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Post by GTCGreg on Nov 11, 2012 4:05:36 GMT
{Reposted here from Kiddies Corner; citadelofmyths.freeforums.net/index.cgi?action=display&board=kidstuff&thread=142&page=1 - CM}
*All experiments posted in this section should be done with adult supervision in a controlled environment and with reasonable safety precautions.* As this was one of the most frequently posted questions on the old Discovery boards, I thought I might bring it here and explain how I tested it. The results were surprising. I took 2 regular sized coffee cups, filled one with 100 degree water, and one with 68 degree water, and put them in the freezer, on the lower shelf, dead center in the middle. Then I took the temperatures every 20 minutes. If you do this test, I'd love to see what results you get.  Well, I don't know what your results were, but I did a similar test almost 4 years ago and had posted the results on the old board. In my test, I took 3 equal sized plastic glasses (about 8 oz. each)of water. One was filled with cold water (around 40ºF.) The second with room temperature water at about 68ºF and the third with 120º water. I put all there in my freezer which was at 1ºF. I checked on the glasses every half hour. As expected, the cold water cup froze first followed by the room temperature cup and finally the 120º glass. There were a couple of others that also tried it. I think that crazy German (C128) was one of them. We all had similar results. The hot water took the longest to freeze. BTW, I'm just kidding with my comments about C128. We stay in frequent email contact.
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Post by blazerrose on Nov 11, 2012 5:16:13 GMT
I found that the cold water froze first, but it was interesting to see how quickly the hot water cooled. If I were to have charted them, I would have noticed a completely different arc for each temp.
About an hour in, the hot water cup was down to 42 degrees, and the cool water cup was frozen on the top - I had to break through a layer of ice to get the thermometer into the water, and the underlying temperature was 37 degrees.
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Post by GTCGreg on Nov 11, 2012 14:36:38 GMT
I found that the cold water froze first, but it was interesting to see how quickly the hot water cooled. If I were to have charted them, I would have noticed a completely different arc for each temp. If you had plotted them, what you would have observed is that the hot water and the cool water would have dropped at the same rate until the cold water just started to freeze. While freezing is taking place, the temperature stops to drop until just about the entire container of water is completely frozen. At that point, the frozen water's (ice) temperature will again continue to drop. While the cold water's temperature is holding during the freezing process, the hot water's temperature continues to drop. Once the hot water gets cold enough to start freezing, it's temperature will also stop dropping until the freezing is complete. If the temperature of the two containers were not taken often enough, it may appear that the hotter liquid was cooling at a faster rate. Except for the possibility of very slight additional cooling by evaporation of the hotter water, they cool at the same rate and any evaporative cooling effect would fall off very rapidly as the hot water cooled off. The opposite happens when ice melts. If you were to plot the temperature of an ice cube heating up, you would find that the ice heats at a linear rate until it hits about 32ºF (0ºC). At that point, the temperature holds until the ice is completely melted. Additional heat energy is absorbed by the ice when it changes state from a solid to a liquid. It's the same energy that was released while the water was freezing and kept the water's temperature from dropping during the freezing process. This is why ice is more effective at keeping a drink cool than something that doesn't change state (melt) such as a cube of metal the same weight as the ice cube. The ice will cool the drink faster and maintain the cool temperature longer because it is absorbing more energy as it melts. Here's a graph of what I'm talking about:  |
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Post by GTCGreg on Nov 11, 2012 14:46:13 GMT
I believe Mpemba effect is really more sloppy experimentation than actually hot water freezing faster. The Wicki article you link to seems to agree. Here's the conclusion from that article. "Although widely mentioned there are very few, if any, modern descriptions of exactly what the effect is and how it may be observed. It is not clear whether 'freezing' refers to the point at which water forms a visible surface layer of ice, or the point at which the entire volume of water becomes a solid block of ice. Some experiments have instead measured the time until the water reached 0 °C.[5]
There are no reliable sources that indicate exactly how to demonstrate the effect and under exactly what conditions it occurs."
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Post by GTCGreg on Nov 12, 2012 1:07:27 GMT
Privatepaddy, I didn't intend to imply that you believed this. It's just that everyone who says hot water freezes faster always points to this Mpemba effect like it's a proven fact. It's not.. When we first started discussing this on the MB boards almost 5 years ago, I spent a fair amount of time trying to track down any real information on experiments that tested it. There are very few and most of the few that did try to verify the effect were so full of holes that the information was pretty much useless.
With that said, maybe this would be a good one for the Mythbusters to test. That is, if they ever return to testing myths again.
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Post by blazerrose on Nov 12, 2012 4:24:03 GMT
It'd be perfect for the next mini-myth special.
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Post by freegan on Nov 12, 2012 11:02:16 GMT
It'd be perfect for the next mini-myth special. Or for a themed show on the subject of "Ice", along with the Ice Cannon. |
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Post by cijay on Nov 12, 2012 17:18:27 GMT
Hey! This is supposed to encourage kids to do their own experiments, not give them results!
