Charged Magazine

Tag: physics

Why Does Hot Water Freeze Faster?

PROFESSOR: Hello class! Welcome to “Advanced Application of Math and Logic in Natural Philosophy” class. Look to your left and look to your right. One out of five in this room will not pass the class.

STUDENT A: Sir, but there are only four of us.

PROFESSOR: (ignoring the student) Ahem. During our first lecture, we will exercise the use of logic and thought experiments to unveil one of the greatest mysteries in the history of science. Even Aristitotle struggled solving this mystery. For centuries many scientists attempted to solve but failed. (drum roll) Why does hot water freeze faster than cold water?

STUDENTS B & C: (smile nervously not knowing whether he is joking or not)

STUDENT A: But sir, hot water needs to be cooled down before it can freeze. So shouldn’t cold water freeze faster?

PROFESSOR: Good point. However, that is not always true. It’s called the (writing on the board) Mpemba effect. Can anyone guess why? It’s okay even if you are wrong. Don’t be shy – that is why I am here.

STUDENTS A, B, C, & D: (silence)

(Satisfied with his authority and the full attention of his students, the professor continues)

PROFESSOR: Actually, experiments showed that hot water will cool faster than water at 60°C. But this is not the case with water below 60°C. Mpemba effect is a combination of multiple factors. Now let’s figure out each of them together. First, tell me what are the characteristics of hot water?

STUDENT B: It has higher average kinetic energy.

The water molecules move faster, which means that they occupy greater volume, similar to how an air balloon expands at higher temperature.

STUDENT C: Wait, that means that if cold water and hot water have the same volume, the mass of hot water will be less. This is because the evaporation is taking place and dissolved gases in hot water escape at a higher rate.

PROFESSOR: Great answers. Water is most dense at 4 degrees Celsius and its density changes according to its temperature. Yes, so 1L of water does not always hold the same amount of mass. 1L of hot water will weigh less compared 1L of water near 4°C. However, the mass difference is slim even when we considering the evaporation of water molecule. So there has to be another factor that contributes to Mpemba effect.

STUDENT: But sir? What about supercooling of water and nucleation? Doesn’t lack of impurities and gases in water lead to supercooling, in which water forms crystal ice at temperatures below 0°C? It has been observed that water can remain liquid at -40°C. If hot water doesn’t have dissolved gas, shouldn’t it be hard to make it freeze?

PROFESSOR: (as if expected) Good point. Except Mpemba effect has been observed in both hot water and cold water removed of dissolved gases. Dissolved gas does not seem to contribute to the process of freezing. You could, however, make beautiful and clear crystal ice by removing dissolved gas before freezing. So what else? What else does hot water do?

STUDENT C: It evaporates faster than cold water..?

PROFESSOR: Yes, the water evaporates faster. But why? And so what?

STUDENT B: Well umm… the temperature of water is the average kinetic energy. At any time, liquid water has molecules with a range of energies. Although most of them have the average kinetic energy of the system, very few have enough energy to escape intermolecular forces such as hydrogen bonds and dipole-dipole attractions. If we were to look at a graph of number of molecules versus energy, it would look similar to a current IQ graph, in which the majority of the population has an IQ of 100 and few have over 140 and under 40.

STUDENT A: (towards student B) Hydrogen bond and dipole-dipole forces are too weak to contain water molecules in liquid form. It’s actually the atmospheric pressure doing all the work.

PROFESSOR: Please let him continue.

STUDENT B: So molecules with higher energy escape, lowering the average energy within the system, and this is evaporative cooling.

STUDENT C: Oh! So the hot water is not only being cooled through evaporative cooling but also from heat conduction and convection inside the freezer.

PROFESSOR: Good answers class. But that is not enough. According to Dr. Takamasa Tkahashi at St. Norbert College, it takes 540 calories to vaporize one gram of water. At above 80°C, the rate of cooling by evaporization is high as vapor draws massive energy from water left behind. To give you an idea how much 540 calories worth, it takes 100 calories to heat up 1 gram of water to 100°C from 0°C.

STUDENT A: (excited) 1 Calorie is the energy required to raise 1 gram of water 1°C! In fact, scientists measured food calories by burning them and heating up water back in old days.

STUDENT D: Wow…is that why greasy foods have high calories?

PROFESSOR: If we neglect heat transfer through radiation or conduction, water will freeze on a desert purely from evaporation. Interesting, isn’t it? We can use zeolites which facilitate evaporation through adsorption to make ice as well. A team of engineers at Rice University built a prototype of ice machine that uses nothing but zeolites and vacuum pump.

STUDENT D: (perplexed) But professor, when hot water reaches the temperature of the cold water, don’t both of them have the same evaporation rate?

