Slashdot Mirror
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."
No.
A guy walks into a bar... well, I forgot the joke, but the punchline is that he's an alcoholic.
I guess I thought we woulda had this one nailed down by now! What will scence reveal that we don't know next?
I am not left-handed, either!
I can finally sleep at night!
This is somewhat akin to boiling really, at least from my perspective.. small nucleation points, that spread throughout the liquid or crystal, effecting an overall phase change when the energy distribution reaches a point such that the majority of atoms prefer the gaseous or liquid state (depending on the phase change).
Wait wait wait, let me get this straight. We put a man on the moon, developed flying machines composed of several hundred tons of steel, and we just now BARELY explain why Ice Cubes melt in our drink? You know, sometimes humanity really is....scary. What'll be truly frightning is if scientists come out with an explanation as to why Ice Cube still gets movie roles.
a. Summary is plagiarized from the article, unless I've missed some nested quotes.
b. These guys took this problem because "the earliest phase of melting has never been seen" but they didn't do that either! All they did was make "see-through crystals that are like small beads and are visible in an optical microscope." Doesn't sound like a hell of a lot of progress to me; anyone care to elaborate?
c. Their main result seems to be that the melting process starts at crystal defects and spreads to create liquidy regions within the crystal. Again, can anyone explain why the melting might not start at defects - the weak points?
I'm sure there's something neater here than I'm seeing; it would be nice if the article had more info.
Speaking of ice, have folks here ever heard of Pykrete? And would this explain why Pykrete melts so slowly?
Supposedly tissue paper works as well as sawdust. So you can tell all your friends you know how to beat someone to death with a wet paper towel.
--grendel drago
Laws do not persuade just because they threaten. --Seneca
Now how long till they can whip-up a batch of Ice-Nine and freeze the whole planet?
From the article submission:
And from the actual article itself: Those look pretty similar to me! Given that the article submission is word-for-word exactly from the article itself, it's fair to assume that the submitter, Killer Instinct, is the same person as the author of the article, Robert Roy Britt. How else could the same text be attributed to two supposedly different people?If you're going to submit an article, summarize it in your own words. If you're just going to paste in the first few sentences of the article, attribute them to the proper author by using a phrase such as, "Quoted from the article: 'insert quote here'." Removing line breaks is not enough to satisfy the "summarize in your own words" criteria.
Here's an example of what the submission should've looked like if Slashdot cared at all about given proper attribution for written text:
Source
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
...smartass slashdotters crack jokes about a new discovery to hide their own insecurities. I, for one, freely admit I have no idea how ice melts.
1+1=2 anyone?
At least the way the article describes the study.. it doesnt seem like it models the problem well.. but something tells me these arent the greatest writers here... For instance:
"So Yodh's team made some big atoms. Specifically, they made see-through crystals that are like small beads and are visible in an optical microscope."
By "see-through crystals" i'm assuming they mean optically transparent crystals constructed from small beads, not crystals that are like beads that then form a larger crystal structure, although from the wording, it's impossible to tell.
"The spheres swell or collapse significantly with small changes in temperature, and they exhibit other useful properties that allow them to behave like enormous versions of atoms for the purpose of our experiment,"
As far as I know.. atoms dont significantly change size when temperature changes.... they change how fast they move. I dont really see how size-changing beads model water molecules here, unless it's on a macroscale where a molecules are considered to expand as a group with increased temperature... but that sort of would defeat the pupose of the whole study...
On the other hand.... I think that the research is probably solid, espcially if it's being published in Science, a extremely selective journal. I think the article just fails to explain it well, and takes quotes out of context. Sadly, this is all too common in scientific journalism.
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 article is too bad; there's probably an interesting result here, but it appears to be shrouded in vagueness and analogy.
It's true that the *exact* mechanism for melting has not been "seen", but the concepts really are well known. Our models are good enough that computer simulations can be very accurate. I have seen several which show features such as surface melting, for instance.
Also, it is absolutely expected that melting begin at defects, but this does not mean that "melting begins below the melting point" as the article suggests. These areas are locally amorphous and there is no reason that they should begin melting at the crystal's melting point. Really, it's all in the free energy equations.
I'm guessing that the real result has been butchered by the article.
4096R/EF7BAFA6 79E1 DF98 D09D 898F 9A11 F6F0 DDDC 23FA EF7B AFA6
Wow... I just realized that I read a whole article about ice melting... And I was interested. I guess that's what you're reduced to when you have nothing to do but read Slashdot at midnight on a Friday...
Sometimes you've gotta roll the hard six.
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.
Is thrilled to know exactly how he will die come spring.
If you could reason with religious people, there would be no religious people
An understanding of how it works is crucial to gaining a firm grasp on the physical world.
I did not RTFA, and now I feel like I am tripping on acid - swallowing colors of the sound I hear, I am just a crazy guy.
Slashdot, it's better than drugs!
It will make you innn-sane!!
When information is power, privacy is freedom.
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).
the aliens will make contact. It would have been emberassing to make contact with planet that couldn't quite pin down the subtleties of how ice melts.
