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How Ice Melts
Killer Instinct writes "Ever wonder how ice melts? Until now, scientists could not explain why ice cubes in your drink melt. They've known the basics, but the details remained elusive. A breakthrough new study, announced yesterday, supports a leading theory that melting starts when the fundamental structure of matter begins to crack. Melting is considered a basic phenomenon in physics. An understanding of how it works is crucial to gaining a firm grasp on the physical world."
jam it up your ASS !!!
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Dear Cecil:
I have a friend who insists that filling an ice cube tray with warm water will cause the cubes to form more quickly than they would if you started with cold water. He said it had something to do with the air circulation around the trays being affected by the temperature.
Not knowing much about frigidity myself, but being contrary, not to mention skeptical, by nature, I expressed doubt. Cecil, was I right, or is there indeed some basis in fact for this foolishness? --Mary M.Q.C., Santa Barbarba, California
Cecil replies:
You were smart to let me handle this, Mary. God knows what would happen if you tried to experiment with ice cubes on your own.
Needless to say, I conducted my research in the calm and systematic manner that has long been the trademark of Straight Dope Labs. First, I finished off a half a pint of Haagen-Dazs I found in the fridge, in order to keep my brain supplied with vital nutrients.
Then I carefully measured a whole passel of water into the Straight Dope tea kettle and boiled it for about five minutes. This was so I could compare the freezing rate of boiled H20 with that of regular hot water from the tap. (Somehow I had the idea that water that had been boiled would freeze faster.)
Finally I put equal quantities of each type into trays in the freezer, checked the temp (125 degrees Fahrenheit all around), and sat back to wait, timing the process with my brand new Swatch watch, whose precision and smart styling have made it the number one choice of scientists the world over.
I subsequently did the same with two trays of cold water, which had been chilled down to a starting temperature of 38 degrees.
The results? The cold water froze about 10 or 15 minutes faster than the hot water, and there was no detectable difference between the boiled water and the other kind. Another old wives' tale thus emphatically bites the dust. Science marches on.
AN ANOMALOUS SITUATION ARISES
Dear Cecil:
Just a few days after I read your column on whether hot water freezes faster than cold water (you said it didn't), I happened to come across an article in Scientific American entitled "Hot Water Freezes Faster Than Cold Water. Why Does It Do So?" What gives? I hope we will see another column soon resolving the issue. --Ellen C., Chicago
Dear Ellen:
I know it must unnerve you to find that a supposedly infallible source of wisdom can make mistakes, so let me hasten to reassure you: Scientific American did not screw up. My results and theirs (specifically, those of Jearl Walker, author of SA's "Amateur Scientist" column) are consistent--we were just working in different temperature ranges.
I found that cold water (38 degrees Fahrenheit) froze faster than hot water out of the tap (125 degrees F). I chose these two temperatures because (1) they were pretty much what the average amateur ice-cube maker would have readily available and (2) I couldn't find a mercury thermometer that went higher than 125 degrees.
Jearl, who is not afflicted with penny-pinching editors like some of the rest of us, was able to get his mitts on a thermocouple that could measure as high as the boiling point, 212 degrees F. He found that water heated to, say, 195 degrees would freeze three to ten minutes faster than water at 140-175 degrees. (There were differences depending on how much water was used, where the thermocouple was placed, and so on.)
Jearl suggested that the most likely explanation for this was evaporation: when water cools down from near boiling to the freezing point, as much as 16 percent evaporates away, compared to 7 percent for water at 160 degrees. The smaller the amount of water, of course, the faster it freezes.
In addition, the water vapor carries away a certain amount of heat. To test this theory
Stop modding me Insightful. I was fucking joking!
But if people really didn't know that the Celsius scale was defined with 0 as the freezing point of water and 100 as the boiling point; well glad I could be useful. There is no mysterious alien mathematical connection, us humans defined the "connection".