PROFESSOR: (impressed and amused) Very good point. I might actually consider allowing one A+ in this class. To answer your question, let’s first consider extreme cases to facilitate our thought experiments. Imagine placing an ice cube onto a very hot plate. What happens?

STUDENT A: (desperate to not embarrass himself) Uh…The ice melts immediately? And while some water molecules evaporate, others boil around the remaining ice cube, making noise from gases and vapor escaping. Noise is also another form of energy transfer.

PROFESSOR: Yes but melting the process does not happen instantly. While the ice is melting, what is the average kinetic energy? It is somewhere in between that of boiling water and ice.

Now, we learned that the energy of liquid water is not perfectly uniform. The faster the change of temperature is applied the greater inconsistency of the energy. Hence, hot water being cooled down to cold temperature has a higher evaporation rate than cold water that remained cold for a prolonged period of time. I think you’ll need to elaborate on this one…

STUDENT A: Also, Newton’s law of cooling dictates that hot water will cool down faster regardless of the evaporation.

PROFESSOR: Yes, of course, faster moving molecules will bounce off of each other more frequently, giving and losing energy through these interactions.

STUDENT D: So that means… initially, cold water has a more uniform temperature. But when hot water is cooled, although the average temperature is cold, there are greater discrepancies of temperature among molecules?

STUDENT A: (being impatient) Yes, and that is why the hot water will be more likely to have molecules with higher energy, which escapes and cools the liquid faster.

PROFESSOR: Like washing a car with hot water during winter, evaporative cooling is more evident with larger surface area. So what else? Does anyone else have any more idea?

STUDENT C: Well… this may seem stupid, but what if Mpemba effect is caused by other effects such as the environment?

PROFESSOR: (intrigued) What do you mean?

STUDENT C: The freezer keeps the temperature below zero all the time. But when the temperature increases, cold air is introduced in order to keep the temperature constant. So if we place hot water inside the freezer, and due to heat conduction, radiation, and evaporation, the internal temperature increases, it will trigger the freezer to blow cold air. You need to elaborate on this… And thus, hot water will freeze faster.

PROFESSOR: Hmm…I like your creative way of thinking.

STUDENT B: Also, freezers have ice dusts inside. And hot water may melt these ice particles within the compartment at contact points of the bowl and the surface. Since water is dense and thus conducts heat faster, hot water will cool down at higher rate. The effect is similar to how ice water cools beer bottles faster than ice cubes.

PROFESSOR: Excellent point! Water is 829 times denser and transfers heat 25 times faster than air. Hot bowl of water sitting on slightly melted ice will definitely cool faster than cold bowl of water sitting on ice dust. Water covers more surface area as well. But aren’t you slightly underage to know how to cool beer bottles?

STUDENT A: (hungry for attention) Sir, what about infrared radiation? We know hot objects emit electromagnetic radiation and thus release energy at a faster pace. A combination of this and everything we discussed today will make hot water cool faster.

PROFESSOR: Now, do you guys remember how water changes its density? Due to heat conduction at contact points, a bowl of water will be colder at the bottom. Let’s consider a cold bowl of water. Cold water near freezing point at the bottom of the bowl will have lower density than water at near 4°C on top. Low density water will rise up switching position with water near 4°C. Neither upper nor lower part of water will freeze until that 4°C temperature is brought to 0°C.

Now what about hot bowl of water? Hot water at the top has lower density than cold water at the bottom. Thus, cold water near freezing point won’t rise up. This means the water at the bottom can freeze before hot water is cooled to 4°C. This effect will freeze hot water faster.

STUDENT C: Whoa… there seem to be a lot of factors that play in Mpemba effect. We discussed mass difference, evaporation rate, freezer compartment effects such as more frequent cold air and melting ice at contact points, and how density can slow or speed up the freezing rate. I don’t think we can come up with anymore explanations.

PROFESSOR: Aha but we are not done! There are more. What are we forgetting class?

STUDENT D: (as if he had more than enough to learn) Not a clue sir…

PROFESSOR: (excited) The internal air pressure of the freezer compartment~!

STUDENT D: But how..?

PROFESSOR: (showing a video clip on a TV) Do you guys watch a BBC TV series called “Bang Goes the Theory”?

STUDENT C: Oh my gosh! I absolutely love that show!!

PROFESSOR: Let’s look at season 2 episode 4. In this episode Dr. Yan shows that when you quickly open a bottle there’s a cloud of white air at the opening. This effect is the opposite of how diesel engine ignites a spark by compressing air. (Professor demonstrates by opening a beer bottle). The air between the liquid and the cap is quickly expanded and loses energy. At this very instant, the temperature drops to around -30°C and this white gas you see is the condensed water molecules in the air.

STUDENT A, B, C&D: (intrigued) wow…

PROFESSOR: As the ideal gas law states, hot air has lower density than cold air under equal pressure and volume. If hot water and cold water were placed in a separate freezer compartments, the one with hot water will have less air molecules inside.

