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jgourlay
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How do you know an element is an element?
My wife and I are homeschooling our kids. My son is 7 and I'm starting his science curricula with chemistry and basic mechanics (physics).
So far, I've been trying to teach him "element" "compound" "mixture". We've done a number of experiments demonstrating reactions between elements to
make compounds (H2 + O2), element swapping (thermite and NaNo3+KCL-->KNO3+NaCl), and off course we've been growing crystals from everything he can
lay hands on that dissolves in water. Also, for the mixture side, we've done various things mixing iron filings and flour, sugar and salt, etc.
He asked me a question the other day which has me totally flummoxed: how do you KNOW something is an element? As you can probably see from the above,
he now has enough understanding of chemical transformation that he's asking "how do I know this gas you call an element (hydrogen) and this other gas
(oxygen) are not really just compounds or mixtures that yeild some other result like water?" Note that I have shown him the electroplating of copper
from copper sulfate and resulting acidifying of the solution.
Wanting to maintain my fatherly position of all-knowing wisdom, I shot back at him, "well, what test would satisfy you." He immediately shot back me,
"you know chemistry, i don't, you need to tell ME the test."
What is/are the test(s) to establish that, say, iron is an element while hematite is a compound though both are magnetic? Or that oxygen and hydrogen
are not both colorless oderless mixtures whose reaction "precipitates" water ala the fertilizer experiment precipitating saltpeter and table salt?
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12AX7
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"Ye Olde Chymists" did it by performing various reactions and determining which compounds were essentially indivisible, always forming compounds of
similar sorts, like salts, when mixed with e.g. acids and oxidizers. This means many pure metals (only a few of which were known around the 18th
century), and some oxides: among them, CaO, MgO, Al2O3 and etc. -- which were indivisible, but only with current technology. Given the similarity of,
say, MgO to ZnO, where ZnO can be reduced, with carbon (and with difficulty), to a metal, but MgO cannot, it was a reasonable suspicion that it
contained an undiscovered element, which electrochemistry of course proved soon after.
One instantaneous test of purity might be x-ray fluorescence, measuring the atomic properties of the sample. Obviously this is not something the old
chemists had.
Tim
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blogfast25
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Firstly it's perhaps useful to consider the definition of a chemical element: Wikipedia's definition is as good as most and quite a lot can be gleaned from it.
But proving that a particular so-called 'pure substance' is in fact a chemical element and not something that can be broken down into constituent
elements (like a compound can) is not an easy task at the level of a home laboratory. Your son is definitely asking the right questions in that
regard.
Greater minds than you and me have in the history of chemistry sometimes mistaken compounds for elements, notably some very stable oxides that are
pure substances but very difficult to reduce or break up in their respective constituents.
There isn't therefore a single, definitive test that can easily be applied at the homeschooling level to determine whether or not a given pure
substance is a chemical compound or an actual chemical element.
On an 'absolute' level, physics comes to our aid. Chemistry has a fairly limited window of temperature in which it can exist, in the sense that above
a certain temperature chemical bonds can no longer exist and all chemical compounds break down into their constituent chemical elements. Such
conditions exist in plasma (typically in stars - but plasma can exist at much lower temperatures, typically a few thousand Kelvin) and this state of
matter could be considered 'element soup'. Creating such conditions allows to study the chemical elements as isolated atoms. In analytical chemistry
this 'trick' is used to identify chemical elements either by their atomic mass or by their emission/absorption spectra.
But on the homefront, backyard chemists often identify elements by studying the chemistry of whatever pure substance they may have prepared and
checking whether that chemistry corresponds to what is known (and well corroborated) about the element in question. For instance, the iron you
mentioned would distinguish itself from any of its oxides by the fact that in contact with acids, the iron generates hydrogen gas, whereas the oxides
don't.
[Edited on 9-7-2008 by blogfast25]
[Edited on 9-7-2008 by blogfast25]
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ScienceSquirrel
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Mass spectroscopy provides a definitive answer.
A sample is ionised and the ions are accelerated in a magnetic field to separate them.
This technique answered the questions about isotopic composition and also has enabled the detection of isotopes with very small half lives.
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jgourlay
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Thanks.
