|MadSci Network: Earth Sciences|
The freezing temperature of salt water depends on how much salt you put in. Oceanographers define the "salinity" of water as the number of grams of salt per 1000 grams of water. (Since 1000 grams of water is almost exactly 1 liter, it's also grams of salt per liter).
Here's a table which gives the freezing point of water at a number of different salinities:
Away from rivers and glaciers, the ocean has a salinity of about 35. The Atlantic is about 1 salinity units saltier than the Pacific. So ocean water freezes at about -1.91 degrees C.
The results of your experiment are strange. Usually, fresh water freezes at 0 C. Did you make sure that your thermometer was completely surrounded by the ice? Don't freeze the thermometer into the ice: ice expands when it freezes, and you could easily break the thermometer. Instead, take the ice out of the freezer, put it it a styrofoam cup, and let it melt, and put the temperature in the melt water. (the meltwater will be the same temperature as the ice until the ice has melted completely). If your salt water didn't freeze at -18 C, you must have put an awful lot of salt in! Be sure to measure the amount of salt you put in. As long as the amount of salt isn't too large, you should expect the freezing temperature to drop by .054 degrees for each gram of salt. Try using 35 or 100 grams per liter.
Another thing to watch out for: when salt water freezes, the salt can't join with the ice crystals, and tends to stay in the liquid water. As a result, ice made from salt water has less salt in it than the water that's left behind. The unfrozen water gets saltier and saltier, and eventually becomes so salty that your freezer isn't cold enough to freeze it. So there will be a little bit of very salty brine left in the bottom of the cup. This is called "brine rejection", and is very important for the study of sea ice in the Arctic and Antarctic oceans.
You can do an experiment to demonstrate this. Take a 100 milliliters of water and put about 10 grams of salt in it (if you do the math, this means the salinity is 100). Put it in the freezer until it's half ice, half water. Take it out, and pour the liquid into a cup labeled "brine", and put the ice into another cup labeled "ice". Let the ice melt, and then taste the water in both cups. (only a sip! if you drink a lot, it'll upset your stomach!) The "ice" cup should be a lot less salty than the "brine" cup.
I confess I haven't actually done this experiment, but it should work. You might have to play with the amount of salt to get it to freeze right.
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