STUDENT D: But professor, why?

PROFESSOR: Because hot water will heat up the air inside the compartment. Of course, the longer you wait before closing the door, the more evident the effect would be. You all saw how quickly air cooled down when I opened the beer bottle. (Takes couple of sips of beer). You can create the same effect by opening the freezer compartment. Now when you open both hot and cold water compartments, the one with hot water will experience greater decrease in temperature; air with lower density will lose more energy when expanded at the same volume as air with higher density. This gives the last minute boost of freezing air to hot water compartment.

But even if you don’t open the freezer, lower pressure will facilitate freezing. Can anyone guess why?

STUDENT A: As we can see on the phase diagram, the freezing point of water is higher at lower pressure. Hence, by lowering the pressure, hot water will freeze at higher temperature than cold water.

PROFESSOR: Excellent! Although the boiling point is lower at low pressure, freezing point is higher. Since ice occupies greater volume than liquid water, applying pressure to ice cubes will turn solid into liquid. Not only does lowering pressure increase evaporation rate, it helps to crystalize ice!

STUDENT C: I see… Mpemba effect isn’t magic. It’s science.!

STUDENT D: (frustrated) But professor, none of these answers answer any questions at all! Why must water evaporate? How come its temperature is not uniform? How come its density changes with the temperature? Why all this?

PROFESSOR: This class is a natural philosophy class for a reason. Science is a study and observation of the universe, finding pattern in its chaotic behavior and thereby hypothesizing the answer with logic. And we test our hypotheses to come up with a prediction. We, scientists, ask the question ‘why?’ and dwell on solving the mystery. But the answer we find is the answer to the question ‘how?’ We observe the mechanism and apply it, but we still don’t know why the universe behaves in the way it does. For example, why does Earth create gravitational attraction? Einstein explained that it is because mass curves the space. But why does mass create a distortion of space and why does the distortion cause attractive forces? We don’t know. All we know is how mass creates gravity

Class, I encourage all of you to pursue knowledge with an inquisitive mind. Scientific knowledge is what shapes our future.

Class dismissed. Write me 50 pages on Mpemba effect by tomorrow. It should be easy now that we discussed it in class.

June 16, 2013
New Houston: We have a problem

If you are a technology enthusiast, NASA observer or all around space geek, you don’t need me to point out the connection between two recent news events. First off, SpaceX has successfully launched, maneuvered and docked their spacecraft “Dragon” with the International Space Station. That’s ISS for you acronym junkies. This is impressive not only in the sense that the actual feat is an accolade shared by very few extra-atmospheric vehicles – to my knowledge only the NASA Shuttle, and respective Russian, European and Japanese space agency crafts have ever docked with the ISS – but the Dragon was the first vehicle produced by an independent company. Deeming Space X as “independent” may give some readers pause, as at least at this point, a sizeable portion of their total funding comes from the US government to replace the lost Shuttle capabilities through the “Commercial Orbital Transportation Services” program run by NASA. The second event is the announcement by Dutch company Mars One revealing their plans to colonize Mars. This, again, is really only significant when considering how Mars One has proposed funding their journey to the red planet – namely, and in their own words, via “media spectacle”. I imagine that will include some kind of space Snooki doing shots of Jägermeister while orbiting Phobos or something, but the point is Mars One plans on going where no man (or woman, so as not to exclude Snooki) has ever gone before without being bankrolled by tax dollars. Ironically, the Dutch could be considered experts in the field of corporate colonialism. If memory serves, if you were getting your spice on about 400 years ago you had the Dutch East India Company to thank for that. It does deserve mentioning however that Mars One is not the first organization to ever propose such an undertaking. In fact, independent organizations have been in the planning stages of colonizing Mars for years, with one particularly prominent group being the Mars Society led by Dr. Robert Zubrin, but these organizations are focused on the technical aspects of a journey to and eventual colonization of Mars, rather than how to fund the colonization.

So, taken in concert, the literal launch of the SpaceX’s Dragon spacecraft and the metaphorical launch of Mars Ones’ venture to fund the colonization of Mars, I would argue that the likelihood of anyone even setting foot on Mars within the next 50 or so years has gone from “not going to happen” to “it might actually happen but still probably not”. Why you ask? Because it’s too expensive, too political, and too dangerous for a single government to do alone. I have never done any book learning on the Cold War, but from my perspective the whole point of the space race (which as a direct consequence gave us global telecommunications, the above mentioned ISS and the Hubble telescope among others) between the United States and the former Soviet Union had less to do with legitimate exploration of orbital and sub-orbital space or going to the moon and more with “if my adversary has the capability of global surveillance and strategic space bound nuclear weapons, I want it too.” But now that there are no other communist super powers to compete with (oh wait), why bother going on to Mars? Do the scientific motivations outweigh the risks of sending humans instead of rovers or other autonomous vehicles? I don’t know the answer to that, and I don’t think any government space agency does either, which makes trying to pitch, fund and actually complete a government organized, manned mission to Mars dubious. Thus, as crazy as it sounds, corporate settlement of Mars might be the best option. However, for their part, NASA has not completely given up the ghost (of mars?) on exploring the Martian surface, and has unleashed a parade of satellites and rovers, with an upcoming landing of the curiosity rover celebrated with a recent and admittedly kind of bad ass promo , over the past several years.