Thanks for the thoughtful answers. You all have confirmed my suspicion. I'll have to correctly phrase, "you're just going to have to trust me"
without blunting his tendency to ask basic questions.
You should see the dance I put on trying to prove to him that there is mass as distinct from the effects of gravity.
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garage chemist
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The term "chemical element" has a rock solid definition:
A chemical element is a substance that cannot be resolved into further components by chemical methods.
This is what our (excellent) chemistry teacher has told us when chemical elements were the subject of the schoolday.
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Sauron
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A century ago, classical spectroscopy. Lines out of a prism. That's how the Curies did it for example.
Now, Mass spec, AA, NAA.
And just when you thought you were comfy with the Periodic Table, go check out the isotopic version of same.
Sic gorgeamus a los subjectatus nunc.
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jgourlay
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| Quote: | Originally posted by Sauron
A century ago, classical spectroscopy. Lines out of a prism. That's how the Curies did it for example.
Now, Mass spec, AA, NAA.
And just when you thought you were comfy with the Periodic Table, go check out the isotopic version of same. |
Sauron: can you give more details on the "century ago" way? Specifically--can I do that method for at least some elements at home by way of
demonstration?
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woelen
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You could buy some gas discharge tubes for different elements. An example of such a tube is the one shown here:
http://cgi.ebay.ca/High-Voltage-Gas-Discharge-Tube-Neon_W0QQ...
Another example is shown here:
http://cgi.ebay.ca/High-Voltage-Gas-Discharge-Tube-Argon_W0Q...
I have such tubes for He, Ne, Ar, Kr, Xe, N2, O2, H2, D2
They also exist for Hg, I2, and even S.
The spectrum from such tubes can be studied with a simple spectrometer, which you could even make yourself. I have pictures of such tubes on my
website, for the elements mentioned above. Click the elements in the periodic table chart of my website to see the pictures of them:
http://woelen.homescience.net/science/chem/compounds/index2....
You need a neon transformer for powering these tubes. I use a 10 mA (shortcircuited), 3300 V (open circuit) transformer. Be careful with these
transformer, they can give you a REALLY NASTY electrical shock.
[Edited on 9-7-08 by woelen]
<!-- bfesser_edit_tag -->[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: fixed
external link(s)]
[Edited on 7.1.14 by bfesser]
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Sauron
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Some of those put out an awful lot of UV and can seriously damage your eyes.. And a neon transformer is not a toy. Those voltages are very hazardous.
Sic gorgeamus a los subjectatus nunc.
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IrC
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"You should see the dance I put on trying to prove to him that there is mass as distinct from the effects of gravity."
jgourlay, can you explain the thought here in greater detail.
As to high voltage, I do not see where this is very much of a worry as long as you are supervising carefully. HV bothers me not at all, whereas many
of the experiments I read about here such as working with CN's scare me much more. Or Bromic's love of the pretty red liquid he makes from time to
time.
Of course it could be that the HV I know is gone when I turn it off, while when I am playing with chemicals the fact remains that they are still just
as dangerous at all times. (no off switch as to toxicity)
IIRC it was a few years ago here I read the best statement on the subject, while I do not remember the member name I do recall it was a girl who said
it, "there comes a time when working with very dangerous chemicals where you have to decide if this is what you are going to do (here she was
referring to the fact that if you decide chemistry is where you are going to spend your life), and if so, deal with it as safely as you can". Here she
was talking about both dealing with your fears, and dealing with proper methods and safety in chemical handling.
As to the transformers being dangerous, think about finding or building an inverter such as those used to power neon tubes from a battery. You can
scale it up somewhat while still having a less lethal source of gas excitation voltages. Or, you could build what was in my front yard?
[Edited on 7-9-2008 by IrC]
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Sauron
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Please edit that image, it is way too large for the forum.
As is, it will probably get deleted by the management.
Sic gorgeamus a los subjectatus nunc.
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IrC
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It seems easier for me to make another post rather than figure out how to add a second image. This is another view of the same coil. I am on the left,
and the man on the right some of you have seen before on Nova or a similar show about Project Tesla. You guessed it, Robert Golka in the flesh. We
also had a good time partying while he was in the Phoenix area, the year was 1989.
