Why can't there be salty ice?

sequoianoir

When water freezes, it excludes almost all the salt, leaving behind either little pockets of very salty brine or, at low enough temperature, little salt crystals. Any salt on the ice will make it begin to melt at a somewhat lower temperature than freshwater does. The initial melting temperature will be -21 degrees C if there are some actual salt crystals around. Salt on a small block of ice will cause the whole block to melt at -21 C, before all of the salt even dissolves. If you have a big block of ice, the salt water will start to get diluted, more like pure water. How warm it has to get before the last bit of ice melts depends on how much ice there is, because that's what determines how dilute the saltwater gets. With a very small amount of salt on a very big block of ice, it won't finish melting until it's almost at the freezing point of pure water, 0 degrees C. At any temperature in between, some of the water will be in the salt solution and some in the frozen ice. Aug 08 03, 1:54 PM

sequoianoir

Two things happen when ice and water are placed in contact: Molecules on the surface of the ice escape into the water (melting), and molecules of water are captured on the surface of the ice (freezing). When the rate of freezing is the same as the rate of melting, the amount of ice and the amount of water won't change on average (although there are short-term fluctuations at the surface of the ice). The ice and water are said to be in dynamic equilibrium with each other. The balance between freezing and melting can be maintained at 0°C, the melting point of water, unless conditions change in a way that favors one of the processes over the other. The balance between freezing and melting processes can easily be upset. If the ice/water mixture is cooled, the molecules move slower. The slower-moving molecules are more easily captured by the ice, and freezing occurs at a greater rate than melting. Conversely, heating the mixture makes the molecules move faster on average, and melting is favored. Adding salt to the system will also disrupt the equilibrium. Consider replacing some of the water molecules with molecules of some other substance. The foreign molecules dissolve in the water, but do not pack easily into the array of molecules in the solid. Then there are fewer water molecules on the liquid side because the some of the water has been replaced by salt. The total number of waters captured by the ice per second goes down, so the rate of freezing goes down. The rate of melting is unchanged by the presence of the foreign material, so melting occurs faster than freezing. Aug 10 03, 6:22 AM

sequoianoir

To re-establish equilibrium, you must cool the ice-saltwater mixture to below the usual melting point of water. For example, the freezing point of a 1 M NaCl solution is roughly -3.4°C. Solutions will always have such a freezing point depression. The higher the concentration of salt, the greater the freezing point depression. But won't any foreign substance cause a freezing point depression, according to this model? Yes! For every mole of foreign particles dissolved in a kilogram of water, the freezing point goes down by roughly 1.7-1.9°C. Sugar, alcohol, or other salts will also lower the freezing point and melt the ice. Salt is used on roads and walkways because it is inexpensive and readily available. It is important to realize that freezing point depression occurs because the concentration of water molecules in a solution is less than the concentration in pure water. The nature of the solute doesn't matter. One might expect that solutes with large molecules are better at blocking water molecules travelling towards the surface of the ice. The hypothesis that solutes with large molecules cause a larger freezing point depression than those with smaller molecules is not in accord with experimental data! The misconception arises because any diagram can't be drawn to scale; the size of the molecules is very small compared to the distance between them. Aug 10 03, 6:27 AM

sequoianoir

As ice begins to freeze out of the salt water, the fraction of water in the solution becomes lower and the freezing point drops further. This does not continue indefinitely, because eventually the solution will become saturated with salt. The lowest temperature possible for liquid salt solution is -21.1°C. At that temperature, the salt begins to crystallize out of solution (as NaCl·2 H2O), along with the ice, until the solution completely freezes. The frozen solution is a mixture of separate NaCl·2H2O crystals and ice crystals, not a homogeneous mixture of salt and water. This heterogeneous mixture is called a eutectic mixture. Aug 10 03, 6:27 AM