So what would the possible goal of colonizing another planet be? If undertaken by a privately funded company, the obvious answer is money. If Mars One leads the way, it might not be “New Houston” that is home to the future inhabitants of Mars but New Amsterdam (again). Are vast quantities of resources the goal of Mars One and like minded would-be-colonists, or is simply the prospect of settling a new world, unfettered of governments, geopolitical conflict and a breathable atmosphere the ultimate goal? Either way, it’s difficult to grok.

image source: http://phoenix.lpl.arizona.edu/postcards.php

June 29, 2012
Supermassive Black Hole Causing a Stir in the Milky Way

Have you ever wondered what goes on in outer space, especially around black holes? I know as a kid, viewing black holes as giant vacuum-like structures, I wondered about the kind of things that might be getting sucked into them while drifting through space. Well, a few weeks ago, I heard a talk by Dr. David Ballantyne about what is actually happening in space around supermassive black holes (SMBHs). (cue Twilight soundtrack)

Black holes are categorized based on their number of solar masses, where one solar mass is equal to 1.989×10³⁰ kg. In order for a black hole to be considered supermassive, it must be greater than 10⁵ solar masses. Goodness gracious! That’s the equivalent of several million or, in some cases, several BILLION suns. A BILLION suns?!? Yes, folks. A billion. And while the formation of black holes isn’t completely understood, it’s possible for black holes to add to one another and form new, supermassive black holes (known as merging).

One location of particular interest is the center of the Milky Way galaxy at Sagittarius A* (pronounced “Sagittarius A star”) where a SMBH potentially resides. Abbreviated Sgr A*, this radio source is about 2.5×10⁴ light years away from our solar system. It can be hard to see due to the murky clouding around it, but with advancements in adaptive optics and the use of telescope arrays, astrophysicists have been able to track young stars orbiting around the area— a key indicator that a black hole is present. In the animation below, you can see various stars orbiting around Sgr A* (the stationary star shape). Star S0-2 is particularly cool to watch as it can be seen making a complete orbit:

Another interesting thing scientists have found is a dense cloud of gas which is moving directly toward this same SMBH. As the gas nears the black hole, part of its structure gets sucked inside. As you can see in the video below (at around 3:22), the cloud will have its closest encounter with the black hole in 2013. That’s next year! As the gas gets sucked into the SMBH, the black hole will gain energy and begin to glow brighter.

Black holes are typically found at the center of galaxies organized in structures referred to as active galactic nuclei (AGNs), which are comprised of the black hole and any surrounding material, in addition to any gas or radiation present. As the material near the black hole swirls around, it creates what is known as the accretion disk. Like our sun, whose corona emits x-rays, black holes can also emit various forms of radiation, including x-rays. What?! Yessir! In a process called magnetic reconnection, magnetic field lines interact and reconnect with one another, causing (x-ray radiating) plasma to be emitted. These x-rays are then dispersed into space, some of which hit the accretion disc and create a ‘reflection spectrum’ which scientists can study.

Now, while supermassive black holes weigh millions (sometimes billions) of suns and have the ability to tear apart nearby objects in space, you don’t have to worry about being sucked into one any time soon. Unless you’re this guy, in which case you may have some issues…

MUSE’s “Supermassive Black Hole”:

Sources:

UCLA Galactic Center Group (http://www.astro.ucla.edu/~ghezgroup/gc/)

Special thanks to Dr. Ballantyne

May 18, 2012
Badass Biology: The Pistol Shrimp

“Wtf, shrimp!!? I make it all the way to this website from who knows what part of the internet, and I see ‘Badass Biology’. Man, Kevin, you flip my brain out with possibilities of flying sharks, rabbits of doom, or a half-honeybee half-unicorn rainbow pollinators. And what do you deliver? A stupid shrimp??! I hate you, and I quit this website. (╯°□°）╯︵ ┻━┻)” To which I reply, “Charles Barkley, I think you’re over-reacting. Please repair my table at once”.

It was hard to find a chart that even went up to 150dB, let alone the 220 these guys produce.

