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Kilogram Reference Losing Weight
doubleacr writes "Ran across a story on CNN that says the "118-year-old cylinder that is the international prototype for the metric mass, kept tightly under lock and key outside Paris, is mysteriously losing weight — if ever so slightly. Physicist Richard Davis of the International Bureau of Weights and Measures in Sevres, southwest of Paris, says the reference kilo appears to have lost 50 micrograms compared with the average of dozens of copies.""
It's not losing weight, it's losing mass!. The kilogram is not a measure of weight, but mass. Silly pound-centric editors :p
I thought that originally the kilogram was defined in terms of water, the mass of 10 square cm of water.
We can't use water as a reference since the molecules in the water are constantly splitting into ions and reforming as molecules. So it is essentially impossible to get 1000 cm^3 of "pure" water. It will be some mixture of H2O, H+ and O-- ions. Also, it would be incredibly hard to prevent other molecules from being disolved in the water. A few stray molecules hitting the surface will ruin your reference mass. Not to mention you need a container to keep it in...
The meter is defined in terms of the speed of light so that gives an empirical way to define the kg independent of anything else.
As mentioned above, we could measure a 1000 cm^3 volume, but we couldn't guarantee the purity of the water in that volume.
That's one reason we are trying to make a perfect sphere to replace the reference kilogram. Then we will have a definition of the kilogram in terms of number of silicon atoms.
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A meter is defined as the distance light travels in a vacuum in 1/299,792,458th of a second.
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The density of water changes when you vary the temperature or pressure, so you'd need an accurate measure of distance, temperature, and pressure in order to get your 1Kg of water.
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I think you mean that the density of water is essentially invariant to pressure.
It very much fluctuates with temperature.
This entire story (which has appeared on a lot of general news sites, but no science news sites) is probably just a case of a reporter misunderstanding something a scientist said. According to the UK NPL site, fluctuations in the physical objects used to define fundamental metric units has always been a problem. Back when they were created, the ideal material for them seemed to be a hard, dense iridium-platinum alloy. This turned out to be a nasty mistake: the alloy is slightly radioactive, which means that some of its mass flies off into space all the time. No mystery there.
This is why most fundamental units are now based on natural constants. For example, the meter used to be the distance between two notches on a platinum-iridium stick. (Before that, it was defined as 1 ten-millionth of a line that goes from the equator to the north pole; except they miscalculated the length of the line!) Now it's based on how far light travels in some tiny amount of time. But there's no consensus as to the best way to get rid of the physical kilogram.
In other words, all we have here is a clueless reporter trying to fill up a slow news day.
It will be some mixture of H2O, H+ and O-- ions.
I really doubt you'll see O-- ions in water. H2O actually splits into H+ and OH- and the H+ often ends up (IIRC) forming an H3O+ ion.
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There's currently a project (which was reported on Slashdot months ago) to make a new reference kilogram of a specific element and geometry. From that, they can define the kilogram as a certain number of atoms of a certain element.
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"You could specify the density of water at $PRESSURE and at its maximum density (somewhere around 4 C)."
Self-referential. Pressure is (force)/(length^2), or breaking it down further, (mass)/[(time^2)(length)]. This is why BIPM abandoned the "cubic deciliter of water" definition in favor of the current platinum-iridium artifact (less compressible, less affected by temperature, etc).
"The only problem with doing this for high-precision measurements is: what is water? Some fraction of the hydrogen will be deuterium, and that'll throw off the density. What fraction of the hydrogen should be deuterium for "standard water"?"
Not an issue, as the average rates of naturally occurring isotopes in the universe is already known (hence the non-integer masses in periodic tables). You'd have a greater problem establishing the purity of the water sample in question, at least if you insist on using it in its liquid state; they don't call it the "universal solvent" for nothing.
OK, exactly how far up your ass did you have to reach to pull that one out?
See, we have this thing called "The First Law of Thermodynamics." At the molecular scale, water molecules don't just decide to break up and go their own way willy-nilly, not the least because both elements involved (hydrogen and oxygen) really don't like being alone (the two hydrogen atoms can go off on their own merry way as a diatomic molecule, but the oxygen will be lonely). Breaking molecular bonds in water takes energy, otherwise cracking water to produce hydrogen would be more cost-effective than cracking methanol (the carbon atoms have a more independent personality and are better able to get over any rejection issues it might have).
Beyond that, even if the energy to crack an individual water molecule were as trivially small as you believe, the energy would have to come from somewhere. Cracking water is endothermic, but so is making it (oxygen atoms, at least, need to be pried apart against their will first, assuming they're not in some kinky threeway), but even if one of those two reactions was exothermic, the energy required to do one act must necessarily equal the energy released by the other, meaning a net change in energy, and a net change in the number of water molecules, of zero.
The real reasons we don't use water are:
- Corrosiveness (which you already covered)
- Compressibility (there is no such thing as an incompressible substance, but liquids are more susceptible than solids)
- Thermal expansion (something else solids are less susceptible to)
- Last, but not least: evaporation
"So it is essentially impossible to get 1000 cm^3 of "pure" water."Very easy, actually; the problem is maintaining its purity after it cools down from superheated steam.
"That's one reason we are trying to make a perfect sphere to replace the reference kilogram. "
Actually, there are a number of different proposals. One involves fixing the Avogadro constant as you say, but the other involves basing mass in terms of an electrical current through a device called a watt balance, which would reverse the current relationship between mass and electric current.
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Remember this useless thing called pH, used for measuring acidity and basicity? You may remember that the neutral point is 7, the pH of pure water at 25 degrees Celsius, when the amounts of H3O+ and OH- are almost equal (yes, those ions exist even in pure liquid water). See this for more information. Thermodynamics is all right but some of its laws get quirky at sufficiently small distances.
The meter has a long history and was in fact once defined as "one ten-millionth of the length of the Earth's meridian along a quadrant, that is the distance from the equator to the north pole". Then it was a number of standard wave lengths and not until 1983 that the meter was defined as how far light travels in a very short time. Wiki has a good article on the meter.
In a vacuum the speed of light is constant - even in a gravitational field as long as your are freely falling.
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