h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hey, If anyone knows offhand the freezing point of saltwater, i.e. the temperature it has to be at to turn into a chunk of ice, please humour me and give me this paltry scrap of info that you alone posess. thank you in advance.

It depends on the salinity, but water with a similar concentration as sea water freezes at about -3 C , I think. Hope this helps....
BCNU - Crescent

Can anyone tell me why its less than 0 C??? I need it for my physics homework...

Simply put, when there is a nonvolatile solute added to a solution, the vapor pressure of the solution will be lower than that of the pure solvent. However, it is usually the pure solvent that will freeze first (the frozen ice on seawater is relatively pure water). For that reason, the vapor-pressure curve for the solid (pure ice) is unchanged. So, the point where the solid vapor pressure curve meets the solution vapor pressure curve is the new freezing point of the solvent out of the solution. Some chemical separations are accomplished this way.

It's important to note, however, that the lower freezing point of salt water (and thus seawater) is due to the *concentration* of ions from the dissolution of the nonvolatile solute, sodium chloride. In English that means when you dissolve nonvolatile salts (of any kind, not just sodium chloride) or ionic compounds in water, the freezing point will be lowered. This is why throwing rock salt on your driveway will melt the water, and also why you can use salty water cooled to a temperature below the normal freezing point of water to make ice cream.

For dilute solutions, there is a simple expression for determining the freezing point depression given the molal concentration of the solute. For water, this can be written as T(freezing) = T(normal freezing) - 1.858 * c(m) ... where c(m) is the molal concentration and the constant K(f) (the 1.858 for water) is in degrees Celcius per molal of solute. It's important to note, however, that it isn't just the amount of salt that is added, but the molal concentration that effects the temperature depression. Therefore, there are some salts that are better than others at creating a temperature depression. Some substances, like ethylene glycol (a common ingredient for automotive "anti-freeze" compounds) have greater molar mass than, say, sodium chloride, so they will provide less freezing point depression per gram. However, ionic solutions will also have boiling point elevation which is another colligative property of solutions. Sometimes, this can be more useful than freezing point depression, as in the case of anti-freeze (which is sort of a mis-nomer, as it's more often-used function is anti-boiling, or as many put it, "coolant").