It is very easy to notice that things dissolve easier in warm liquid than in cold liquid, and even easier still in hot liquid. But surely the chemical composition of these liquids is identical. So what gives?
I suppose that what I’m really asking is this: Exactly what does the process of dissolving involve, and why does the temperature affect it?
Chemically they are the same, but the water molecules are moving faster in hot water. (That’s what heat is - kinetic energy of the particles.) Therefore, on a molecular level, the solution will be mixed up faster and more thoroughly. As an analogy: take a big bucket full of white marbles and pour in some black marbles. Jiggle the bucket slightly and you’ll see the black marbles start to mix in with the white ones, but only slowly. Now shake the bucket vigorously (i.e. add more energy, or “heat”). Soon you’ll see the black marbles are thoroughly mixed through the white ones. That’s your solution.
To answer the other part of your question, things dissolve in water because their molecules or ions are polar or electrically charged, and they are attracted to the polar water molecules.
Not everything is more soluble in hot water than in cold. Gases (oxygen, carbon dioxide) are more soluble in cold water. I believe some salts are more soluble in cold water also though I don’t know which ones.
Correct. Calcium carbonate has a solubility curve in water that decreases with temperature. What a nuisance.
I used to have an argument with my wife, who had been told that using cold water was as effective for soaking dirty dishes as hot water, due to the low solubility curve of baked-on food.
I agreed that the solubility wasn’t much influenced by temperature, but the rate of solution was much higher with hot water, and we didn’t have to soak the bloody things in the sink overnight in cold water to get them clean.
I won, eventually. And I am still married.
Si
I think there’s more to it with food residues. A lot of foods contain fats that have a melting point comfortably within the “hot washing-up water” range - think butter, chocolate etc. If you soak the dish in cold water, the fat stays solid and will take a lot longer to be lifted by the soap. In hot water, the fat liquefies (or at least softens) and is removed a lot faster.
This goes for ionic compounds (salts, basically). When larger covalent compounds, like sugar, dissolve in water the process of dissolving is totally different. That’s one of the little surprises I remember from Chem 101.
I guess dissolving is partially physical and partially chemical. Thanks all!
I did say electrically charged or polar. Sugar is covered with a whole bunch of hydroxyl groups, which are polar, and hydrogen-bond with water molecules. (Simple explanation for non-chemists, oxygen is more “electronegative” than hydrogen, which means it attracts electrons more strongly than hydrogen does. The result of this is that in an O-H bond, the electrons are “pulled” towards the oxygen rather than the hydrogen, making the oxygen atom rather electrically negative and the hydrogen atom rather electrically positive. So, in adjacent molecules, the O of one will tend to attract the H of the other, and vice-versa.)
Senegoid is right in that unlike ionic compounds (which dissociate into separate ions, e.g. NaCl, common salt, into Na[sup]+[/sup] and Cl[sup]—[/sup] ions), covalent compounds remain as complete molecules in solution.
Disclaimer: I did graduate in chemistry, but 14 years ago and haven’t used it in 12. 
very punny.
as mentioned due to the polar nature of water it can dissolve ionic and polar substances. many parameters are involved in how much will dissolve. other substances present will affect things. the speed at which things occur is called chemical kinetics.