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That's a really interesting link, but I don't think it's really the same process, or answer my objection. I don't think "melting point" is the right concept for

That's a really interesting link, but I don't think it's really the same process, or answer my objection. I don't think "melting point" is the right concept for the phenomenom of combining water and sucrose crystals. When sugar dissolves in water, it creates a solution, a chemical mixture characterized in this case by ionic bonds between water molecules and sucrose molecules. In general, the melting and boiling points of a solution will not be the same as the melting and boiling points of any of the individual substances that comprise the solution. When I think about combining substances, I think about suspensions, solutions, colloids. (When I think about combining substances, I'm usually thinking about cooking). A suspension is small particles mixed into a liquid, but it isn't stable; after time or with a centrifuge, you can separate them back out. A solution is a chemical bond between individual particles of each substance, such that it is stable. A colloid is small particles mixed into a liquid or gas, but the small particles are so small that it is stable and cannot easily be separated out. Here's the link I used to refresh my memory enough to explain these concepts: https://lab-training.com/understanding-differences-solutions-emulsions-colloids-dispersions/ https://lab-training.com/understanding-differences-solutions... I'm sure you're aware of all that, but I wanted to write it out so I could be more explicit about my confusion. Adding water to solid rock will not result in a solution, colloid, or suspension. Adding finely ground rock and water can do so; that's kind of what mud or clay is. So I was confused how adding water to rock could result in any kind of mixture that changes the melting point of the rock. Jofer's explanation makes sense to me: it's not actually adding H2O molecules to solid rock. Instead, it's that certain minerals under sufficient heat and pressure change mineral/crystal structure in a way that results in the precipitation/expulsion of various molecules and ions, including hydrogen and oxygen. Those precipitates diffuse through the solid until they react and combine with other minerals, causing transformations of those into new mineral/crystal structures with a lower melting point. The link you provided talked about how solutions of sucrose and water in differing concentrations have different melting points, but I was confused about how rock and water could dissolve at all.

pfdietz 4y ago
The bonds between water and sugar molecules are not ionic, they're hydrogen bonds. These are also the bonds between sugar molecules. The general thing that is happening is this: water allows the sugar molecules to separate without paying as large an energy penalty. This means the higher entropy of disordered sugar molecules allows that energy penalty to be overcome at lower temperature (and/or higher concentration, which reduces the entropy increase.) The same thing is happening with water and rock, if not by exactly the same mechanism. Water reduces the energy cost of breaking bonds in the rock (covalent Si-O bonds, not hydrogen bonds). Adding water to rocks does result in a solution. Rock is very slightly soluble in water. It's just that at normal temperature and pressure the solution is extremely dilute. This is because the energy penalty is so high that a very large entropy increase is needed to overcome it, and the entropy is logarithmic in dilution. Raise the temperature a bit and the solution becomes more concentrated. This is how quartz is deposited to grow artificial crystals for electronics: it goes into solution in hot, pressurized water and is deposited on seed crystals. This also happens naturally with various minerals (including quartz.)
- bcbrown 4y ago
  I guess I thought that since salt water solution was ionic, all solutions in water were ionic. Thanks for teaching me something new.

pfdietz 4y ago

The bonds between water and sugar molecules are not ionic, they're hydrogen bonds. These are also the bonds between sugar molecules. The general thing that is happening is this: water allows the sugar molecules to separate without paying as large an energy penalty. This means the higher entropy of disordered sugar molecules allows that energy penalty to be overcome at lower temperature (and/or higher concentration, which reduces the entropy increase.) The same thing is happening with water and rock, if not by exactly the same mechanism. Water reduces the energy cost of breaking bonds in the rock (covalent Si-O bonds, not hydrogen bonds). Adding water to rocks does result in a solution. Rock is very slightly soluble in water. It's just that at normal temperature and pressure the solution is extremely dilute. This is because the energy penalty is so high that a very large entropy increase is needed to overcome it, and the entropy is logarithmic in dilution. Raise the temperature a bit and the solution becomes more concentrated. This is how quartz is deposited to grow artificial crystals for electronics: it goes into solution in hot, pressurized water and is deposited on seed crystals. This also happens naturally with various minerals (including quartz.)

bcbrown 4y ago
I guess I thought that since salt water solution was ionic, all solutions in water were ionic. Thanks for teaching me something new.