Hello mglim!
You had two questions, this one and the one in your other thread, so I'll answer them both.
*This ended up being terribly long; sorry! I'll try to shorten it a bit.
1. You're right-- this article is very high level! It took me a bit of time to understand, but here's a rundown of what I think the article is saying. (Of course, it's possible I misunderstood something, so let me know if part of the explanation doesn't make sense!)
(Keep in mind that this is only one of many possible explanations for the Mpemba effect.) It does have a bit more oomph, though, since its mathematical model seems pretty good.
We usually think about cooling as heat moving out of the object being cooled. This is what
Newton's Law of Cooling says: basically, how fast something cools depends on how much hotter than the outside temperature it is. So, hotter objects cool faster. At first glance, this might seem to explain the Mpemba effect, but if we do some math, we can show that since the hotter one has to cool down more, if we are cooling two cups of water, one hot and one cold, in the same way, the hot one should always have a higher temperature, and that the colder one should always freeze first.
So, Newton's Law of Cooling is missing something.
The paper suggests an explanation for the effect based on something called molecular orbital (MO) theory. You may have learned about atomic orbitals in your science classes-- if not, an atomic orbital is basically a cloud of electrons in an atom. Atoms have orbitals of different energy levels; electrons will generally want to
go to the lowest energy levels available and
spread themselves out as much as possible, in that order. Molecular orbital theory is a bit more complicated; essentially, it means that in any covalent bond, there are two orbitals, one of which is
bonding (stable, low-energy) and the other of which is
antibonding (unstable, high-energy).
*This paragraph is not essential; if it doesn't make sense, just understand as much as you can and move on.
Manipulating the molecule's temperature, intermolecular interactions (including the hydrogen bond, like O:H), or other properties can change the energy of the molecular orbitals. These relationships aren't well-understood yet; the authors of the paper propose that when the water is cooled, the O:H bond shortens as it releases energy, making the H-O bond lengthen and thus increase its energy state. Because its energy is higher, the water is cooling at a rate faster than Newton's Law of Cooling would suggest.
More simply put, the water thinks it is warmer than it is, and thus cools more quickly than expected. This lengthening of the H-O bond and the resulting Mpemba effect are not present if the water
starts out being colder.
2. You mentioned that you were confused by the contrasting explanations of the effect. Here's what I've gathered:
Bonds store energy in different ways. Generally speaking, the longer bonds are, the more energy they store and the higher-energy the bond is. However, in water, there are really two sets of bonds: hydrogen bonds (O:H) and covalent bonds (H-O). Hydrogen bonds store and release energy more easily; however, they don't store much energy at all. So, when cooling begins, hydrogen bonds are the first to release their energy by shortening. However, this doesn't actually cool the water much. But it does force the covalent bonds into a higher energy state, which makes them more willing to cool down quickly.
3. I assume by boiling you mean heating a pot of water to boiling point, then re-cooling it to your desired starting temperature. I would expect the main effect to be a lower amount of dissolved gases. Mineral concentration might also be slightly higher, but I wouldn't expect it to be that significant.
(As a side note, it's likely that after boiling, you'll have a little bit less water remaining. Be sure to factor this in-- maybe try measuring out equal volumes of boiled and unboiled water for your experiments with the Mpemba effect? Remember that a lower volume of hot water could cool faster than a greater volume of cold water, whether the Mpemba effect comes into play or not.)
The Mpemba effect is very poorly understood; scientists disagree on what is really going on. This makes it a great topic for a science fair project! But it also means that your project, unlike most middle school projects, isn't guaranteed to give simple answers. You'll be observing a complicated effect
and varying the type of water at the same time. And you could figure out an important key to the Mpemba effect! But there's also a chance that things won't work out the way you want them to. Scientific investigation can be hard, but it's important to remember that no matter what result you get, you've learned something.
As a bit of advice on the experimentation itself-- I've read a bit on this effect, and it seems to be hard to make it work. So I'd suggest trying to get the effect to happen a couple of times before boiling any water. If you practice getting it to work in unboiled water, you'll already have a solid grip on the procedure by the time you start the real experiment!
Good luck, and if you have any more questions, just ask!
Vysarge
so I can't experiment until I get a new one soon.