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InvisibleP-O


Registered: 10/08/11
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Re: Hot water freezes faster than cold water? [Re: RasJeph]
    #722308 - 03/20/14 03:36 PM (5 years, 9 months ago)

comments say they are not sold.


We need a growerite to do the experiment for the boys


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InvisibleDataM
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Re: Hot water freezes faster than cold water? [Re: P-O]
    #722361 - 03/20/14 08:07 PM (5 years, 9 months ago)

I'm not sold either.

Covalent bonds are formed by a sharing of electrons b/w two atoms in order to complete their valence shells and achieve a lower net energy state. This shared pair of electrons is not shared equally in H2O. The oxygen atom is more electronegative, meaning it has a stronger postive charge influence from the protons in its nucleus at the point where the electron pair is being shared, and attracts the electron pair more strongly than the Hyrdogen atoms. This leaves the hydrogen atoms' nuclei exposed more often, resulting in a slightly positive charge on the hydrogen atoms, and a slightly negative charge on the oxygen atom.

At the same time, the electron distribution of a water molecule looks like this:


The two unshared pairs are repulsed by the electrons being shared b/w the hydrogens and oxygen. This pushes the two electron pairs and the two hydrogens into a tetrahedral shape around the oxygen, like this:


Each of the four bonds are equally repulsed because the electrons forming the bonds all have equally negative charge. This results in this uniform shape that maximizes the distance b/w bonds. But in water, the hydrogens are slightly postive, or rather their electron shell is less negative, so they are not as repulsed by each other as the other two unshared pairs are from each other. This results in the hydrogen atoms being forced closer together as the unshared pairs push farther apart. This results in the 104.5 deg. bond angle seen in water molecules.

This bent angle results in a slightly positive side and a slightly negative side of each water molecule. When water molecules are close to each other, this slight attractive force begins to influence their motion. This is the hydrogen bond. It is significantly less powerful than the covalent bonds that bind the water molecule together, and is in fact considered more of a pseudo-bond. These hydrogen bonds are so weak, that they are constantly being formed and broken and reformed b/w liquid water molecules, even at cooler temperatures.

This is the kicker, the longer the hydrogen bond, the more energy it stores (according to the article). So, as the molecules cool and slow down (temperature is average kinetic energy of molecules), they are drawn closer together by these hydrogen bonds. The potential energy stored in the attractive force of the hydrogen bonds acts to accelerate the water molecules as they fall towards each other, thus imparting kinetic energy on the molecules. In effect, the stored energy in the hydrogen bonds actually ADDS HEAT to the water molecules, which must then be removed.

What I'm trying to say is, the explanation in that article do not consider the FACT that in order for water to freeze, it must first be cold. When the hot water becomes cold, its intrinsic properties are the same as those of the cold water sample. This means that some sort of mechanism acting independent of the intrinsic properties of liquid water must be causing this effect. At this point, the only explanations that I would be willing to accept are mass loss by evaporation and a stronger natural convection current that persists after the water has cooled down.


--------------------
“The Universe is under no obligation to make sense to you” -NDT


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OfflineThebooedocksaint
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Re: Hot water freezes faster than cold water? [Re: Data]
    #722382 - 03/20/14 10:38 PM (5 years, 9 months ago)

Your use of the term Hydrogen bond makes me unsure if you are talking about the covalent O-H bond, or the intermolecular dipole bond known as the Hydrogen bond.

I'd presume it has to do with the fact that when water is in solid for its crystalline shape is organized by the intermolecular Hydrogen bonding, of which each water molecule tries to have 4 of. I'd say the water originally being hot allows the molecules to move around more, and thus orient themselves by chance into hydrogen bonds. Once the molecules have low enough energy, and chance to be correctly oriented to freeze,  they freeze. It starting with very little energy might just make it take more time for the Hydrogen bonds to orient themselves correctly.

I know this is also a well documented fact that these molecules freeze faster when hot, i don't know how PH may factor in though. That may or may not lead to different conclusions. But frankly most chemistry exceptions are just exceptions.


--------------------
"To say that nothing is true, is to realize that the foundations of society are fragile, and that we must be the shepherds of our own civilization. To say that everything is permitted, is to understand that we are the architects of our actions, and that we must live with their consequences, whether glorious or tragic."

"Je pense, donc je suis (I am thinking, therefore I am)." -Rene Descartes

I am tired of Earth
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InvisibleDataM
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Re: Hot water freezes faster than cold water? [Re: Thebooedocksaint]
    #722398 - 03/21/14 12:18 AM (5 years, 9 months ago)

I'm talking about both in my last post, but any reference to hydrogen bond is referring to the inter-molecular dipole bond.

The problem is that in liquid water, a large portion of the molecules exist in a 2 hydrogen bond arrangement (they only share 2 hydrogen bonds with neighboring molecules). These bonds typically last on the order of femtoseconds. These weak and temporary bonds also are not confined along O-H--O inter-molecular lines, and may be bent along a curving path between the 3 atoms. This constant formation, stretching, bending, and breaking of these bonds is what allows the liquid state of water to exist. The strength and lifetime of hydrogen bonds further decreases with the increased kinetic energy and increased inter-molecular distances associated with higher temperature. This would mean that hot water samples will be less likely to be governed and organized via hydrogen bonding than cold water samples.

Consider a hot water sample. A high temperature equates to high average kinetic energy. This would mean that all molecules have the necessary kinetic energy to break any hydrogen bond that tries to impart influence it. Because inter-molecular hydrogen bonds are attractive in nature, and because energy is conserved within the bulk sample any change in kinetic energy of a molecule due to hydrogen bonding would mean an equal yet opposite change in kinetic energy to the other molecule at the other end of the hydrogen bond. Coupled with the extremely short duration and weak interaction of the hydrogen bonds, organization of molecules via hydrogen bonding becomes very nearly impossible.

