Kitchen Myth Busters
Warning, the following entry contains mathematics and science. If you are not a geek, please do not read on.
SPAGHETTI INCIDENT - True
I wasted half a packet of Barilla No.7 in my pantry and could not figure out why. But today, the Rheologeewoldt* shed light to this problem by giving me a recent article from Physical Review Letter. It is about how spaghetti snaps into pieces. As some of you might have observed, spaghetti does not break into two pieces when you bend it, most of the time it snaps into three to ten pieces. Apparently this was a problem which confounded the great Feynman himself. So recently some nerds in France has managed to model this phenomenon. The mechanics behind this is so: At the instant of the first fracture, the suddenly freed end suddenly realizes that it should have no curvature (check your undergrad structures notes), while the remainder of the spaghetti has not had time to react and thus remains in a curved shape. The information that the end is freed travels along the spaghetti in the form of a burst of flexural waves and generates a local concentration in curvature, causing another breakage further downstream, which generates yet another burst of waves, causing another breakage…
This was the first myth I looked at which actually turned out to have some truth in it. While we are on this topic, let me waste your time on a few more of my silly home experiments, unfortunately, none of the original claims seem to have any scientific basis at all.
COLD WATER BOILS FASTER THAN HOT / HOT WATER FREEZES FASTER THAN COLD - false
Matlove and his girl brought my attention to this myth and commissioned me to investigate the truth in it. I did some experiments at home and found this myth to be completely bogus, at least in the conditions of my kitchen. I expect the kitchen conditions to be similar to mine, so don’t try to be smart and freeze hot water.
KITCHEN SINK VORTEX - false
Some people think that the sink vortex always spin clockwise in the Northern Hemisphere because of the coriolis effect. This is not true, the coriolis force is so small, to see it, you will have to leave the water in an extremely still environment for days to rid it of any initial eddies.
SCREAMING LOBSTERS - false
Some redneck once told me lobsters scream in pain when dunked into boiling water. That’s bogus, lobsters has no vocal chords, apparently that hissing noise is the heated air trapped under the shell expanding and whooshing out of the gaps.
In fact lobsters feel no pain, they don’t have a brain.
*Rheologeewoldt is another excellent friend. For a living, he pleasures snails to obtain their fluids.
posted by oof @ 5:33 AM
8 comments
8 Comments:
Once I torched a huge spider in my army camp and it "screamed" really loud. It must be the vortex created by the collapsing spider-organs causing fluids to gush out resonating an audible sound-wave. 100-plus boy, you should model the fluid mechanics behind this.
The tests are kelong!
Ah ha! I was wondering about the hot water freezes faster in the freezer. I remember seeing that in my primary school textbook. But I just never did understand the explanation given. So the bloody textbook confirm wrong right??
All things being equal, cold water freezes faster.
It takes time for the energy contained in a hot object to be transferred to a cold object. However, the rate of heat transfer is proportional to the temperature difference between the two objects, so hot water will lose heat faster than cold water. In other words, if you have water at 90 degrees C and water at 10 degrees C and the freezer is at -10 degrees C, the hot water will lose heat five times faster than the cold water; however, the cold water will still win the race. As the hot water cools it's rate of heat transfer will decrease, so it will never catch up to the cold water.
Some people claim that hot water freezes faster because a pot of boiling water can be thrown into the air on a cold winter day, and it freezes in mid air creating a shower of ice crystals. Whereas a pot of cold water thrown into the air comes down as large blobs of water. This happens because the hot water is so close to being steam, that the act of throwing it into the air causes it to break up into tiny droplets. (hot water is less viscous than cold water, listen to the sound it makes when you pour it in the sink) The small water droplets have a large surface area which allows for a great deal of evaporation, this removes heat quickly. And finally, the cooled droplets are so small, that they can be easily frozen by the winter air. All of this happens before the water hits the ground. Cold water is thicker and stickier, it doesn't break up into such small pieces when thrown into the air, so it comes down in large blobs.