This means that a different process is responsible for macroscopic melting. Since macroscopic chunks of ice tend to be imperfect crystals, it stands to reason that the weak unions between crystalline structures facillitate melting.
After all, I am strangely colored.
This is a particularly pervasive myth. Of course, the folklore is incorrect: according to basic thermodynamics, a quantity of warm water will invariably take longer to freeze than an equal quantity of cold water.
Note that key phrase, "an equal quantity" -- in an experiment with two uncovered containers of hot and cold water, you'll find that the resultant mass of water in each of the containers is anything but: a good deal of water from the hot water container is lost to evaporation. So, with a decreased mass, it's easy for the originally hot water to cool more quickly than a significantly larger mass of cold water.
Essentially, hot water does cool faster than cold water in an uncovered container, but you end up with significantly less ice than if it were originally cold.
It turns out that, at the molecular level, nodody knows the answer to this question, either, especially in the presence of impurities. In fact, in general, the subject of "Phase Change" is something of a black art, full of "empirical models", a great dissapointment for a mind that lusts for explanations in terms of hard mathematics. Unfortunately, as a graduate EE taking this course in Chemical Engineering, my grade reflected my disappointment. (Aside: my grad work was done in connection with the Army Corp of Engineers Cold Regions Research and Engineering Lab, thus my unnatural interest in the topic. As the cold war with the USSR gave way to the hot wars in the Mideast, funding for research in the associated topics has dropped off).
It is called the Mpemba Effect.
More on this phenomenon (history en possible explanations) here
Abstract from the actual Science article:: Much more informative than this silly article. Premelting is the localized loss of crystalline order at surfaces and defects for temperatures below the bulk melting transition. It can be thought of as the nucleation of the melting process. Premelting has been observed at the surfaces of crystals, but not within. We report observations of premelting at grain boundaries and dislocations within bulk colloidal crystals using real time video microscopy. The crystals are equilibrium close-packed three-dimensional colloidal structures made from thermally responsive microgel spheres. Particle tracking reveals increased disorder in crystalline regions bordering defects, the amount of which depends on the type of defect, distance from the defect, and particle volume fraction. Our observations suggest interfacial free energy is the crucial parameter for premelting, in colloidal and atomic scale crystals.
IIRC, the explanation for the ice-cube-trays-in the-freezer 'anomaly' seems to involve the specific temperatures of the two samples, the insulating sides of the tray (minimising heat loss via conduction), enthalpy of vaporisation and the temperature gradient in the water. But don't quote me.
What appears to be a comprehensive exposition on the matter can be found here here.
.:the truth is a lie undiscovered:.
The key is that water has a high specific energy, so it can absorb a lot of energy without actually increasing in temperature. The other types of molecules in your gravy solution can happily be heated to over 212 degrees without boiling; only the water boils. As more and more water cooks out of the gravy, there becomes less water to absorb the energy through evaporation so the energy begins heating the remaining non-water liquid to a higher temperature than water's boiing point.
:-)
This is the entire methodology of fudge making. Create a sugar-water solution. Apply heat. It gets to 212 slowly. Water begins to evaporate. Sugar continues to heat, driving the temperature of the solution above 212 degrees. The less water there is the less resistance there is to moving above 212. At the appropriate temperature (235 degrees; soft ball stage) you remove the solution from the heat and let it cool. You now have fudge. Ideally you would also add corn syrup, chocolate, cream instead of water, butter and vanilla extract (at the end) to improve the flavor. And hopefully you would stir vigorously once it drops below 150 or so so that the sugar crystals that are created are as small as possible and your fudge has a smooth texture.
Caramel is made in the same way. Heat the sugar-water solution to just before the burning point for sugar (around 350), add cream, boil for 3 minutes then cool. Youv'e got caramel!
Mmmmm.... food....
Justin Dubs
If you don't think that matters, then you certainly don't qualify as a "nerd". :P Basic science is all about finding out how the world works, without necessarily having any obvious utility for that knowledge. A couple of days ago was the 100th anniversary of Einstein's publication of the theory of special relativity. Did that "matter" at the time?
