Forget what you know about H2O

Forget what you know about H2O
By Robert C. Cowen

We take the properties of water for granted. Yet scientists assure us that we have a lot to learn about our biologically essential old friend. Research supporting the counterintuitive claim that hot water freezes faster than cold illustrates this point. The claim made news in 1969 when Erasto Mpemba, a Tanzanian schoolboy, said that his ice cream mixture froze faster when it started out hot than when it started out cold. Never mind that others have reported this strange behavior of water for centuries. Skeptics scoffed. The boy's teacher spoke derisively of "Mpemba's physics."

It's time to rethink the derision. Jonathan Katz at Washington University has studied the "Mpemba effect" and finds the claim valid. He explains that it has to do with the hardness of water. Dissolved bicarbonates of calcium and magnesium, which make water hard, also depress the water's freezing point and slow its rate of cooling compared to softer water that has less of the bicarbonates. Heating water causes the bicarbonates to precipitate out. This forms the deposits in hot water kettles, for example. Thus, preheating water (or Mpemba's ice cream mix) softens the water, allowing it to freeze faster and at a higher temperature than cold, hard water.

Dr. Katz speculates that the Mpemba effect doesn't always show up because experimenters are using soft water to begin with, according to a report in New Scientist. In that case, there would be no advantage to preheating the water. Reviewing the physics of water in Science two years ago, Yan Zubavicus and Michael Grurze at the University of Heidelberg in Germany explained why "liquid water is one of the most mysterious substances in our world." Water can form 13 known forms of ice crystal "or probably infinite number of its amorphous modifications," they wrote. Research at the molecular level is showing that the traditional image of water molecules as pyramid-like tetrahedrons holding hands with one another is grossly oversimplified.

For example, Greg Kimmel and colleagues at the Pacific Northwest National Laboratory in Richland, Wash., have made a film of water that repels other water like a water-repellent layer of paint. They spread a layer one molecule thick on a platinum surface. Instead of leaving attachment points free where other water molecules could bind, the molecules in the first layer fixed all their attachment points to the platinum. Any new incoming water was repelled as decisively as raindrops bouncing off a well-waxed automobile.

more:
http://www.csmonitor.com/2006/0615/p16s01-stss.html

world is a mysterious and wondrous place to be... despite its human population!

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Thank you.

EVERY time I boil water. Thanks for the post!

(asked by a kid)

If hydrogen if flammable

and oxygen is flammable

why isn't water flammable?

We usually define flammability as being in the presence of oxygen, and refer to things that are flammable as things that will react rapidly with O2. Actually, "flammability" is a relative thing. There are certain chemical fires that you do not want to try and dowse with water as it will increase the flames. Water will "burn" with certain, highly reactive substances such as sodium metal, which is why solid Na is always stored under a blanket of oil. Toss a chunk of it into a bucket and BOOM!

I think it was on the show Braniac that I saw this done. Hand grenade in a bathtub. Would have loved to have seen it done in an old cast iron tub rather than the fibreglass one that was used.

The process of burning something is the process of oxidizing it -- ie., combining it's atoms with oxygen atoms. If there is no fuel available to combine with oxygen, oxygen will not catch on fire by itself.

Water is sort of like 'burnt hydrogen', if you want to think about it that way. If you burn pure hydrogen, you get pure water as a side product.

you get pure water as a side product."

Interesting! Thanks for the info.

burn pure hydrogen with pure oxygen.

I thought about saying that, but then I thought, "Oxygen is implied by the term 'burning'", so then I thought, "Aw, heck with it -- I'll leave it."

:)

"burning" implies an oxidizer, which is often but not always oxygen.

What else does hydrogen oxidise with?

(chem :dunce:)

I was speaking of oxidation in general, not hydrogen in paricular, so I don't know of any specifics.

But we could hypothesize a reaction of hydrogen with sulfur: 8 H2 + S8 -> 8 H2S

wherein the sulfur gets reduced and the hydrogen oxidized.

