"I never really thought about how salt dissolves in water. It seems so obvious."
Kana stared at the beaker in the lab.
"But do you know why it dissolves?" Rei asked.
"Um... because water breaks down the salt?"
Milia quietly approached and placed a molecular model down. A water molecule model.
"Water is a polar molecule," Rei began explaining. "The oxygen side is slightly negative, the hydrogen side slightly positive."
"Partial charges?"
"Yes. This asymmetry makes water a special solvent."
Milia added sodium and chloride ion models.
"In a crystal, sodium chloride has Na⁺ and Cl⁻ arranged in a regular pattern," Rei continued.
"What happens when you put it in water?" Kana asked.
"Water molecules surround the ions. This is hydration."
Milia slowly moved the models. Water molecules arranged themselves to envelop the salt ions.
"The negative oxygen parts face Na⁺, and the positive hydrogen parts face Cl⁻."
"Like an embrace," Kana murmured.
"Good expression," Rei acknowledged. "It's called a hydration shell. It stabilizes the ions."
"But why does it stabilize them?"
"Electrostatic interactions. Positive and negative attract. When hydration energy exceeds crystal lattice energy, dissolution occurs."
Milia drew a diagram in her notebook. An energy diagram.
"Dissolution is a balance of entropy and enthalpy," Rei explained.
"Sounds complicated..."
"Simply put, it's a balance between disorder and energy. If dissolution is energetically favorable overall, it dissolves."
Kana suddenly thought. "Living bodies are mostly water, right?"
"Exactly," Milia nodded. She spoke, which was rare. "More than 60 percent of body weight is water."
"Why do we need so much water?"
"Water is a solvent for biomolecules," Rei answered. "Proteins, sugars, ions. Everything functions dissolved in water."
"Without water?"
"Reactions don't occur. Enzymes don't work. Life activities stop."
Milia took out another model. A protein surface.
"Protein surfaces are also hydrated," Rei explained. "Hydrophilic amino acids on the outside, hydrophobic ones inside."
"There are parts that like water and parts that don't?"
"More precisely, parts that interact easily with water and parts that don't."
Kana said excitedly, "So protein shape is determined by water too!"
"Exactly. A phenomenon called the hydrophobic effect. To avoid water, hydrophobic parts cluster inside."
Milia smiled. "Water is the sculptor of life."
"That's poetic," Rei said.
Kana stared at the water in the beaker. "Such a simple molecule does such complex things."
"H₂O. Just three atoms," Rei continued. "But its properties are remarkably diverse."
"Hydrogen bonds," Milia added. "Water molecules also connect with each other."
"That's important too?"
"Extremely. Water's high boiling point, surface tension, why ice floats on water. All thanks to hydrogen bonds."
Kana started writing in her notebook. "Hydration = water embracing solutes."
"Good summary," Rei acknowledged. "And that embrace makes life possible."
Milia said last. "No life without water. No biochemistry without hydration."
Outside the window, rain began to fall. Countless water molecules pouring down on the earth. Each preparing to dissolve something, carry something, support life.
"Next, let's talk about solubility," Rei proposed.
Kana and Milia nodded. The story of water has only just begun.