十万个为什么·科学启蒙30篇（3）

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为什么热水比冷水更容易溶解糖

Hot water dissolves sugar faster because heat gives water molecules more kinetic energy to break apart sugar crystals.
Sugar molecules are held together by weak attractions, and warmer water molecules collide with them more forcefully and frequently.
Higher temperatures also increase the space between water molecules, letting more sugar particles fit between them.
Dissolving is a physical change—not a chemical reaction—so the sugar remains chemically identical whether hot or cold.
Stirring helps speed up dissolution in both cases by bringing fresh solvent into contact with undissolved sugar.
However, stirring alone cannot overcome the fundamental limit set by temperature and solubility.
At 100°C, water can hold over twice as much sugar as it can at 0°C—about 487 g per 100 mL versus 179 g.
Once saturated, adding more sugar won’t dissolve—it’ll just settle at the bottom regardless of temperature.
This principle applies to many solids, but gases like CO₂ actually dissolve *less* in warm water, which is why soda goes flat faster when warm.
Understanding solubility helps cooks, pharmacists, and environmental scientists predict how substances mix in nature and industry.