"Does it really freeze at zero degrees?"
Toma asked while shaking a cold pack.
Rei shook his head. "Not necessarily. Pure water sometimes doesn't freeze even below zero degrees."
"What? But I learned that in science class," Kana was surprised.
"That's a nucleation problem," Rei began explaining. "For water to become ice, an initial small ice crystal, a nucleus, is needed."
"Nucleus?"
"Like a seed of ice. There's a moment when water molecules begin to gather in a certain arrangement."
Toma asked with interest. "So without a nucleus?"
"You get a supercooled state. It stays liquid even below zero degrees."
Kana opened her notebook. "Isn't that unstable?"
"Very unstable. It freezes all at once with a slight shock. Supercooled water is a metastable state."
Rei drew molecular structures on the whiteboard. Water molecules forming hexagonal patterns.
"Ice crystal structure has beautiful symmetry. Hydrogen bonds are regularly arranged."
"Hydrogen bonds?"
"Weak bonds between the hydrogen of an OH group in a water molecule and the oxygen of another water molecule. This determines ice structure."
Kana stared at the diagram. "Why hexagonal?"
"Determined by the geometric arrangement of water molecules and the directionality of hydrogen bonds. Around an oxygen atom, four water molecules are arranged tetrahedrally."
Toma placed the cold pack on the desk. "But liquid water has hydrogen bonds too, right?"
"Sharp. In liquid, hydrogen bonds constantly break and form. When it becomes ice, they become fixed."
Rei continued. "Nucleation has a critical size. Ice clusters that are too small are unstable and melt quickly."
"Critical size?"
"Balance of surface energy and volume energy. Small clusters have large surface area, and surface energy works unfavorably."
Kana tried to calculate. "How big?"
"Depends on temperature. Near zero degrees, several tens to hundreds of water molecules."
Toma thought. "So making ice with pure water is difficult?"
"Theoretically. In practice, container walls or minute impurities become nucleation sites."
"Impurities help?"
"Called heterogeneous nucleation. On foreign surfaces, water molecules can arrange more easily."
Rei drew another diagram. "Organisms use this. There are ice nucleation proteins that promote ice formation."
"Proteins make ice?"
"More precisely, protein surfaces guide water molecule arrangement. They have structures similar to ice crystal lattices."
Kana was surprised. "So it freezes more easily near zero degrees?"
"Yes. Some plants and microorganisms in cold regions intentionally control ice nucleation."
Toma asked. "Conversely, can you make it harder to freeze?"
"Antifreeze proteins do exactly that. They inhibit ice growth."
"How?"
"They adsorb to ice crystal surfaces and prevent water molecule addition. Antarctic fish have them."
Kana summarized. "The moment ice forms is when molecules cooperate to align their arrangement."
"Beautiful expression," Rei smiled. "Phase transition is a collective phenomenon. It doesn't happen with just one molecule."
Toma looked out the window. "Snow crystals too?"
"Same principle. Nucleation and growth produce complex patterns depending on humidity and temperature."
"Like magic," Kana said.
"Physical chemistry magic," Rei answered. "Intermolecular interactions create macroscopic beauty."
The three pondered for a while about the mysterious behavior of water, the most familiar substance.