托福阅读资料之How Do You Make A Diet-Coke Float?

　　Take a can of “classic” Coca-Cola and a can of Diet Coke, and without opening either one submerge them in water. A deep sink or a bucket will do the trick. You’ll find the can of Diet Coke rises to the top, while the Classic Coke stays sunk. Can you guess why?

　　Why?

　　The answer lies in the chemical contents of the Diet Coke —in fact, in the very distinction that makes it “Diet”.

　　Although their ingredients are otherwise similar, Diet Coke is not made with real sugar: instead, the makers of Coca-Cola products have replaced the sugar with a synthetic chemical called Aspartame.

　　Aspartame, which is commonly known as Nutrasweet, is a kind of substitute sugar —intended to mimic the distinctive taste of sugar without carrying the associated calories.

　　High Molecular Density

　　Real sugar has a high molecular density. In other words you can think of sugar molecules as being thickly packed together.

　　This also means sugar molecules themselves will be relatively heavy, even when dissolved in solution. In the case of the sinking Coke, the real sugar content is acting as an anchor for the can.

　　Coca-Cola Taste

　　There’s also the fact that Nutrasweet is much sweeter than sugar, and so less is needed per can to achieve that recognizable Coca-Cola taste. Thus the bobbing Diet Coke not only has a lower average density, it also has a lower mass —there’s less “stuff” dissolved in the mixture. If you were an advertising executive, you might want to market diet sodas as “Light” —it wouldn’t be far from the truth.

　　你可以在家做这样的实验。

　　把没打开过的两听可乐(含糖，无糖)放在深槽或者高桶中，里面的水淹没两听可乐，你会发现无糖可乐浮在水面;而含糖可乐却沉下去了。你能猜到原因吗?

　　奥秘就在于无糖可乐的成分，这也是“无糖”的原因。虽然，无糖可乐和普通可乐的成分很相似，无糖可乐中添加的并非真正的糖，而是一种做“阿斯巴甜”的合成化学物。

　　“阿斯巴甜”是一种常见的代糖甜味剂，不含糖的热量却有糖的甜味。

　　高分子密度

　　真正的糖分子密度高，也就是说糖分子紧密地排列在一起。

　　即使溶解在溶液中，相同体积的糖更重。实验中沉下去的普通可乐，正是因为糖成分像锚般让罐子下沉。

　　可乐的味道

　　代糖比糖甜，所以更少的代糖能达到普通可乐的甜味。这样水中摇摆的无糖可乐不仅平均密度更低，质量也比普通可乐轻——因为更少的代糖溶解在可乐中。如果你是广告商，你可以这样宣传——无糖碳酸饮料的优点：轻。这可没有说大话。