上一片，小烧瓶把酸和碱调成了中性。这次它系上围裙回到厨房实验室——往水里加糖、加盐，玩转溶液与浓度。Last leaf, our flask neutralised acids and bases. Now it ties on an apron and heads back to the kitchen lab — dropping sugar and salt into water to play with solutions and concentration.
先看最平常的一幕：把糖放进水里。Start with the most everyday scene: sugar goes into water.
糖放进水里，搅拌后"消失"了——它没有真的消失，而是均匀散开在水里，变成糖水。糖是被溶解的"溶质"，水是"溶剂"，合起来就是"溶液"。盐水也一样：盐 + 水 = 盐水。Drop sugar into water and stir — it seems to disappear. It hasn't: it spreads evenly through the water as sugar water. The sugar is the solute, the water is the solvent, and together they make a solution. Salt water works the same way: salt + water = salt water.
那要是一直加下去呢？小烧瓶试了试。But what if you just keep adding more?
第 2 步：加呀加，溶液会饱和Step 2: Keep adding — and the solution saturates
一勺一勺地加糖，开始都能溶。加到某个量，怎么搅都化不了，杯底留下晶体——溶液"饱和"了。温度还能帮忙：热水能溶更多糖；热糖水放凉，多余的糖又会重新析出来。Spoon after spoon dissolves — until one spoonful just won't. Stir all you like, crystals stay at the bottom: the solution is saturated. Temperature helps too: hot water dissolves more sugar, and when the syrup cools, the extra sugar crystallises back out.
饱和看温度，浓淡看配比——那浓淡怎么量出来？Saturation depends on temperature; strength depends on the mix — so how do we measure it?
溶质质量分数 = 溶质质量 ÷ 溶液质量 ×100%。医院挂的生理盐水就是 0.9% 的食盐水：每 100 克溶液里含 0.9 克盐。浓度太低或太高都不行，0.9% 刚好和身体里的体液相配。Mass fraction = solute mass ÷ solution mass × 100%. Medical saline is simply a 0.9% salt solution: 0.9 g of salt in every 100 g of solution. Too weak or too strong won't do — 0.9% matches the fluids in your body.
🎮 你来当配液师（30 秒）🎮 Your turn: the solution lab (30 seconds)
道理讲完了，动手配一杯：拖滑块，把 4 个任务都完成。Time to mix: drag the sliders and finish all 4 tasks.
一句话记住它：溶液 = 溶质 + 溶剂；浓度 = 溶质 ÷ 溶液；再也溶不下的时候，就叫饱和。Remember it in one line: a solution is solute + solvent; concentration is solute ÷ solution; when nothing more will dissolve, it is saturated.
溶液 = 溶质 + 溶剂（盐水 = 盐 + 水）Solution = solute + solvent (salt water = salt + water)饱和：怎么搅都溶不下，杯底留晶体；热水能溶更多Saturated: no more dissolves, crystals stay at the bottom; hot water dissolves more浓度 = 溶质 ÷ 溶液 ×100%；生理盐水是 0.9%Concentration = solute ÷ solution × 100%; medical saline is 0.9%
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