Yes, it’s a shrimp. A measly two-inch-long shrimp. But you don’t understand. This one single stupid shrimp is just about the coolest thing I’ve ever heard. Trust me, and when you hear one too, you’ll probably go deaf, because this one little shrimp IS ONE OF THE LOUDEST ORGANISMS ON THE PLANET. I’m talking 220+ decibels, much louder than a gunshot and louder than standing next to a jet engine at take-off. O_o.

First, I want to talk about some other wicked cool stuff this little guy does apart from being a tiny walking firework of sonic pain. These shrimp garner lots of attention from ecologists because of their symbiotic, or mutually-beneficial, relationship with gobies, a type of small marine fish. The coral reef populations of this shrimp build burrows and recruit the help of a goby to stand guard out front while being no more than an antennae’s distance from the shrimp, using the goby’s well-developed vision to watch for danger. Once danger comes, the goby flicks its tail to signal a threat, and both the goby and the shrimp retreat into the same burrow. Then they start slapping five. It’s flippin sweet.

The mutualisms don't just end at gargoyle-ing.

From an ecologist’s point of view, this is totally mutualistic, and some scientific literature lists this interaction as “totally f-ing amazing” [citation needed] (Good luck with finding THAT reference -ed.). Yeah, both the goby and the shrimp live to see another day should a shark, barracuda, squid, rogue buoy, really fast snail, very-misplaced land predator, sinking boat part, cloud of wasps, or sentient shrimp-hating spear come around to threaten them. The goby gets a burrow, and the pistol shrimp gets a gargoyle. Selection at work.

I like to think of the goby as the pistol shrimp’s pet, kinda like me buying a falcon and parading it around the neighborhood on a leash. And once danger comes, the falcon goes “Hey yo, Kevo, you might need to reconsider this route”, we split, we don’t find danger, and I buy the falcon ribs for a job well done (and because I really want ribs right now). So in a sense, pistol shrimp are danger shepherds equipped with sun-blasting cannons of glory that we’ll get to in a moment.

I need to take a break to collect myself. Holy balls, these shrimp own everything own-able. Next time I see one, I’m buying it ribs. The whole rack.

Let’s get to that “pistol” thing, why don’t we? Pistol shrimp have one very distinct set of front appendages, like two arms extending outwards from its body. Only, one of these arms is disproportionately larger than the other, and it’s quite a contrast. Like no foolin’ man, this one arm alone can be as long as half the entire length of the shrimp’s body. Imagine walking around with one of your arms the size of the Hulk’s and you get the idea. This may look pretty stupid, but this powerful musculature operates one of the most badass weapons nature can offer.

Yeah, kinda like this

When in serious danger or when hunting, the pistol shrimp cocks its massive claw backwards into a 90^o position. Once the claw snaps shut, it produces a jet of water propelled forward at 60mph. Yeah, woo, a fast water jet. Big deal. But here’s the amazing part. Are you ready to have your mind blown? If you have ever listened to me before, now is a pretty good time, too: This jet of water creates a bubble that cavitates, or implodes, generating a pressure wave that creates a sound loud enough to break glass in an instant. Not only that, but once the bubble cavitates, the heat generated by the force of the implosion causes the temperatures inside the bubble to reach about 18,000 degrees Fahrenheit. Yes. Eighteen. Thousand. Oh, and to top it off, the whole process takes three tenths of a second.

THIS TINY SHRIMP MAKES A BUBBLE SO AWESOME THAT IT’S LOUDER THAN A JET AND MORE THAN TWICE AS HOT AS THE SURFACE OF THE SUN.

You can imagine the horribly bubbly doom the pistol shrimp’s enemies must face. Stunned (or killed) by the force of that kind of pressure change, the pistol shrimp’s prey has basically just become lunch in an instant. The shrimp pulls its quarry into its burrow, and goes to town, just how I could go to town on some ribs right about now. I dug up a video about this whole process in case what I said wasn’t convincing enough:

http://www.youtube.com/watch?v=eKPrGxB1Kzc

If you want me to write about any other badass biological bosses, leave a comment. I may talk about them, or I may just tell you that your suggestions are awful. Probably not the latter though. I like people.

Well, I like most people except you, Charles Barkley. I really miss that table. Now I’ve got nothing to rest my wine on when Glee’s on.

Photo credits:

http://www.airpowered.co.uk/images/dbachart.jpg

http://loyalkng.com/wp-content/uploads/2009/11/arm_wrestling_champion_03.jpg

April 26, 2012
Water: Prankster of the Natural World Part 2

If you haven’t read part 1 (why would you do that?), here is a review of the scenario we are considering in part 2:

Two identical containers of water (same volume, same container size and material) are placed into a freezer. One container contains water at just below its’ boiling point (< 100 °C) and the other at roughly room temperature (21 °C). Which will freeze first?

Well I have two jars right here!