You mean Kurt Vonnegut . Strange error to make in a correction :-)
I believe posters are recognized by their sig. So I made one.
meaning that bucket 1 would reach temp 0 cel at half the time
Unless you measured and timed the temperatures of both buckets from start all the way to freezing temperature, you really haven't really timed it..
One should expect a substance will tend to change in temperature faster, the farther it is from the temperature of the environment, and indeed in proportion to that difference, except where the material actually changes states between solid/gas/liquid/etc, the change of temperature is expected to be be a fairly continuous thing predictable thing: if you have an approximation the proper coefficient for the substance, then how long it will take to cool is expected to be fairly predictable, and having a greater temperature normally means it will cool quicker at first and slower later (in the long run, taking longer).
Hot water has to become cold water before it can freeze. It won't "skip temperatures", it will eventually pass through the same temperature your cool water was at.
But 100 degrees is special, because it is water's boiling point, and you ought have tried with hot water that was not boiling so that you could be consistent about the volume of fluid you are working with, without having big portions of the hot water evaporate: it makes a difference how quickly it freezes whether you heat a gallon of water or heat water and then pour out a gallon. (Of course the amount of material matters also)
The temperature of the container matters. Depending on the substance, whether it is a good insulator or not and if it's covered would effect the rate at which the water freezes.
Water needs to be measured out in equal amounts before heating for results to be comparable, since for all you know, water might change density when you heat it.
Depending on how cold it is outside, the difference between hot water and cold water is possibly quite tiny compared to the difference in energy between cold water and frozen water.
One of your buckets could have simply frozen first out of pure luck or small factors other than its starting temperature, or because it was so enormously cold outside that the buckets' difference in temperature was insignificant and less important than the position and direction of the wind with respect to the bucket, for example.
Is the king of Sweden's daughter hot? Prolly.
Yes. Zee for yourzelf. We are proud of her.
http://www.princess-madeleine.com/
You are right in the broad overview, but wrong on the details. The boiling temperature of a mixture is not necessarily due to the boiling temperatures of its two components. The boiling temperature of a solution is not a linear combination of the boiling temperatures of its constituents. It's often close, which is why we have Raoult's law (although it technically deals with vapor pressure, not boiling temperature).
Ethanol and water, for example form an azeotrope, a constant boiling solution, at something like 96% ethanol. It's why you can't distill alcohol to 100% purity. At the boiling temperature of the azeotrope, ethanol and water molecules are evaporating at the same rate, even though the solution is not at the boiling temperature of either.
Actually, explaining various behaviors of water is one of the hardest problems remaining in science. Water is less dense when frozen (ice floats) but this is the opposite of most solid/liquid pairs. Water has a 'critical point' at 4C, where its density is at a maximum (even more impressive is that it has a density maximum and minimum within 4C!) If you remember chemistry, you always treated acids as making H+, or maybe they were more rigorous and wrote H3O+. In actuallity, we don't know how many water molucles are surrounding the H+ from the acid - I've seen papers that strongly suggest 13, and others suggesting more and less. Because of its small size, water molcules exchange very quickly - I can't recall the exact value this early on a Saturday, but if a particular water molecule stays around an object for more than a nanosecond, it would be the first time - waters exchange on the order of femtosconds (10^-12 s). Large dipole, small molecule, hydrogen bonding, big symmetry - all this makes water one heck of a wierd special case.
Be careful of your thoughts; they could become words at any minute...
Its called the Leidenfrost effect
I meant something stronger than mere thermodynamic reversibility. There are many ways for water to freeze and ice to melt, and as long as the end results are the same, they're thermodynamically equivalent. The research presented here examines the reasons why one path is chosen over another. Instead of dealing with statistics, the researches are investigating particulars.
After all, I am strangely colored.