Step 2: Warm ice turns into liquid water.
Step 3: Profit!
Oh, and don't forget, you can use distilled water to make 2" long ice spikes on your cubes!
No, in fact they are talking about boiling water; a.k.a. 100 degrees Celsius. As people have pointed out, there are two reasons. Evaporation is a cooling process (that's why you sweat), and during evaporation liquid mass takes a gas form, thus there is less of your ice cube to freeze. And distilled water doesn't freeze "faster" it freezes at a higher temperature than water with impurities. Hence, people in the north applying salt to their streets.
Until now, scientists could not explain why ice cubes in your drink melt.
Scientists does not explain why things happen. Only how.
thomasdamgaard.dk.
It is called the Mpemba Effect.
More on this phenomenon (history en possible explanations) here
using hot water makes it faster than using cold water, right?
Water that's really hot will loose heat more rapidly than cool water in the same surroundings. What people don't get is that once the hot water has cooled off, it now cools at the slower rate.
What actually IS useful about freezing hot water is that there are a lot less air bubbles so the ice doesn't crack and throw shards out when you pour freshly brewed tea over it on a hot summer afternoon.
super cooled liquids and vapors are easy to make.
I have a small fridge here (absocold) it is kind of like the small fridges students use in dorm rooms.
If I put a bottle of water in the freezer compartment
most of the time it will not freeze.
what is fun is to hand it to someone and ask them to shake it or even let me them drink the water.
I will suddenly turn to slush. It is very strange to have water freeze in you mouth.
Other planets are more stable (climate wise) then earth, but their normal temperatures are probably too extreme to sustain life.
Venus' orbital eccentricity: 0.00677323
Neptune's 0.00858587
Earth's: 0.01671022
Venus' surface temperature ranges from about 820 degrees to nearly 900 degrees F
Earth's surface temperature ranges from about -80 degrees to around 130 degrees F
Neptune's mean cloud temperature ranges from -315 to -307 degrees F (Temperature varies vastly by cloud elevation, but probably little across the same cloud level)
------ Take away the right to say fuck and you take away the right to say fuck the government.
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:.
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Here's how it works:
http://www.nuklearpower.com/daily.php?date=041125
And seen in action:
http://www.nuklearpower.com/daily.php?date=041102
This reminds me of a similar effect that I often observe while cooking, particularly while stir-frying (or any other high-heat method). That is: a drop of water will evaporate more quickly in a pan on medium heat that it will in a pan on high heat.
The reason? When a drop of water hits a pan on very high heat, the underside is instantly tranformed into a layer of vapor which then acts as a buffer between the pan and the liquid on top. So insulated, the water droplet will then "dance" and roll around the pan like a ball bearing. The drop can remain in the pan for a surprising amount of time, though I have never personally measured.
There they were, sitting in the van with all those dials, and the cat was dead. -V. Marchetti, CIA
If "pre-melting" truly begins at the defect sites, it would be interesting to see whether ultra-low defect containing crystals melt at a higher temperature. Say, purify and grow a chunk of ice through the same procedure used to fabricate semiconductor grade silicon (Czochalski style or epitaxially), and then see if it holds together through warmer temps.
You could imagine two closed cylindrical containers, each initially filled with a substance in a liquid state. The liquid of container A is at a temperature such that the density is the minumum. The liquid in container B is initially at a higher temperature than container A. For simplicity's sake, the only extremum of density with respect to temperature is the minimum I mentioned. You begin cooling both liquids at the top of the cylinder.
As heat is transferred from container A, the density will always be increasing from the top to the bottom of the container in a predictable fashion, i.e., the "heavier" substances will always be on the bottom of the container. This doesn't promote convection. With container B, there are good opportunities for convection, due to the varying density gradient and the effect of gravity. Solid forming on the top of container B could even sink. The convection leads to a higher sustained temperature gradient at the cylinder boundary, leading to faster heat transfer and faster cooling.
Note that this mechanism doesn't require an open system. There are, of course, other possible mechanisms, but this is the simplest one I could come up with.
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
News flash!! Still no cure for cancer, but scientists are hard at work discovering how ice melts. Quick somebody start polishing that noble prize!
...that's generally how I get the ice to melt.
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.
The underlying point here is the techniques materials scientists normally use to examine material properties. Techniques like FTIR, SEM, STEM and x-ray diffraction work well on materials in one state but any time phase change occurs they are too simple to examine the change as it occurs. Even an environmental SEM that can examine certain materials at higher temperatures tends to still be too simplistic to examine a phenomena like melting closely at the atomic scale. For melting energy really one of the few useful techniques is DSC (differential scanning calorimetry) and that still won't let you observe the melting mechanism itself, only detect the energy needed to reach the melting point. In this area, the physicists actually have us beaten because they at least have particle detectors that can observe the effect of high energy collisions at the sub-atomic scale. That's why this experiment is important, they are developing techniques to circumvent the limitations of the instrumentation.
What would Richard Feynman do, if he were here right now? He'd do some math and he'd follow through!