Molecular-level explanation of macroscopic phenomena: dissolving

[dextercioby]

TL;DR

What really happens when sugar is mixed with liquid water and then the solution is heated to the boiling point

What I know and please correct me: a macroscopic probe of raw sugar you can buy from the store can be modeled to be an almost perfect cube of a size of 0.7 up to 1 mm. Let's assume it was really pure, nothing else but a conglomerate of H12C22O11 molecules stacked one over another in layers with van de Waals (?) "forces" keeping them together in a macroscopic state at a temperature of let's say 20 degrees Celsius. Then I use 100 such tiny pieces to throw them in 20 deg water.

I stir the mixture and 20 seconds later, all these tiny cubes are gone, at least at the level of the human eye. What happened at molecular level, where did they go?

Then I take this sweet liquid and heat it until it boils. Surprise or not, the mini crystals seem to appear again. What happened at molecular level, is the original process really fully reversed? I don't have liquid water anymore, only recrystalized sugar, right?

 

[.Scott]

No.

The process is described in Science Insights.

Each sucrose molecule possesses eight hydroxyl groups. These groups allow the sucrose molecule to develop multiple regions of partial positive and negative charge. When solid sucrose is introduced to water, the polar water molecules are strongly attracted to these charged regions through hydrogen bonding.

When sucrose is dissolved in water, I would expect each sucrose molecule to be surrounded by H20 molecules - almost like armor.

When the water is boiled away, don't expect tiny, nicely formed cubes of sugar. The best you can hope for are crystals that form much in the shape of the diminishing puddles that exists just before the last of the water evaporated.

There could be some destructive distillation. It is likely that many of the sucrose molecules will have separated into the glucose and fructose that makes up sucrose. The temperature at which water caramelizes is 160C - so with care. that can be avoided. But fructose caramelizes at 110C, so if any of it is freed from the glucose, it is likely to break down further.

Here is a video describing the sugar refining process. Regular boiling it used until the sugar concentration is 60%. After that, it is further dried in large vacuum pans. Beyond that, the crystals are dried in a air-heated tumble dryer. It isn't clear what creates the crystal size, but just before the syrup is pan dried, an alcohol sugar mixer is added - presumably a form of seeding.

 

[Borek]

To add to that: the process of surrounding dissolved molecules by solvent molecules is called solvation. Sometimes there are several solvation layers - they separate dissolved molecules from each other. When you remove the solvent sucrose molecules combine to build the crystal lattice again.

Basically is is an interplay between intermolecular forces - in a crystal sucrose molecules are "glued" to each other, in the solution they are surrounded by water (or some other solvent). Whether a substance is easily soluble depends on which of these forces dominate. More precisely they are part of a standard mix of thermodynamic effects: dissolution can be both endo- and exothermic (depending on how strong are intermolecular forces substance-substance, substance-solvent and solvent-solvent) and solution has a higher entropy, ΔH-TΔS.