I don't know anything about the kinetics of such a reaction; I supposed it'd be exothermic but I couldn't comment on the spontaneity.

> Oxidation and reduction, complementary chemical reactions characterized
> by the loss or gain, respectively, of one or more electrons by an atom
> or molecule.
>
> Originally the term oxidation was used to refer to a reaction in which
> oxygen combined with an element or compound,
>
> e.g., the reaction of magnesium with oxygen to form magnesium oxide
> or the combination of carbon monoxide with oxygen to form carbon dioxide.
> Similarly, reduction referred to a decrease in the amount of oxygen in a
> substance or its complete removal, e.g., the reaction of cupric oxide
> and hydrogen to form copper and water.
>
> ...
>
> Thus oxidation has come to be defined as a loss of electrons or an increase
> in oxidation number, while reduction is defined as a gain of electrons or
> a decrease in oxidation number, whether or not oxygen itself is actually
> involved in the reaction.

http://www.infoplease.com/ce6/sci/A0837179.html

I had this vague memory that oxidation didn't require oxygen (though couldn't
remember the details) but when I read this, had a flurry of decades-old
memories from Chemistry O level ... the storage system is still working
but the retrieval subsystem is getting a bit rusty in places!

easily be ionized in water from the halogen compounds.

One might argue that carbon oxidizes hydrogen as well under certain circumstances, though one should note that methane cannot formally be considered acidic, and is covalent. The extent to which hydrogen can be considered oxidized has to do with polarity, which is a distribution of electrical charge on a molecular level. Compounds in which hydrogen is oxidized have formal positive charges on the hydrogen.

In some cases, some elements are oxidized by hydrogen, boron for instance, and lithium. In these compounds, the formal charge on hydrogen is negative.

As noted by others, oxidation often has nothing to do with oxygen. One of the most powerful oxidizing agents known is hexafluoroplatinate. It oxidizes oxygen. (It also oxidizes the nobel gas, xenon, as discovered by Neil Bartlett in 1962.)

they freeze faster, and the ice is clearer.

When water freezes any disolved gasses "precipitate out" creating the streaks, pits and bubbles that often appear in ice.

Hot water can hold less in the way of disolved gasses than cold water. Thus hot 'flat' water will form clearer ice than cold. The aerators on most modern taps make the effect even more pronounced.

No fancy name.

The explanation given was that the hot water had fewer dissolved gases.

Essentially the same explanation: Raoult's Law.

What I like is watching drops of water skitter across the surface of water. Sometimes they go for many inches before they coalesce with the surface. A temperature difference helps, I think--no actual data here, just ad hoc observations--but I have no idea why the temperature should matter.

dismissed it on the grounds that hot water had to pass through exactly the same thermodynamic state between heating and cooling.

However this article gives a very plausible explanation for the occurence of just such an effect under certain circumstances.

Fascinating and interesting.

A similar kind of issue lead to the discovery of the noble gases in the atmosphere, beginning with argon. It was discovered that air with a cyrogenic source had a very slight difference in density from air obtained via the chemical decompostion of nitrogen compounds like hydrazine. When nitrogen was removed from air by chemical means, it was found that the residue contained argon and other constituents, later found to be neon, krpyton and xenon.

there is ultimately less water to freeze.
One explanation I've read.

Despite being chemically identical to 'normal' water, heavy water is poisonous to a great many biological processes. Subtle differences in the hydrogen bonding of the deuterated water (deuterium instead of hydrogen) are enough to alter the catalytic properties of proteins and enzymes.

Or to sum it all up.

Water is freaking wierd stuff.

Which all in all is a good thing since all those wierd properties contribute to the complexity of life.

Usually isotopic differences have little or no effect on heavier elements.

But in the case of hydrogen, deuterium is twice the mass. This can have quite an effect on the chemistry, and not just when it comes to biological processes. Even simple reactions like acid/base equilibria are effected by a high deuterium content.