"Hey, what's happening right now?"
Kana stared at the beaker. The solution was slowly changing color.
"It's reacting," Rei answered. "Molecules are changing."
Toma's eyes lit up. "But how? Does molecule A suddenly become molecule B?"
"Not suddenly," Rei drew a diagram on the whiteboard. "In between, there's an incredibly brief 'critical moment.'"
Kana tilted her head. "Critical?"
"It's called the transition state. An intermediate, unstable state that's neither reactant nor product."
Toma picked up the beaker without permission. "Can't we see it?"
"No. It exists for about 10 to the minus 13 seconds."
"What!" Kana was surprised.
Rei continued. "If one blink takes 0.1 seconds, this is one trillionth of that."
"I can't imagine..."
"But this instant determines the reaction rate," Rei emphasized.
Kana wrote in her notebook. "The transition state is key?"
"Yes. You need to overcome the 'mountain' from reactant to transition state."
Toma folded his arms. "Mountain?"
"Energy mountain. It's called activation energy."
Rei drew a diagram. The horizontal axis showed reaction progress, vertical axis showed energy.
"Reactants are at this position. Products are here. But you can't go directly."
"Why not?"
"You must pass through the transition state. This is the energy peak."
Kana understood. "Like crossing a mountain pass?"
"Exactly. If the pass is high, it's hard to cross. So the reaction is slow."
Toma got excited. "What if we raise temperature?"
"Good idea. When temperature rises, molecular kinetic energy increases."
"It becomes easier to cross the pass!"
Rei nodded. "The Arrhenius equation expresses that. Reaction rate depends on activation energy and temperature."
Kana asked. "What does the transition state look like?"
"That's interesting," Rei's eyes sparkled. "Bonds are half-broken, and new bonds are half-formed."
"In limbo?"
"Yes. In SN2 reactions, you get pentacoordinate carbon. Usually it's tetracoordinate."
Toma was confused. "Rule violation?"
"Not a violation. It's allowed for an extremely short time."
Kana pondered. "But why is it unstable?"
"Because the electron configuration isn't optimal. Bonds are halfway."
Rei gave another example. "Enzymes stabilize this transition state."
"Stabilize?"
"They bind to the transition state, lowering the energy mountain. So reactions become faster."
Toma looked at the beaker. "Right now, transition states are being born and disappearing every instant?"
"Yes. Countless molecules are passing through that 'critical moment.'"
Kana murmured. "The invisible moment determines everything."
"That's the essence of chemistry," Rei said quietly. "Transition state theory explains why reactions occur and how fast."
"It's a beautiful theory."
"Beautiful but harsh. Transition states can't be observed. We can only infer."
Toma laughed. "Studying something invisible?"
"Much of science is like that," Rei admitted. "But from indirect evidence, they certainly exist."
Kana stared at the beaker. The reaction proceeded quietly.
"Countless dramas in an instant, repeated endlessly."
"Yes. Chemistry is also the science of the moment."
Toma looked around the room. "Even now, transition states worldwide?"
"In quantities of quintillions, sextillions."
Kana took a deep breath. "Critical moments certainly exist."
Rei smiled. "Understanding that is understanding chemistry."
The three quietly imagined the invisible moments of reaction.