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Facts and fiction about food and cooking, by Peter Aitken
Cold water boils faster than warm water
April 4, 2011Posted by on
This is another myth that falls into the “suspend the laws of physics” category. That hasn’t happened yet in my kitchen, and if it has in yours then you can probably get on TV. Seriously, to illustrate how ridiculous this idea is without getting into physics and formulas, think of it this way. If you put cold water on to boil, at some time before it boils the water will have become warm. Let’s say it takes time “A” for the water to go from cold to warm. Then after some additional time it will boil – call the time it takes to go from warm to boiling “B”. So, the time it takes the cold water to boil is “A + B” and the time it takes the warm water to boil is “B.” If this myth were true then time “A + B” would be less than time “B” and there’s just no way this could be no matter how many martinis you’ve had.
A variation of this myth claims that cold water boils faster than warm water if it has been boiled previously and then cooled off. The “explanation” in this case is that the first boiling drives dissolved air out of the water, which is true enough. However, dissolved air does not affect the boiling of water, at least not in any significant way, so this one is nonsense too.
Apparently this myth has its origins in the fact that cold water heats faster than warm water. A pot of water at 40 degrees will reach 60 degrees faster than a pot of 7o degree water will reach 90 degrees, given the same heat source. This is because the rate of heat transfer is proportional to the temperature differential between the heat source and the item being heated. But the cooler water will always take longer to boil.
It is true, however, that warm water sometimes freezes faster than cold water. This happens only under very specialized conditions, and has nothing to do with boiling water.
Note, however, that many people generally avoid using hot tap water for cooking because hot water may not be as clean from sitting in the water heater or from leaching substances from the pipes (a particular worry in houses with old plumbing).
It’s really not specialized conditions needed for hot water to freeze faster than cold. Mind you, we’re talking a very minor time difference. I think in our science lab it was never more than 3 minutes. The molecules in hot water move slower than in cold, which is why this happens.
Now, you can’t take gallons of hot water and throw it in the freezer to get this result as this will raise the temperature of your freezer, but for a single tray of ice cubes it does work.
Thanks for your comment. The reasons are actually related mostly to cooling by evaporation. It is well explained here: http://www.scientificamerican.com/article.cfm?id=is-it-true-that-hot-water
If you have bad tap water, it is possible that hot tap water has enough dissolved material in it to elevate the boiling point, which could conceivably make cold water boil faster. It does seem like that would be unusual though.
You should start with cold water for anything where water quality is important (tea, coffee, etc) to minimize the amount of dissolved metal ions.
I am not going to get into the chemistry and physics of the water add how it boils.(boring for a lay person that is not an engineer).. But there are characteristics that effect how water boils.. including the mineral content, what type of mineral contents elevation in which you live, etc….I would still recommend using cold water (depending on the age of your pipes in you house) to reduce the sediment that is dissolved in your water.
This “myth” actually has more than a grain of truth in it. You should read about the Mpemba Effect: http://en.wikipedia.org/wiki/Mpemba_effect
It has been observed a number of times, and, while it’s certainly not true that cold water will always boil faster than hot water at all, it has been seen enough times that scientists know that the results aren’t a fluke.
You seem to be confusing two totally different things. As you note, the Mpemba effect has been observed under certain circumstances, but that’s warm water freezing faster than cold, which has nothing to do with cold water boiling faster than warm water.
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I have never heard claims that cold water boils faster than hot water, but cold water does absorb heat faster than cold water. Actually the temp of the water doesn’t really affect how long it takes to boil (as I demonstrated in 78th grade science fair). So, filling your chafing dishes with hot water does not heat tour good faster or save stereo. If you start with hot water, the sterno will just produce heat and until that heat collects and finally exceeds the temp of the water, the water will just sit there and the stereo will just use fuel. If the waters in the chafing set is cold, it will start absorbing that heat from the get go.