Yes it can -- a general explanation
Hot water can in fact freeze faster than cold water for a wide range of experimental conditions. This phenomenon is extremely counter- intuitive, and surprising even to most scientists, but it is in fact real. It has been seen and studied in numerous experiments. While this phenomenon has been known for centuries, and was described by Aristotle, Bacon, and Descartes [1-3], it was not introduced to the modern scientific community until 1969, by a Tanzanian high school student named Mpemba. Both the early scientific history of this effect, and the story of Mpemba's rediscovery of it, are interesting in their own right -- Mpemba's story in particular provides a dramatic parable against making snap judgements about what is impossible. This is described separately below.
The phenomenon that hot water may freeze faster than cold is often called the Mpemba effect. Because, no doubt, most readers are extremely skeptical at this point, we should begin by stating precisely what we mean by the Mpemba effect. We start with two containers of water, which are identical in shape, and which hold identical amounts of water. The only difference between the two is that the water in one is at a higher (uniform) temperature than the water in the other. Now we cool both containers, using the exact same cooling process for each container. Under some conditions the initially warmer water will freeze first. If this occurs, we have seen the Mpemba effect. Of course, the initially warmer water will not freeze before the initially cooler water for all initial conditions. If the hot water starts at 99.9° C, and the cold water at 0.01° C, then clearly under those circumstances, the initially cooler water will freeze first. However, under some conditions the initially warmer water will freeze first -- if that happens, you have seen the Mpemba effect. But you will not see the Mpemba effect for just any initial temperatures, container shapes, or cooling conditions.
This seems impossible, right? Many sharp readers may have already come up with a common proof that the Mpemba effect is impossible. The proof usually goes something like this. Say that the initially cooler water starts at 30° C and takes 10 minutes to freeze, while the initially warmer water starts out at 70° C. Now the initially warmer water has to spend some time cooling to get to get down to 30° C, and after that, it's going to take 10 more minutes to freeze. So since the initially warmer water has to do everything that the initially cooler water has to do, plus a little more, it will take at least a little longer, right? What can be wrong with this proof?
What's wrong with this proof is that it implicitly assumes that the water is characterized solely by a single number -- the average temperature. But if other factors besides the average temperature are important, then when the initially warmer water has cooled to an average temperature of 30° C, it may look very different than the initially cooler water (at a uniform 30° C) did at the start. Why? Because the water may have changed when it cooled down from a uniform 70° C to an average 30° C. It could have less mass, less dissolved gas, or convection currents producing a non-uniform temperature distribution. Or it could have changed the environment around the container in the refrigerator. All four of these changes are conceivably important, and each will be considered separately below. So the impossibility proof given above doesn't work. And in fact the Mpemba effect has been observed in a number of controlled experiments [5,7-14]
It is still not known exactly why this happens. A number of possible explanations for the effect have been proposed, but so far the experiments do not show clearly which, if any, of the proposed mechanisms is the most important one. While you will often hear confident claims that X is the cause of the Mpemba effect, such claims are usually based on guesswork, or on looking at the evidence in only a few papers and ignoring the rest. Of course, there is nothing wrong with informed theoretical guesswork or being selective in which experimental results you trust -- the problem is that different people make different claims as to what X is.
Why hasn't modern science answered this seemingly simple question about cooling water? The main problem is that the time it takes water to freeze is highly sensitive to a number of details in the experimental set- up, such as the shape and size of the container, the shape and size of the refrigeration unit, the gas and impurity content of the water, how the time of freezing is defined, and so on. Because of this sensitivity, while experiments have generally agreed that the Mpemba effect occurs, they disagree over the conditions under which it occurs, and thus about why it occurs. As Firth [7] wrote "There is a wealth of experimental variation in the problem so that any laboratory undertaking such investigations is guaranteed different results from all others."
