Adding xylitol to hot water lowers the boiling point and makes it boil

[Jonathan212]

This probably occurs with salt too but let's concentrate on xylitol: you heat water till it boils, put the hot water in a cup and add a tsp of xylitol. It boils again violently for a couple of seconds. The high school explanation is that the boiling point of the water is lowered. I'd like to know what happens in terms of energy, and at the molecular level: as in, you have 1000 molecules of water vibrating and a molecule of xylitol gets close. Why would the neighbouring water molecules vibrate more next to xylitol?

 

[DrClaude]

You can't really get a good picture of collective effects (like phase transitions) by focusing on individual molecules.

What is at play is the Gibbs free energy (G) of the different phases. Water will go from liquid to gas at 100 °C because at that point the G curves of the liquid and gas cross, and G_gas < G_liquid for T > 100 °C.

Adding a solute only affects the liquid phase (since the solute will not be present in the gas), so it changes the point where the curves cross. See the nice graphs at
https://chem.libretexts.org/Bookshe..._Some_Applications_of_Entropy_and_Free_Energy

 

[symbolipoint]

One might guess that some energy is released when dissolving xylitol in water, raising the temperature of the water solution. Enough to make liquid reach the boiling temperature for the mixture? Not sure myself.

I would imagine better, that adding any solute to solvent (water) would INCREASE the boiling point and DECREASE the freeze point. We can wait for other members who have a better familiarity with xylitol-water solutions.

 

[Jonathan212]

Exactly the same thing happens with salt. Is it really a physics problem if you're trying to predict what will happen if a single molecule of salt falls through air molecules till it hits water molecules?

 

[Borek]

It is more about activation energy required for bubble building, and not limited to xylitol.

Saturated vapor pressure depends on the interactions between liquid molecules. When the phase interface is flat, molecules interact with others occupying exactly half the full solid angle. Depending on whether the surface is convex or concave there are more or less intermolecular interactions and saturated vapor pressure goes up or down (think capillary condensation).

When you add any solid into the water it initially has plenty of convex and concave places, were water gets curved, hence it starts to boil quickly.

If you have a good stainless steel pot with smooth walls put some water into, heat it till it almost boils, then add a pinch of salt. Pufff! And if you keep heating, adding rice or groats later will produce the same effect again. Just keep some rag ready

Edit: Jonathan212 beat me about the salt part.

 

[Jonathan212]

What exactly does dissolving mean when we're talking about xylitol? It does not turn into ions does it?

 

[Borek]

No it doesn't. Dissociation is not an obligatory part of the dissolution process.