In part 1 I discussed the Leidenfrost effect, named after a guy named Leidenfrost. In part 2 I will discuss the Mpemba effect named after, you guessed it, a guy named Mpemba. Interestingly, Erasto Mpemba was a young student when he made the initial observation that hot water can freeze faster than cold water. While he was not the first to make this discovery, his observation led to the first scientific study of the topic so it bares his name.

At first glance, the second scenario is decidedly less exciting. No boiling, no sizzling, and unfortunately no shoddy videography, but unlike the Leidenfrost effect the mechanism behind the Mpemba effect is still hotly debated. During the boiling experiment the lower thermal conductivity of a trapped steam layer prevented efficient heat transfer to the remaining liquid droplet. Several competing explanations are offered for the Mpemba effect, but I will only consider the influence of variation in heat transfer upon freezing. In general “heat transfer” can be considered to explain both the Mpemba and the Leidenfrost effect, and there are three modes of heat transfer. Last time I described conduction which is the transfer of heat within a solid material or between two physically connected bodies. Conduction assumes that the atoms involved in the transfer of heat are fixed in space, like they would be in the solid pan from the Leidenfrost scenario or the walls of the container for the Mpemba effect. Thermal conductivity as described in part 1 is a measure of heat conduction. In a fluid convection is the major mode of heat transfer and implies that atoms or molecules are free to move around. Convection is the reason a bath of cold water with hot water poured into it will eventually reach a single homogenous temperature or opening the front door on a cold day will “let the cold in”. When dealing with both solids and “fluid” phases (a fluid means a non-solid phase) both conduction and convection can be considered. A more detailed explanation of conduction and convection can be found here.

The third mode of heat transfer is radiation, which is the transfer and absorption of electromagnetic radiation. This was indirectly discussed in a previous article but is not important for these scenarios. A nice animation of each of the three modes mentioned here can be found on the web.

To observe the Mpemba effect I took two identical salsa jars and ate their contents. This did not result in the Mpemba effect so I cleaned the jars and set about a second experiment. In one jar I put ¼ cup of tap water and allowed the water to reach room temperature by setting it on the counter for a few minutes. The temperature of my room was about 24 °C (75 °F). In the second jar I put ¼ cup of boiling water. By the time I poured the water it had cooled slightly from the boiling point (100 °C or 212 °F) to about 88 °C (190 °F). I checked these temperatures with a meat thermometer intended for porkchops and roasted chickens so the accuracy is not perfect but for this experiment it doesn’t matter – we only want to confirm that one container has water at a significantly higher temperature than the other. I placed both containers into my freezer which according to the same thermometer is somewhere around -15 °C (5 °F). After two hours I took the containers out and had a look-see. The room temperature water was mostly frozen except for a cavity between the ice at the top of the jar and the bottom of the jar itself. The boiled water was frozen solid. If I moved both jars around motion of the remaining liquid water in the tap water jar was obvious. From left to right below is both jars from the top after freezing, the room temperature jar from the bottom, and the boiling water jar from the bottom. The opening image for this article shows both jars unfrozen.

Both jars after freezing – tap water on left

Boiled water frozen

Frozen tap water

The heat transfer explanation that fits my results goes something like this: due to a difference in density between hot and cold water, hot water in the boiled container rises to the top and delays freezing. When the hotter water cools slightly it sinks to the bottom of the jar and stirs the rest of the container, keeping the temperature in the container homogenous. These movement within the water due to relative density differences are called convective currents. The convective currents are strong enough to keep the average temperature in boiled hot water jar above freezing until the entire container freezes simultaneously. Convective currents due to temperature differences are responsible for many naturally occurring events like the movement of an atmosphere around a planet. Meanwhile, the initially cooler tap water freezes at the top of the container preventing convective heat transfer between the water and the cold air in the freezer. Convective currents are also present in the tap water jar, but the remaining heat distributed by them is not enough to prevent a top layer of ice from forming.

I feel obligated to admit that I had some difficulty getting this experiment to work. At first I tried using a whole cup of water in the same jars, checking the temperature in each container every five minutes. In this test I found that the boiled water dropped in temperature very rapidly at first whereas the tap water changed temperature very slowly. Both jars reached around 34 °F, just above the freezing point, at about the same time. The tap water jar froze on the top first making subsequent measurements of the underlying liquid difficult. Eventually both containers were mostly frozen but I couldn’t tell when the interior of either jar had frozen. Repeatedly opening the freezer door to inspect the jars also manipulates the experimental conditions, so when trying this at home I recommend you only check the freezer once an hour or so to see if anything is frozen. Transparent containers like my glass salsa jars make a quick inspection easier, but using containers of different materials could lead to some interesting results. It is this apparent sensitivity to experimental conditions that makes the mechanism responsible for the Mpemba effect subject to question. You can read more about some of these on the Wikipedia article for the Mpemba effect, but I encourage you to try some of your own tests and see what you find!