Because of this, I still don't think that any properties that are intensive to H20 can explain why a hot water sample will freeze faster than a cold water sample. I'm not saying that this Mpemba effect doesn't exist, I'm just saying that it cannot be due solely to an intensive property of water being different at one temperature or another. The effect must be caused by some change in the extensive properties of the water sample like mass, dissolved solids, convection characteristics, surroundings, etc.


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“The Universe is under no obligation to make sense to you” -NDT


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OfflineThebooedocksaint
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Re: Hot water freezes faster than cold water? [Re: Data]
    #722459 - 03/21/14 01:37 PM (5 years, 9 months ago)

H20 actually can hydrogen bond 4 times. Donating twice (two Hydrogen, accepting twice (two lone pairs). Each one is a force of attraction between these molecules. The assumption I entertain, and the one any chemistry professor told me, was that these Hydrogen bonds are probably the reason. I don't know how hot ammonia freezes, but it's the only low weight molecule I know of that can do a lot of Hydrogen bonding like that... I guess (but ammonia is actually worse if it's the only liquid present).

Hydrogen bonding might seem weak, but the Strands of DNA are held together through Hydrogen bonds. They are stronger than your normal Dipole-Dipole intermolecular interaction. Perhaps once the Crystal Matrix begins forming the slightly warmer (than cold water) interior is able to become part of the matrix faster. With all these collisions occurring more often, because of increase velocity, the chance occurrence of molecule hitting each other correctly to cause a reaction is increased.

There indeed probably are other physical characteristics, since chemists themselves don't actually know why. Sometimes you have to shrug and move on when there isn't much more to be done experimentally.


--------------------
"To say that nothing is true, is to realize that the foundations of society are fragile, and that we must be the shepherds of our own civilization. To say that everything is permitted, is to understand that we are the architects of our actions, and that we must live with their consequences, whether glorious or tragic."

"Je pense, donc je suis (I am thinking, therefore I am)." -Rene Descartes

I am tired of Earth
I am tired of these people


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InvisibleDataM
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Re: Hot water freezes faster than cold water? [Re: Thebooedocksaint]
    #723126 - 03/24/14 07:54 PM (5 years, 9 months ago)

Quote:

Thebooedocksaint said:
Sometimes you have to shrug and move on when there isn't much more to be done experimentally.




I hear ya, and yea from a practical perspective there is no reason to be discussing this. My whole reason for starting this thread was to discuss a topic that has no definite answer. Conversations about topics that have already been ironed out and well documented are very practical, but can get boring sometimes. I have actually learned something from this thread, because I had no clue that the Mpemba effect was an observed and documented phenomena :eek: that shifted my personal opinions on the matter from "this crazy shiz can't even happen" to "maybe I should think about this more..." :cool::thumbup:


Quote:

Thebooedocksaint said:
H20 actually can hydrogen bond 4 times. Donating twice (two Hydrogen, accepting twice (two lone pairs). Each one is a force of attraction between these molecules. The assumption I entertain, and the one any chemistry professor told me, was that these Hydrogen bonds are probably the reason.




I understand that H20 has the ability to bond 4 times, and indeed it does in liquid water. 4-bonded H20 (2 strong, 2 weak) makes up about 15% of a bulk sample of liquid H20 at 25oC. However, 80% of that sample is composed of 2-bond H20 (one strong, one weak bond), and 5% of that sample is non-bonded H20. When that same sample is at 90oC, that makeup shifts to roughly 10% 4-bond, 85% 2-bond, and 5% non-bonded.  [Citation Abstract]



Quote:

Thebooedocksaint said:
Hydrogen bonding might seem weak, but the Strands of DNA are held together through Hydrogen bonds. They are stronger than your normal Dipole-Dipole intermolecular interaction.




This is very true. However, the reason DNA strands are solid at room temperature is because of the strong covalent phosphodiester bonds linking the "backbones" of each strand. There are also localized charges along the outside of the double helix which further increases its effective melting point. Additionally, in the center of the double helix you have 2 to 3 hydrogen bonds per base pair, all pulling toward the same central axis, this will result in a force multiplication all acting in a common direction. This is not true in a sample of H20, where all hydrogen bond forces are acting in random directions with highly variable force, meaning that the net forces acting on a single H20 molecule will be randomized and not nearly as constraining as the concerted effort of the hydrogen bonds between DNA base pairs, thereby less likely to pull and constrain a molecule into a lattice pattern. Since hydrogen bonding decreases in water at higher temperatures, this lack of effect seemingly becomes more pronounced.

The reason I'm pointing this stuff out is to provide a point-counterpoint discussion. I like discussing why a certain idea could or could not work. This was the kind of discussion I was looking for on this topic. :highfive:


Quote:

Thebooedocksaint said:
Perhaps once the Crystal Matrix begins forming the slightly warmer (than cold water) interior is able to become part of the matrix faster. With all these collisions occurring more often, because of increase velocity, the chance occurrence of molecule hitting each other correctly to cause a reaction is increased.




This may very well be the case, and in reference to your comment on the lack of experimental evidence, I believe this would be a good topic for researchers to focus on for future experiments. IMO it seems like the only reason we currently don't know why/how this effect is accomplished is simply because its not a very pronounced effect that works all of the time. This leads to lack of interest, and thus a lack of research funding. It could very well be the case that 5 years from now I'll have to say "Man that dude told me thats what was going on..." :gethigh:


--------------------
“The Universe is under no obligation to make sense to you” -NDT


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