or is that a myth? scientifically speaking, does it really matter if it’s cold or hot water?
Here’s a Wikipedia article that uses this question to illustrate the scientific method, and which strongly indicates that the answer is Yes.
Yes it does. In very simple terms, the molecules in warm water are moving around more than they are in cold water (and at 212ºF, move fast enough to break free of the liquid form altogether). So they are able to dissolve the sugar crystals more easily.
This is true for most solutions, however there are a few compounds which become less soluble at higher temperatures, e.g. sodium sulfate (see graph on Wikipedia page).
It does dissolve faster in hot water. I’m looking for a cite but couldn’t find anything more compelling than the article cited above. I remember seeing a graph in freshman chemistry that showed the dissolution rate of sugar as a function of temperature, and it increased. For some reason that I don’t understand, the same graph for salt was nearly flat, probably because it’s a simpler molecule with an ionic bond (but I didn’t do that well in chemistry). Because of the behavior of sugar some people develop the incorrect intuition that all solids dissolve faster in warmer water.
All good cooks know that the best way to make lemonade (or any sugar syrup) is to boil the water for a quick and thorough dissolution of the sugar (but don’t turn it into caramel!).
This study guide suggests that the vast majority of solids do in fact dissolve better at higher temperatures.
The best possible cite is to do the experiment yourself. Remember, this is science: When the authorities disagree with the experiments, it’s the authorities that have to change. And it’s not like sugar and water are hard to come by.