Food "science"

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OK, this is just sweet. Literally.

One thing doesn't sound entirely right to me, though:

After fielding guesses about carbonation and evaporation rates, Dr. Jellinek explained: "You have an ice cube, and the ice cube is in contact with water or alcohol. The first thing to realize is the ice is melting because there is heat transfer from the water to the ice."

Colder water is denser, so water from a melting cube sinks, stirring up warmer water, which causes the ice cube to continue melting. Alcohol, however, is less dense than water, so the water from the cube sinks faster in the Scotch, stirring the water more vigorously and causing the ice cube to melt more quickly.

Dr. Jellinek then dropped two ice chips, roughly the same size, one into a cup of water, the other into Bell's Scotch Whiskey. "The main thing is what's coming off the bottom is superfast," Dr. Jellinek said, pointing to the ice and Scotch. "Just in the last 30 seconds, this guy is now 30 percent smaller than the one in the water."

Solutions exhibit a phenomenon called freezing point depression. The freezing point of a solution of alcohol and water is substantially lower than that of pure water. That's why you can keep vodka in your freezer without having it freeze. So, the effect of dropping an ice cube in whiskey is like putting salt on ice--you get fast melting. 1 It's not clear you need to invoke convection to explain this phenomenon.

1. Note that this is a general property of solutions, not one that depends on the freezing point of the solute. After all, NaCl is solid at room temperature but NaCl in water exhibits more freezing point depression than alcohol in water because the NaCl dissociates when dissolved, providing twice as many ions in the water.


Yes, you have freezing point depression... but the convection makes ice even faster in a solution that has a substantially different density than plain water.

So far as i can tell, Ekr is right. I see no reason to invoke convection at all.

Try it - an icecube in a glass of water, one in a glass of salt water, and one in alcohol.

Salt melts ice by preventing any melted ice from refreezing. But it doesn't work if no ice melts at all -- in really cold temperatures salt is of no use and you have to switch to calcium chloride or something else with an exothermic reaction.

Salt water will melt the ice cube a bit faster than water, because it prevents the small amount of refreezing that normal water will do. But it won't work as fast as alcohol. Whatever solution you put the ice cube into doesn't change the melting point of the ice itself, only the liquid around it. So to melt the cube you have to transfer heat to it.

Also note you need a generous amount of water & alcohol in the test, to negate the difference in specific heat between the two. Otherwise the alcohol will cool to a much lower temperature than the water does, slowing the rate of heat transfer.

Isn't there some heat released when you add water to alcohol (or vice versa)? This is due to the entropy increase. I wonder if melting ice in alcohol generates heat in this way that would not happen when melting in plain water.

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