Is honey really a supersaturated solution? Does heating to un-crystalize redissolve it or melt it?

Question

In the SciShow video Honey: Bacteria's Worst Enemy after about 00:30 the narrator says:

Honey is only about 17% water. Most, but not all of what remains is sugar. The two main types of sugar in honey are glucose and fructose. Like all sugars, glucose and fructose are sticky — they attract water.

Honey is technically a supersaturated solution, meaning it contains more sugar than would normally dissolve at that temperature. That’s why it eventually gets all crystally in the pantry — over time, sugar comes out of the solution.

In my experience when old honey "gets all crystally in the pantry" I've placed the jar in a hot water bath, and eventually the honey's viscous liquidy consistency is restored.

When I do this, am I redissolving the sugar, or melting it?

I'm asking because even hot, with only 17% water it's hard to imagine it can become an unsaturated solution.

Answer 1

Honey is indeed a complex mixture containing more than hundred compounds.

As for Wikipedia and depending on the point of view it is

• a supersatured liquid solution

• a viscous supercooled liquid (in the sense that it can get so viscous as to appear solid, without affecting its status of being a supersatured solution, and undergoes glass transition).

https://en.wikipedia.org/wiki/Honey#Physical_and_chemical_properties

The facts that you describe in the question are less surprising if we consider that:

• with respect to sugars crystallization, of which at least there are two different ones, glucose and fructose, the rest of the non-sugar components must be considered to be impurities

• impurities, even in traces, often hamper the crystallisation of a compound, even in simple mixture of a single compound and the above traces. This is common after organic synthesis, in which often a viscous "oil" is attained that might crystallize only upon prolonged storage or a careful removal of the disturbing trace compound(s)

• finally and most important, the solubility of sugars in water is very high, and very sensitive to temperature. For instance, at room temperature glucose is already soluble in the reason of 90 g per 100 ml of water, that means a saturated solution already contains about fifty percent weight per weight of sugar

A table is here (I didn't cross check the values):

http://www.mpcfaculty.net/mark_bishop/supersaturated.htm

All this, viscosity included, makes the attainment of a supersatured solution particularly easy, as in the kitchen in the case of sucrose:

https://sciencing.com/make-supersaturated-solution-sugar-6199355.html

As such, heating crystallized honey does indeed dissolve the sugar, and a supersaturated solution is attained upon subsequent cooling.

Answer 2

Melting and dissolving are all the same when you look at mixtures close to saturation.

You can say water lowers the melting point of the sugar, or that the solubility of sugar increases with temperature. Different description, same fact.

What makes this seem different from e.g. a salt water solution is that the molten (i.e. non-crystalline) sugar is fully miscible with water, which is only possible because the m.p. of sugar is not so far above the Bp. of water. If you go to high pressures (like in earth's mantle), the situation between (supercritical) water and rock is probably very similar.