So with the limited number of experiments done, often under very different conditions, none of the proposed mechanisms can be confidently proclaimed as "the" mechanism. Above we described four ways in which the initially warmer water could have changed upon cooling to the initial temperature of the initially cooler water. What follows below is a short description of the four related mechanisms that have been suggested to explain the Mpemba effect. More ambitious readers can follow the links to more complete explanations of the mechanisms, as well as counter- arguments and experiments that the mechanisms cannot explain. It seems likely that there is no one mechanism that explains the Mpemba effect for all circumstances, but that different mechanisms are important under different conditions.
1. Evaporation -- As the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the water to cool and freeze. Then the initially warmer water can freeze before the initially cooler water, but will make less ice. Theoretical calculations have shown that evaporation can explain the Mpemba effect if you assume that the water loses heat solely through evaporation [11]. This explanation is solid, intuitive, and evaporation is undoubtedly important in most situations. However, it is not the only mechanism. Evaporation cannot explain experiments that were done in closed containers, where no mass was lost to evaporation [12]. And many scientists have claimed that evaporation alone is insufficient to explain their results [5,9,12].
2. Dissolved Gasses -- Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling. So the initially warmer water may have less dissolved gas than the initially cooler water. It has been speculated that this changes the properties of the water in some way, perhaps making it easier to develop convection currents (and thus making it easier to cool), or decreasing the amount of heat required to freeze a unit mass of water, or changing the boiling point. There are some experiments that favor this explanation [10,14], but no supporting theoretical calculations.
3. Convection -- As the water cools it will eventually develop convection currents and a non-uniform temperature distribution. At most temperatures, density decreases with increasing temperature, and so the surface of the water will be warmer than the bottom -- this has been called a "hot top." Now if the water loses heat primarily through the surface, then water with a "hot top" will lose heat faster than we would expect based on its average temperature. When the initially warmer water has cooled to an average temperature the same as the initial temperature of the initially cooler water, it will have a "hot top", and thus its rate of cooling will be faster than the rate of cooling of the initially cooler water at the same average temperature. Got all that? You might want to read this paragraph again, paying careful distinction to the difference between initial temperature, average temperature, and temperature. While experiments have seen the "hot top", and related convection currents, it is unknown whether convection can by itself explain the Mpemba effect.
4. Surroundings -- A final difference between the cooling of the two containers relates not to the water itself, but to the surrounding environment. The initially warmer water may change the environment around it in some complex fashion, and thus affect the cooling process. For example, if the container is sitting on a layer of frost which conducts heat poorly, the hot water may melt that layer of frost, and thus establish a better cooling system in the long run. Obviously explanations like this are not very general, since most experiments are not done with containers sitting on layers of frost.
Finally, supercooling may be important to the effect. Supercooling occurs when the water freezes not at 0° C, but at some lower temperature. One experiment [12] found that the initially hot water would supercool less than the initially cold water. This would mean that the initially warmer water might freeze first because it would freeze at a higher temperature than the initially cooler water. If true, this would not fully explain the Mpemba effect, because we would still need to explain why initially warmer water supercools less than initially cooler water.
In short, hot water does freeze sooner than cold water under a wide range of circumstances. It is not impossible, and has been seen to occur in a number of experiments. However, despite claims often made by one source or another, there is no well-agreed explanation for how this phenomenon occurs. Different mechanisms have been proposed, but the experimental evidence is inconclusive. For those wishing to read more on the subject, Jearl Walker's article in Scientific American [13] is very readable and has suggestions on how to do home experiments on the Mpemba effect, while the articles by Auerbach [12] and Wojciechowski [14] are two of the more modern papers on the effect.
dude, all I can tell you is I tried it at home in the SP common kitchen freezer. approximately 2 cups of water in what used to be nescafe glass jars.
I think the explanations given in mpemba effect article are a bit exaggerated, and applies to rather "extreme" conditions. If you can replicate it at home, you should publish it!
Let me do some dimensional analysis to confirm it.
Giorgia just informed me that in italy it is a cardinal sin to snap spaghetti into half, only those frogs with no respect for italian cuisine can perform such experiments.
mamba effect..
btw, my blog disappeared. WTF happened?
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