We, as members of the EPIA, are all ice men and women in one way or another. Some of us administer the operation of the association, others build the machines and materials needed to produce packaged ice, while others perform the actual manufacturing and distribution of packaged ice. Therefore we should know a little or even a lot about the product we make and sell. That includes information about how cold it needs to be to remain ice and not become water before we can sell it, as well as, have sufficient knowledge about its chemical makeup and properties to make safe ice for human consumption. After all, we are “ice experts” and our customers expect us to know more about the product we sell than they do.
We should even be aware of many interesting well-known properties of ice. Not just the ice we make and sell, but ice encountered every winter where we live. For example, “why is ice slippery?”
How many of us learned in school that water freezes at 0˚C? Hopefully all of us. Now how many of us also learned that pressure on a small area of ice will raise the temperature and make the ice melt? Again, hopefully, all of us. Now take that a step further and apply this principle to something we have probably all done, ice skating. If like me, your butt got to known the ice up close and personal.
We all agree that the transition from liquid to solid state (or water to ice) and then reverse happens at 0°C. Continuing with grade-school science, we also learned that as you apply pressure on the ice, the ice will melt and become water. And this small layer of melted ice (i.e. water) on the surface of the ice below the blade of the ice skate will allow the skater with very little friction to gliding across the surface of the ice. The water between the blade and the ice is what we actually glide across. When we stop exerting pressure, that water freezes almost instantly due to the cold temperature surrounding it.
Yep, that is what we all learned. Just like today’s “urban myths”, we believed what our parents and teachers told us because we were little and ill-informed and they were the all-knowing authorities. Even today with immediate access to the all-knowing internet, we still often are led to believe urban myths that really are not true. After all, the common assumption is that if something is on the internet, it must be true.
Let’s continue with ice skating. Here is an example of one student asking others to enlighten him on something he expects to be a final exam question. Here is the actual question on this self-described expert web site about physics. It promotes itself as follows; “Physics Stack Exchange is a question and answer site for active researchers, academics, and students of physics.”
QUESTION
“Okay, a homework assignment I had to do, suggested that the reason we can skate on ice is the peculiar p(T)-curve of the ice-water boundary. The reasoning is that due to the high pressure the skates put on the ice, it will melt at temperatures above 0˚C and thus provide a thin film of liquid on which we can skate. My calculations suggest to me that the pressure would not be nearly high enough to lower the melting point to even something like −0.5˚ Celsius. I suppose it is some other mechanism, probably related to the crystal structure of ice, but I’d really appreciate if someone more knowledgeable could tell something about it.”
This question got lots of useless and counter-productive help from believers in common knowledge until one person actually had done some homework in the past and pointed the questioner in the right direction:
- Respondent #1:
- “I’m pretty sure this was a problem on the final exam for my undergrad thermodynamics class 😉 so at least I think your calculation is reasonable. I don’t know/remember what the real reason is though.” – David Z, Dec 7 ‘10 at 5:31
- Respondent #2:
- “Well, this analysis completely ignores that when you skate you are not standing, but you are actually moving. There should be some friction between the skates and ice and this should provide enough heat to melt the ice and create a thin water film. At least this is my intuition (perhaps completely wrong).” – Marek Dec 7 ‘10 at 11:44
- Respondent #3:
- “The binding energy near a surface is different than the binding energy in bulk, and it is possible that you melt a thin surface layer without melting the bulk. – Ron Maimon Aug 21 ‘11 at 14:14
- Respondent #4:
- “Yup, this is true that the pressure is too small, but the true explanation is not justified yet. Nevertheless, the common sense is that there is a lubricating film of water or at least anomalous ice. For an overview, see: http://lptms.u-psud.fr/membres/trizac/Ens/L3FIP/Ice.pdf
Very interesting read. In summary with notations from people that really do study and know ice, this is really why we slip on ice.
Charles Petit, as a Chronicle Science writer, published his results on December 12, 1996. He agrees that everybody knows ice is slippery, but he says that scientists at Berkeley say they have now learned why skaters, skiers, and hockey pucks slide, slip, and slide some more — and it is not the reason most people think.
Even at 200 degrees or more below zero, seemingly solid ice is — in a sense — all wet, the researchers say. Unlike other solids that are, in fact, solid all the way to their surfaces, an extremely thin, quasi-fluid layer coats the surface of frozen water, they conclude. “People thought that the reason you can ski, or ice skate, is because the pressure or heat from friction turns a tiny layer of ice into water, and it acts like a lubricant,” said chemist Gabor Somorjai of the Lawrence Berkeley National Laboratory. “But that is wrong, and it makes no sense anyway.”
If it were friction or pressure, he said, the slipperiness of ice would vary according to how hard someone pressed on it. Some parts of a ski would slip, others would tend to stick.
And the other Berkeley chemist on the project, theorist Michel van Hove, calculates that skis, skates, and pucks do not generate enough pressure to instantly liquefy ice.
The two, working with colleagues in France and Holland, began looking closely at ice as a sideline to a study of the interaction between water and metals, such as platinum, that act as catalysts by speeding up chemical reactions. “We realized after we started that we could make some very unique measurements on the surface of the ice, too,” van Hove said.
Using a method called “low energy electron diffraction,” in which electrons are bounced off a surface to reveal its shape on an atomic scale, they got results from ice that made no sense. Molecules of water, or hydrogen (H) and oxygen (0), each have two hydrogen atoms and one oxygen atom. But in the Berkeley experiments, the surface molecules in ice appeared to be bonded to the underlying solid by only one hydrogen atom instead of the expected two.
They finally realized, said Somorjai, that “we were being tricked. There was nothing wrong with the hydrogen. It was the oxygen.” It turned out that the oxygen atoms at the surface were vibrating furiously, jittering back and forth three times faster than oxygen atoms buried more deeply in the ice structure.
They do not know why the surface oxygen atoms in ice vibrate so much. That is a subject for future study. But the result of the wildly jiggling oxygen atoms is that the ice surface smooths itself out “like a fluid, and it acts wet even when it is not really wet,” Somorjai said.
“Water is such an interesting material, a unique chemical, and you just assume it is unusual,” Somorjai said. “It is the basis of life, it floats when it is frozen (most solids are denser than their liquid phases). So one more strange thing about it is not such a surprise.” The scientists reported their results late in 1994 in the Journal of Physical Chemistry.
The news about wet ice is not the only such oddity reported in 1996. In October 1996, scientists at the U.S. Geological Survey in Menlo Park and the Lawrence Livermore National Laboratory reported a form of ice that, when coated by a material called gas hydrate, stays solid well above its usual melting temperature. The hydrate appears to prevent pure liquid water from forming on its surface. Whether the two phenomena are connected is not known, «but these are probably not the last new things we will learn about water,» van Hove said.
So there you go. The next time someone tells you to be careful because the ice is slippery, you can tell them the real reason you will slip. Whether they believe you or not is totally up to your ability to convince them to disregard what they know is fact.