April 10, 2012
Water: prankster of the natural world part 1

If you’ve ever taken a life sciences course your teacher probably mentioned one of several properties that make water particularly interesting. Water is cohesive meaning it sticks to itself, adhesive meaning it sticks to other things (superhydrophobic surfaces notwithstanding), and is considered the “universal solvent” because it readily dissolves many other substances. These properties are some of the most critical that allow water to function as the key to most terrestrial forms of life. Another interesting fact about water is that it is one of the few substances that exists in all three states of matter (solid, liquid and gas) in fairly non-extreme locals on earth. Furthermore, these phases exist within a relatively narrow range of temperatures – the entire transformation from solid to gas spans only 100 degrees Celsius (from melting of ice at 0 °C to boiling of liquid water at 100 °C). Compare that to ethyl alcohol (the alcohol in “alcoholic” beverages) which solidifies at -114 °C and boils at 78 °C (a range of only about 200 °C, but with freezing occurring at a temperature below the coldest temperature ever recorded on earth) or a pure metal like iron with a melting temperature of 1538 °C and boiling temperature of 2862 °C – that’s a phase range of over 1000 °C!

It is important at this point in my introduction to define a phase: a phase is one of several atomic or molecular configurations for a given composition. When considering melting and boiling points, we are observing only the transition between the three states of matter mentioned above, however it is equally correct to consider multiple crystal structures for one composition of solid as different phases. Some of these solid phases are not easily distinguishable without sophisticated analytical techniques, while others have characteristic changes in physical properties. No one would confuse graphite and diamond though they are in fact different phases of the same element. Plasma, also known as a superheated gas, is considered a fourth phase of matter but not one normally observed in nature with a notable exception being bolts of lightning. However, when heated to the point of being a plasma the atoms defining a given composition become ionized and for this reason plasma is normally considered separate from the more conventional states of matter. Fun fact for gamers: this is the plasma in all your favorite late game super-weapons.

This is where things get weird for our old friend agua.

Consider the following scenarios:

1) Two identical pans are pre-heated to temperatures above the boiling point of water. The first pan is heated to 150 °C and the second pan to 300 °C. If drops of water at room temperature are placed into each pan, which drop will undergo the phase change from liquid to gas (boil) first?

2) Two identical containers of water (same volume, same container size and material) are placed into a freezer. One container contains water at just below its’ boiling point (< 100 °C) and the other at roughly room temperature (21 °C). Which will freeze first?

If considering only the energetics of each scenario, the logical conclusion is that a higher temperature pan means faster boiling, and a lower starting temperature means faster solidification. But if you were to perform these exact experiments at home (which I hope you do!) the opposite would occur.

So whats going on? Is water playing by the rules (of thermodynamics)? Fortunately for us, Gibbs will remain stationary in his grave. As it turns out, both of these scenarios are good examples of how the properties of matter can change dramatically after a phase change. In both of these cases, variations in heat transfer between phases is likely to blame.

For the water in the pan scenario, there exists a temperature above which water is so quickly vaporized upon hitting the pan a thin layer of steam is trapped below the liquid droplet, causing it to literally levitate above the surface of the pan on cushion of water vapor. Think air hockey table, but droplets of boiling water replacing the plastic puck. This temperature is called the “Leidenfrost point”, which according to unreferenced figure from Wikipedia is about 200 °C. Steam has a thermal conductivity roughly 10-100 times lower than that of liquid water depending on the temperature. In an idealized example, thermal conductivity is a measure of how easily a quantity of heat in a high temperature region can travel from one side of a 100% dense (no porosity) slab of material to the other. Thermal conductivity has units of energy per length per degree per second (Joules/(meter*°C*seconds)), meaning for a fixed length and temperature a material with lower thermal conductivity will take more time than a material with higher thermal conductivity to transfer the same amount of energy.

At temperatures above the Leidenfrost point the Leidenfrost effect is observed and drops of water on a pan will take longer to boil because the steam trapped below the liquid water takes more time to transfer heat to the droplet from the pan. The videos below demonstrate drops of water into a pan below (top) and above (bottom) the Leidenfrost point. Above the Leidenfrost point, the droplets can barely be heard to sizzle as they zoom around on cushions of steam. For the pan heated above the Leidenfrost point, occasional contact with the edges of the pan, which are actually at a lower temperature or placing drops repeatedly in the same area which leads to local cooling will lead to momentary sizzling as the entire droplet boils. For the videos below, I used a nonstick pan and an eyedropper to put a few drops of tapwater onto the pan at a time. For the lower temperature I set my range to setting of 2 out of 10 and for the higher temperature I set it to about 6 out of 10 and waited a few minutes at each temperature for the pan to heat up.

