So my father and brother-in-law were over this weekend for a barbecue. My fridge was stuffed with appetizers and salads so I was only able to fit a 12-pack of beer. Normally, this would have been enough so I didn't worry about it. After a couple hours, the stash was depleted.
I did have some more beer in the garage but the 90+ degree Chicago heat had rendered it useless for at least an hour. Now what? These guys all had designated drivers and, in all honesty, they wanted more beer. Little did I know, the old man had a trick up his sleeve that I had never heard of. If I had a video camera ready, I would have taped this because it was pretty cool (sorry, no pun intended there).
Here is how he took beer from 80+ degrees to (seemingly) 40 degrees in about 3 minutes.
1. He took 6 hot beers from my garage and he placed them into a steel pot from the kitchen
2. He tossed in enough ice cubes to completely cover the beer
3. He then filled the pot with water
4. Next, and this is the trick, he tossed in (what must have been) 2 cups of table salt.
5. He took a large wooden spoon and stirred this thing up to be sure the salt dissolved.
6. He placed the concoction into the freezer and in 3 minutes we had ice cold beer.
Frankly, I wish I knew about this little trick years ago. Apparently this works for wine, soda, or anything. The addition of the salt does something that I am admittedly not qualified to explain. If we have any experts that want to weigh in, feel free. I do however know that this works.
The salt Elevates the melting point of the ice.. Raoults Law.
ReplyDeleteFreezing point depression is very similar to boiling point elevation. By dissolving a solute in a solvent, the freezing point of the solution is lowered. The reason for this lower freezing point is that when a liquid freezes, the molecules form a pure solid sample of solvent. Those molecules still in the liquid phase still have solute molecules at the liquid-solid surface, so more molecules will go from solid phase to liquid phase. Really, it's the same reason that a solution's vapor pressure is lowered (and hence boiling point is elevated). The same happened with the Ice in the steel pot.. it dosen't allowed to melt it even at 3 to 5 degree centigrade .. and your pal stirred it to keep the flow of the cold temperature uniform and agile..
Since the physical reasoning is the same, you can correctly assume that the mathematical formula for freezing point depression is also the same as for boiling point elevation:
Dtfp = Kfp·m·i
The first thing you should note is that since the freezing point is depressed, Dtfp is going to be a negative number. The freezing point depression constants (Table 14.4, page 673) are listed as negative values to show that the freezing point is depressed. The rest of the factors in the freezing point equation are the same as for the boiling point equation.
Freezing point depression is a solution property that is very useful today when we don't want to be inconvenienced by cold weather. Sprinkling salt on icy roads clears up the ice, because the salt lowers the freezing point of the ice. If you think about the equation, you should also be able to figure out why calcium chloride (CaCl2)is used to de-ice roads rather than sodium chloride. Calcium chloride has a van't hoff factor of 3, as opposed to sodium chloride's van't hoff factor of 2. So, one mole of calcium chloride is 1 1/2 times as efficient at lowering the freezing point as sodium chloride.
Antifreeze in your car engine is another prime example of an application of freezing point depression used today. A mixture of ethylene glycol and water in your radiator both protects your engine from freezing and boiling. The concentration of ethylene glycol required to lower the freezing point to -10°C is 5.38m.
So, how do you know that your antifreeze will protect your car in the winter? You can have your antifreeze tested at any garage, be it your local mechanic or the Jiffy Lube down the street. The tester has a chamber with some indicator in it and a rubber bulb to suck up solution. The indicator is comprised of a dial or a bunch of plastic beads. The floating beads indicate how much protection your solution provides. This indicator works, of course, by density. The higher the density of solution, the more beads float (the beads have different densities), and the greater degree of protection from freezing for your engine.
Thanks and Regards
Ranabir Bhattacharjee - India