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."

Comments

Re:freezing water

[guardiangod]

It depends on the fluid's temperature....

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

Re:Hmmmmm...

[poopdeville]

That's a shame. This is a very interesting topic. We've known for centuries that melting is related to heat, and there are molecular models of freezing. Namely, water molecules tend to align themselves in a crystalline structure unless they're stirred up. A region freezes when the average kinetic energy is low enough for the molecules to align themselves. Consider a fairly large volume of water -- in macroscopic scales. Heat conduction through liquid water is faster than through ice, because of convection. So the macroscopic freezing process isn't reversible. (There are other reasons why the process isn't reversible, but one suffices)

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.

Re:freezing water

[Escherial]

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.

Re:freezing water

[tijnbraun]

It is called the Mpemba Effect.

More on this phenomenon (history en possible explanations) here

Re:freezing water

[Serpentine]

It is not a myth although it certainly is pervasive...among physical chemists. Basic thermodynamics is just that: basic. Like all laws of science it makes assumptions that are not always true. Under specific conditions the effect can still be observed once evaporative loss is compensated for; it apparently has even been observed in closed containers.

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.

Re:Anti-Cold

[ToastyKen]

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?

Re:Not Insightful or Interesting

[Kidbro]

Actually, the original value for freezing in the Centigrade scale was 100 and boiling was zero. It wasn't changed until the mid 1700's.

While what you're saying is true, I think that the way you put it may give people reason to exaggerate the life span of the original scale. The original system was proposed in 1742, and modified to its current version in 1747. Both are years I'd say qualify for the being part of "the mid 1700's".

Reference.