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
Compounds dissolve because the water’s molecules are moving about and knocking against the sugar molecules. But a granule of sugar has lots of molecules and the water only knocks on the outside of the crystal.
Dissolving also depends on the water’s capacity to hold the sugar. But if the water isn’t moving, then the water touching the crystal very quickly gets saturated with sugar and can’t hold anymore. Given time, the dissolved sugar will diffuse into the rest of the cup.
But since you want hot coffee, you need to speed the process up. Stirring causes everything to move around, so more water molecules knock against the granule. It also moves the sugar around, so it doesn’t stop the granule from dissolving further.
It will dissolve and disperse on its own eventually, but that might take time.
Left unstirred, the sugar and the slighty denser sugar-water will collect at the bottom of the cup and give you one very sugary sip at the end.
It depends on how hot the coffee is, and how much sugar you’ve adding.
Once you’ve reached a coffee temp and volume of sugar that doesn’t automatically absorb into the liquid, then gravity takes over.
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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