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Post by freegan on Nov 12, 2012 17:30:32 GMT
Don't disillusion the poor kids. They already know that the grown-ups always steal the best toys.  |
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Post by Cybermortis on Nov 12, 2012 19:32:09 GMT
I thought that the myth as so often posted on Discovery was that hot/boiling water would freeze faster if a cup of it was thrown into the air in sub-zero temperatures?
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Post by blazerrose on Nov 13, 2012 1:30:19 GMT
That's a corollary one. The aerosolization that occurs when the water is tossed in the air would make it almost a moot point when it comes to freezing.
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Post by freegan on Nov 13, 2012 2:43:07 GMT
I have not conducted this experiment myself but were I to I would probably use PT100 temperature probes one for each cup and one for the freezer so the temperature could be read remotely without opening the door. Also if the freezer is a fan forced variety I would place the experiment so that forced convection was not a factor. Good plan. Do those probes connect to a laptop for continuous data acquisition? Plus, perhaps, a feed from a thermal camera to show the temperature gradients and convection patterns in each cup. (May need to put the camera in the freezer with the cups but I'm unsure how the electronics would cope with the lower temperatures in the range of freezer settings that could be tested.) |
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Post by srracing on Nov 13, 2012 2:48:47 GMT
I don't seem to ever see any controls on the experiment (Mpemba effect)
A few concerns.
1. Hot water is less dense than cold water, thus mass per cup is less.
2. The hot water will evaporate faster initially decreasing mass even more.
3. How is the hot water heated? In the process of heating tap water, some impurities will distill out and with some water evaporating, some minerals will increase in ratio.
SO the comparisons of "hot water" to cold water are not equal.
Allowing for those things, it does not surprise me at all that hot water could freeze faster than cold water under the right circumstances. If the two waters were identical, other than the temp, IMO the effect is a myth.
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Post by freegan on Nov 13, 2012 3:04:50 GMT
I don't seem to ever see any controls on the experiment (Mpemba effect) A few concerns. 1. Hot water is less dense than cold water, thus mass per cup is less. 2. The hot water will evaporate faster initially decreasing mass even more. 3. How is the hot water heated? In the process of heating tap water, some impurities will distill out and with some water evaporating, some minerals will increase in ratio. SO the comparisons of "hot water" to cold water are not equal. Allowing for those things, it does not surprise me at all that hot water could freeze faster than cold water under the right circumstances. If the two waters were identical, other than the temp, IMO the effect is a myth. 1. Can be compensated for by weighing the samples to ensure equality. 2. Can be compensated for with lidded cups. 3. Can be compensated for by using distilled water. If no Mpemba effect is seen with all 3 compensations in place, the experiment can be repeated with one compensation removed to identify the predominant influence. There shouldn't be any reason for the MythBusters to not get a definitive result. They might even get published or at the least referenced, in a professional journal. |
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Post by GTCGreg on Nov 13, 2012 15:19:52 GMT
I think the idea that, under exactly the right conditions of initial temperatures and environmental and equipment conditions, a container of hot water may freeze sooner than cold water, turned into the myth; hot water freezes faster than cold water.
It's similar to the conclusion that one defective cell phone overheats while charging turns into; Answering your cell phone while it is charging will cause it to EX P L O D E
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Post by the light works on Nov 13, 2012 15:49:14 GMT
I thought that the myth as so often posted on Discovery was that hot/boiling water would freeze faster if a cup of it was thrown into the air in sub-zero temperatures? that may be the ORIGINAL terms of the demonstration - but you know how things get skewed with the short attention spans people have nowadays. |
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Post by srmarti on Nov 13, 2012 18:33:50 GMT
This was posted to death back on the MB forum. My best guess on this was back in the days of metal ice cube trays and freezer shelves there was some grain of truth here. The hot water would melt the frost on the shelf, make better contact for better heat transfer to chill the initially hot water faster and make it freeze faster.
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Post by the light works on Nov 13, 2012 19:00:29 GMT
This was posted to death back on the MB forum. My best guess on this was back in the days of metal ice cube trays and freezer shevles there was some grain of truth here. The hot water would melt the frost on the shelf, make better contact for better heat transfer to chill the initially hot water faster and make it freeze faster. again, possibly skewed. Heloise (as in hints from) said to use hot water in ice cube trays because the hot water would freeze CLEAR, and make prettier ice cubes. |
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Post by Cybermortis on Nov 13, 2012 19:03:41 GMT
Interesting, this might mean that the original claim was that warm water made 'better' ice - which is something MB could test while they are doing this.
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Post by the light works on Nov 13, 2012 19:31:14 GMT
indeed
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*All experiments posted in this section should be done with adult supervision in a controlled environment and with reasonable safety precautions.*
