Why Does Sugar Water Freeze Faster Than Pure Water?

[daniel3777]

when an ice berg freezes in salt water, the ice is actually pure water because the freezing water kind of pushes the salt out. so why does the suger in koolaid freeze with the water. i asked my chemistry teacher and he looked for the answer but he doesn't know why. this may seem like a stupid question but it is really bugging me.

 

[GCT]

Salt and water are stabilized through ion dipole-dipole forces. At low temperatures, that is with a decrease in kinetic energy, hydrogen bonds will predominate. It is also very difficult for a salt-water to freeze simply due to the ardous process of forming a stable crystal structure from a relatively disordered molecular liquid arrangement.

However, water and sugar can form hydrogen bonds. Therefore it relatively little decrease in kinetic energy for intermolecular bond formation. Also the crystal structure relatively easier to form. Nevertheless there will still be a freezing point depression.

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[daniel3777]

thanks, you're so cool

 

[Bystander]

Originally posted by daniel3777
when an ice berg freezes in salt water, the ice is actually pure water because the freezing water kind of pushes the salt out. so why does the suger in koolaid freeze with the water. i asked my chemistry teacher and he looked for the answer but he doesn't know why. this may seem like a stupid question but it is really bugging me.

Simple --- the sugar in koolaid does NOT freeze with the water. For your iceberg, you have three phases, solid water, a more concentrated salt solution, and if concentrated enough, solid salt. The solid phases have sufficiently different densities to settle out or float, and the liquid phase is low enough in viscosity that the solids can separate from it. The koolaid? You get solid water in a network of needles in whatever mold you're using to make popsicles; you get a more and more concentrated sugar syrup that has a viscosity that increases with concentration "poured" over the ice crystal network, and at low enough temperatures a "semi-solid" solution of water in sugar, wet sugar plus food coloring. Does it separate into two separate blobs, ice on the top and sugar on the bottom? Eventually, if you cycle your popsicle up and down in temperature without ever exceeding a temperature 2-3 degrees below the freezing point of water --- go check your mother's/g-mom's/friend's freezer for the two year old package of popsicles and look at the contents --- stickly, gooey syrup on one side and colorless ice on the other.

 

[GCT]

Frozen Koolaid is not a "semi-solid." A semi-solid indicates a colloid. A frozen popsicle (which is easier to break apart than frozen koolaid; frozen koolaid, even concentrated frozen koolaid is "solid" as an ice cube) is easier to break apart, nevertheless it is still a solid. This solid consists of different intermolecular bonds which are weaker than water-water hydrogen bonds.

Of course the molecular bonding structure is not extensive throughout the substance; not similar. With time, water will be able to overcome the unfavorable change in entropy to form a more ordered structure of pure frozen water.

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[radagast]

While everything written is accurate, it does leave out some important aspects. With water freezing onto an iceberg, you have a strong equilibrium situation where as the water molecules form fixed hydrogen bonds, it does so over an extended period of time such that they have enough time to squeeze out the Na and Cl ions. It uses the same mechnism as zone freezing separation techniques. Koolaid, in the freezer freezes relatively rapidly, without a prolonged equilibrium situation, such that sugar and flavors are trapped in the forming ice crystals.

If you've ever kept freezer pops (basically koolaid in long clear plastic package/container), that's refrozen a number of times, the sugar and flavor will concentrate at the end of the freezer pop that always freezes last.

 

[GCT]

Yes, it takes time for the iceberg to form and I accounted for this by mentioning entropy; rate of reaction is slow due to the arduous process of forming an ordered solid while the solution itself is relatively disordered. As for koolaid or another sugar-water solution, crystals form faster due to intermolecular hydrogen bonding which makes it possible for the solution itself to exist as a solid. The solution itself is relatively disordered as compared to a pure frozen water. Relative entropy as well as the strength of intermolecular attraction account for the differences.


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