You may have heard of ole’ Leidenfrost’s namesake before associated with dipping things (namely one’s hand) into boiling lead after placing it in water. The Myth Busters did it after significant deliberation and preliminary testing, but somehow the alpha geeks at PopSci seemed to think it would make a good “DIY” project. I have to say, I can understand why dipping digits is the sexier version of the Leidenfrost effect demonstration, but the pan + water trick seems a little less invasive. In their defense, PopSci also covered this demo and the equivalent effect with boiling Nitrogen instead of steam as well.

However you plan to enjoy the Leidenfrost effect, do so under proper supervision and be careful.

Check out part 2 for a discussion on the second scenario!

March 23, 2012
Sequins + Dress + Spotlights = Laserdress

I had a chance to catch some of the Golden Globes the other night. By had a chance I mean, I elected to watch the Golden Globes. For a little bit. You know how it is, when you have a choice between watching celebrities clap or watching reruns of M*A*S*H you have to ask yourself, “When is the next time I can watch M*A*S*H?”

When I did stick around, it wasn’t just because I think Emma Stone is a stone cold fox (I do), or because I thought Ricky Gervais is particularly funny (I didn’t), and it’s not because I really care what the celebrities are wearing (I don’t). It was all the dang sequins.

Maybe this is just from a fashion uninitiated, but there were a lot of sequins going on at the Golden Globes. But then I thought about it – why not? Sequins do a pretty good job of catching the eye, and that’s kind of the red carpet Modus operandi, right? Sequins’ secret lies in their ability as a reflector of incident light. Much like a signal mirror used to reflect the sun towards a rescue plane, sequins sewn onto a dress (or a pair of hideous shoes at DSW) reflect any light that hits their surface at an angle equal in magnitude to the incident angle reflected through a plane perpendicular to the reflecting surface. This is analogous to bouncing a ball off of a smooth surface. If the ball is dropped straight down the angle of incidence is 0° and so the ball bounces straight back up. Meanwhile, if the ball is bounced at an angle like a basketball being bounce-passed between two players, after striking the ground the ball will travel towards the second player at an angle identical to that thrown by the first player. This is called specular reflection (with light not basketballs). When considering this concept from the viewers perspective of a be-sequined dress, the dress appears to shimmer and flash. If the light-source comes from a single direction (like a spotlight) then the incident angle is fixed and the relative angles of the sequins change with the motion of the wearer. The result is flashes of light as individual sequins pass through an orientation which places the reflection angle directly towards your eyeballs!

Maybe that’s not that surprising to you, given that you have observed this phenomenon first hand dozens of times in mirrors. However consider this – not all light is made equal. Light can be described as wave traveling through space from the source. These waves have the shape of the trigonometric sine function when visualized in two dimensions, and have an associated wavelength that is related to the energy of different forms of light. To humans and many other species, our eyes detect this difference in wavelength as color, but only in a relatively narrow range of wavelengths. This is referred to as the “visible spectrum” , and the acronym ROYGBIV known by many a grade-schooler as the colors of the rainbow actually orders the visible spectrum correctly from longest to smallest wavelength!

Thus, while sequins are reflecting the crap of out the visible spectrum, light waves with appreciably longer or shorter wavelengths will have different interactions. For this reason, reflectors designed to reflect these different wavelengths can require drastically different shapes and materials. Some examples include a satellite dish whose curved edges reflect microwaves (these are of similar wavelength to the ones used in microwave ovens!) towards central antenna (the prong in the middle) increasing the signal, and metal coated Mylar (a type of flexible plastic) used in space blankets and on satellites (the ones in space, not the dish on your roof) to reflect infra-red radiation. In the case of space blankets, these reflect infra-red radiation (light waves with wavelengths just above the color red in the visible spectrum responsible radiative heat transfer – this is why heat lamps in restaurants are red!) emitted by the wearer back to their body. On satellites this wavelength is emitted by the sun and reflected away from heat sensitive electronics. In the case of metalized Mylar, the reflection is diffuse rather than specular, meaning the reflection takes place at many angles.

So we’ve learned that sequins are sparkly because they reflect light at discrete angles. If they operated with diffuse reflection, the entire dress would appear to glow rather than sparkle. Fingers crossed that the fashion community will one-up themselves at next years GGs. A simple experiment you can try at home can be done with a mirror, a yard stick, and a flashlight (or ideally a laser pointer). Measure one yard from a wall and place the mirror on the ground, then place yourself one yard away from the mirror facing the wall. Hold the flashlight at the top of the yardstick pointing towards the mirror. The reflected light should strike the wall at the exact height you are holding the flashlight! This is specular reflection in action!

Image: m_bartosch / FreeDigitalPhotos.net

January 25, 2012