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Old 12-13-2012, 11:48 AM
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Why does sugar dissolve easier in warm water than cold

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Old 12-13-2012, 12:00 PM
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a solid (solute) dissolves in a liquid (solvent) when molecules of the liquid associate (bond) with molecules of the solid. this is done molecule by molecule. when the liquid is warmer those molecules move around faster. when the molecules move faster they can associate and move away quicker leaving space for unassociated liquid molecules to interact with unassociated solid molecules.
Old 12-13-2012, 01:02 PM
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Is there not a little more to it than that? Does that explain the higher saturation point of warm solvents?
Old 12-13-2012, 01:25 PM
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Originally Posted by johnpost View Post
when the molecules move faster they can...move away quicker leaving space for unassociated liquid molecules to interact with unassociated solid molecules.
This is a description of diffusion. It does explain why sugar dissolves faster (which may have been what the OP was asking), but it does not explain why a given quantity of warm water can dissolve more sugar than the same quantity of cold water, no matter how much time is invovled.

Can anyone explain the greater solubility of sugar in warm water vs. cold water?
Old 12-13-2012, 03:16 PM
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Quote:
Originally Posted by Machine Elf View Post
This is a description of diffusion. It does explain why sugar dissolves faster (which may have been what the OP was asking), but it does not explain why a given quantity of warm water can dissolve more sugar than the same quantity of cold water, no matter how much time is invovled.

Can anyone explain the greater solubility of sugar in warm water vs. cold water?
My semi-educated WAG is that it's for the same reason - the molecules moving around faster means that at the molecular level there's more mixing going on, so more sugar can be held in solution without starting to cluster together into crystals and precipitate out.

This relationship doesn't always hold, though: http://en.wikipedia.org/wiki/Solubility#Temperature
Old 12-13-2012, 03:42 PM
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Actually, the solubility of salt doesn't change much with temperature, about 10% difference, from 0 to 100 degrees (at ambient pressures). I don't know the reason, but I doubt it's simple because different salts have very different solubility curves. I'd expect simple answers to apply to most of them.

http://en.wikipedia.org/wiki/Solubility

The bottom line is that solubility is due to the difference between the rate of two processes: salt going from crystalline to liquid (dissolved) form, and vice versa. So, solubility is at least as complicated as crystal formation.

Last edited by Learjeff; 12-13-2012 at 03:43 PM.
Old 12-13-2012, 04:02 PM
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IIRC (and I may not) I believe it's due to volume expansion of the liquid as a whole, as the molecules of water are spaced further apart on average with higher temperatures. Thus there's more room for more sugar (or salt or other compounds/molecules) to fit in between the water molecules.

But it's been a loooong time since I took physical chemistry.

Last edited by Qadgop the Mercotan; 12-13-2012 at 04:03 PM.
Old 12-13-2012, 04:44 PM
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(That may be the 1st time I've seen QtheM come in with anything other than a definitive end-all-be-all explanation. Guess the world really IS coming to an end ... )
Old 12-13-2012, 05:01 PM
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I had this vague image that the greater kinetic energy of the molecules of the solvent made them smash into the solute quicker, which. I don't know makes the solute crack into smaller units, and that, by some process other people know, makes them easier to dissolve, accompanied by heat loss.
Old 12-13-2012, 05:10 PM
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I've also found that Splenda dissolves more easily in room-temperature coffee than hot. And hot produces more foam.
Old 12-13-2012, 06:06 PM
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edumacated speculation.

for something to crystallize (come out of solution) you might reduce the solvent (evaporation) and maybe cool it. the solute get to meet up and bond with each other because of closer proximity or less bouncing around.

if you have dissolved sugar in cold water and wanted to dissolve more then you would warm the water.

a warm (faster moving) water molecule might both knock the sugar around more so that getting next to another sugar slow and steady to crystallize would be less likely. also the water holding the sugar apart would be shared more, fewer water molecules would be needed to hold the sugar molecules apart.
Old 12-13-2012, 10:22 PM
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It has to do with entropy, but I'm not sure if I can come up with the why. I can just point to the equation.

dG = dH - TdS

The Gibbs Free Energy is what determines what state is lower in energy. That determines if solubilized or not solubilized is favored state. The Enthalpy (H) is what is defined by the bonding and electrostatic interactions. The entropy is the number of degrees of freedom. For most things, solubilized ions have a greater number of degrees of freedom, so as the temperature goes up, dG gets more negative. Entropy is funny though. An ion could coordinate or organize water in such a way that the solubilized system is actually lower in entropy. This odd situation can lead to cases where the ions are actually less soluble at higher temperatures.
Old 12-13-2012, 11:02 PM
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All I know is most solutes like warm water. There are those that prefer cold water. Calcite has retrograde precipitation (precipitates in warmer sea water.) That's why you have coral reefs only along the warmer, shallower parts of the ocean.
Old 12-13-2012, 11:08 PM
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You've also got the different bonds to consider.

Salt, table salt (NaCl) forms an ionic bond. When it dissolves in water, the atoms separate, so you've actually got a solution of Na+ and Cl- floating around.

Sugar has a covalent bond, so when it dissolves in water, you've got actual complete sugar molecules floating around.

Some substances dissolve easier in cold water, and precipitate out in hot water. Calcium carbonate is the crud that deposits in tea kettles and hot water heaters, because it comes out of the water solution at higher temperatures.

And then there are supersaturated solutions..

Take a chemistry class. You'll learn more than you ever wanted to know!


~VOW
Old 12-14-2012, 12:24 AM
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Quote:
Originally Posted by johnpost View Post
a solid (solute) dissolves in a liquid (solvent) when molecules of the liquid associate (bond) with molecules of the solid. this is done molecule by molecule. when the liquid is warmer those molecules move around faster. when the molecules move faster they can associate and move away quicker leaving space for unassociated liquid molecules to interact with unassociated solid molecules.
Well, than why doesn't sugar dissolve in cold water when you stir it?
Old 12-14-2012, 12:38 AM
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It does, it just takes longer. Like the others here so far, I can't give a definite explanation why most solutions favor a higher temperature. If you take a look at it, the temperature difference between "hot" and "cold" water is pretty small, less than 60 degrees. It is a strange thing. But I think entropy is a good place to start reading.

Still with the calcium carbonate/calcite exception, I also know there is another factor besides retrograde precipitation: carbonic acid concentration. That part of dissolved carbon dioxide dissolved in sea water has a greater concentration in the colder (and deeper) regions. That's another factor for limiting the ranges and depth of coral reef formation.
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