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That Guy
Registered: 08/12/08
Posts: 4,062
Loc: Southwestern US 
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 Re: Hot water freezes faster than cold water?  [Re: P-O]
#722361 - 03/20/14 06:07 PM (11 years, 1 month ago) |
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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.
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“The Universe is under no obligation to make sense to you” -NDT
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