I was pouring out a soda that had been sitting overnight in my van and when I opened the lid I noticed that there were areas that were more brown (Diet Coke) and areas that were clear (melted ice). I was under the impression that over a relatively shourt period of time, molecular motion should completely diffuse the diet coke into the melt water. So what is going on?
No, liquids diffuse slowly - something we did in chemistry at school. There is Brownian motion in liquids, but it’s way slower than in gases, because liquid molecules just plain can’t travel very far (that’s sort of what distinguishes a liquid from a gas). To get liquids to mix, you need to either stir them or have convection going on (which is just another way to achieve the stirring).
I like lager with Grenadine syrup. It’s not at all hard to wind up with a layer of syrup in the bottom of the glass and the rest of the glass filled with lager. Although the sugary Grenadine syrup will dissolve perfectly well in the watery lager, you need to persuade them to socialise. Otherwise they’ll just sit there one on top of the other going nowhere. I’ve never tried leaving it overnight as I’ve generally been in too much of a hurry to drink it, but I’d be prepared to bet it’d still be substantially unmixed.