"When I put salt in water, it smoothly disappears."
Kana stared at the spoon. Table salt was dissolving in water.
Rei answered. "Rather than dissolve, it scatters."
"Scatters?"
"Separates into ions and diffuses into the water."
Milia supplemented. "NaCl becomes Na⁺ and Cl⁻."
"The ionic bonds break?"
"Yes. Water molecules pull them apart."
Rei drew a diagram. "Salt crystals have Na⁺ and Cl⁻ arranged regularly."
"Lattice structure?"
"Exactly. Held together by electrostatic attraction between ions."
Kana asked. "Then why does it dissolve? Despite the strong bonds."
"Because the energy of hydration is large," Milia explained.
"Hydration?"
"Water molecules surround the ions. Stabilized by dipoles."
Rei talked in detail. "Water's O is δ-, H is δ+. O faces around Na⁺, H faces around Cl⁻."
"Electrostatic interaction."
"Yes. A layer called the hydration shell forms."
Kana drew in her notebook. "Ions wrapped in water."
"When this hydration energy exceeds the lattice energy, it dissolves."
"Lattice energy?"
"The energy needed to turn an ionic crystal into gaseous ions."
Milia wrote an equation. "ΔH_solution = ΔH_lattice - ΔH_hydration"
"For dissolution to occur, hydration must compensate for the lattice."
Kana thought. "But if I add too much salt, it won't dissolve."
"Solubility," Rei answered. "There's an equilibrium state."
"NaCl(s) ⇌ Na⁺(aq) + Cl⁻(aq)"
"Solubility product Ksp = [Na⁺][Cl⁻]"
"If more dissolves, the equilibrium breaks."
Milia experimented. "Let's compare cold water and warm water."
Same amount of salt in two beakers.
"The warm one dissolves faster!" Kana observed.
"When temperature rises, molecular motion becomes active," Rei explained.
"Water molecules move faster and can pull ions apart more easily."
"But does solubility also change?"
"For sodium chloride, not much. Other salts change significantly."
Kana asked. "When dissolving, heat?"
"NaCl is endothermic," Milia answered.
"Gets cold when it dissolves?"
"Slightly. Because lattice energy is larger."
Rei supplemented. "But entropy increases."
"Entropy?"
"Disorder. From solid to solution, degrees of freedom increase."
"ΔG = ΔH - TΔS. The entropy term drives dissolution."
Kana understood. "From order to disorder."
"Natural flow."
Milia said quietly. "The time when salt dissolves is gentle."
"Gentle?"
"Not a violent reaction. Quietly, naturally scattering."
Rei nodded. "Entropy increase. The law of the universe."
Kana stared at the water. "Can't see it, but it's there."
"Ions are transparent. But they exist."
"Conducts electricity?"
"Yes. Freely moving ions carry current."
Milia inserted a wire. The lamp lit.
"Electricity flowed!"
"Evidence of dissolution."
Kana wrote in her notebook. "The gentle time when salt dissolves."
"Chemistry's gentleness," Rei smiled.
"Unhurried, but certain."
The three stared at the solution. Ions drift in the water. The time when salt dissolves flows quietly today too.