry is the first step to understanding."
Milia smiled and filled her sample bottle. Toma seriously collected rain too.
Small actions. But it starts with understanding. Knowing the reason for acid rain's tears. That's the beginning of change.