When sugar is poured in hot coffee and not stirred, should the sugar fully absorb the hot liquid? Why does the coffee taste sweeter when the sugar is physically stirred into the liquid?
In: Chemistry
For a molecule, like a sugar molecule, to be dissolved in water, water molecules need to come and surround it, forming a kind of “water shell” around the sugar. Once a water molecule is involved in one of these shells, though, it cannot participate in making a shell around another sugar molecule. So, as you dump sugar into your coffee, some of the molecules on the surface of the sugar crystals will get scooped up and dissolved by water molecules. But those that don’t immediately get dissolved fall to the bottom of your cup.
The water molecules inside the coffee are constantly moving around. So free water molecules, not yet involved in any of these shells, keep reaching the sugar at the bottom and scooping op sugar molecules, and then drifting away again making room for more free water molecules. But this process is pretty random. A sugar molecule has to get lucky that enough free water molecules make it down to the bottom to scoop it up. This may eventually happen (if there is still free water available, i.e. if the sugar-to-water ratio isn’t too high), but it takes time. Moreover, the water-sugar “balls” that form when a sugar molecule is dissolved also drift away randomly (they don’t deliberately move out of the way towards the top of the cup). So you end up with a high concentration of dissolved sugar (and not much free water) near the bottom, which is slow to distribute throughout the cup.
Stirring the liquid speeds up this process, by mixing the undissolved sugar crystals more evenly throughout the liquid, causing more “meetings” between sugar molecules and free water molecules.
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