I might add this was also nearly his demise. If you look carefully you will see two pole transformers on the ground at the right, he picked up a
shovel I had leaning against the house and was trying to draw arcs to himself. I tried to warn him I had been experimenting with coupling
coefficients, and had the coil very tightly coupled, to the point resonance was being ruined lowering the output to a couple million volts or slightly
less. Usually when the coil "rang" properly I would quite often have to put out fires where the bolts would play against the edges around the roof of
the house. Here you see the bolts not as long (they were about 4 feet but cameras never seem to catch the slender full length ends of the arcs, making
them look not as long as they are when you are standing near them).
Anyway, when the primary is so tightly coupled to the secondary, getting hit with an arc gives you a shock which also contains a large amount of the
power from the pole pigs combined with the resonant rise. The millions of volts in the power arcs carry a large component of the 28.8 KV at several
amperes, making those arcs deadly beyond measure (14.4 KV doubled since I was using 2 single bushing type pole pigs fed out of phase to double the
voltage feeding the rotary spark gap). The rotary gap, .06 uF 120 KV cap bank, and primary coil were all full floating between the pole pig output
bushings. Actually the power Company people call these the input, since the 240 is to them the output. I just love running these things backwards!
[Edited on 7-9-2008 by IrC]
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blogfast25
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| Quote: | Originally posted by IrC
"You should see the dance I put on trying to prove to him that there is mass as distinct from the effects of gravity."
jgourlay, can you explain the thought here in greater detail.
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He's referring to the fact that a mass object, if separated by enough distance from other mass(es) has no gravitational force acting on it. The
heaviest man on Earth would still be weightless in space. Mass, weight and gravitation are related, connected yet distinctly different things. Quite a
challenge to explain that to a youngster: that's where the dancing comes in I guess... 
And instead of HV discharge emission spectroscopy as Woelen suggests, I would play closer to home (pardon the pun). Even when buying some gas
discharge tubes, you're essentially trusting the supplier that the tube contains the element in question (no problem with that of course), as well as
accepting on trust that the lines you look up in standard tables are indeed the result of genuine observation (and not fraud). The method therefore
remains relative to other people's results. (Having said that, spectroscopes based on the diffraction grating of a plain old CD are fun to build and
use.}
No, I think it would be more fun to home-determine the chemistry of some very common (and FREE!) elements and compare it to the textbooks to make sure
the, say iron or carbon or oxygen or whatever, does what it says on the tin...
[Edited on 10-7-2008 by blogfast25]
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Sauron
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I'd be curious to find a description and sketches or photos of a 19th century spectroscope such as the Curies used. I winder if a project existed in
The Amateur Scientist to build such a spectroscope, it sounds like a Stang-ish idea to me.
Sic gorgeamus a los subjectatus nunc.
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ScienceSquirrel
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I should think it is possible to build a gadget capable of showing the existence of the Fraunhofer lines moderately easily.
http://en.wikipedia.org/wiki/Fraunhofer_lines
From there you can talk about the discovery of helium
http://en.wikipedia.org/wiki/Helium
But you may be getting a bit deep for a youngish child.
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Sauron
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I didn't have pedagogy in mind, just amusement.
There certainly are people who collect antique scientific instruments, restore them, build them from scratch as authentically as possible. I am not a
collector, but I would at least like to know more about exactly how these were made and operated. I know the general principle. But not the specifics.
Flame or electric arc? for example. I guess the line spectrum was recorded on a photographic plate. Black and white obviously.
[Edited on 10-7-2008 by Sauron]
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jgourlay
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Blogfast hit it right on the head.
1. you can, for example, APPROXIMATE a mass demonstrate by weighing a wood ball, verses moving that fish-scale horizontally and showing what happens
when you pull the wood ball as it floats in water. But this is unsatisfactory for a number of reasons. Force in motion is difficult to measure--with
the exception of the force of gravity--at home with simple instruments. And force in motion, acceleration if you will, is what you need to measure to
demonstrate mass.
2. Not afraid of high voltage. I used to design 20kW 5kV power supplies based around constant wattage ballasts. However, as blogfast points out,
using that at home to demonstrate elements requires many, many "black boxes". I am trying very hard to teach him not to trust, but to verify.
Could point me in the direction of "diffraction grating from cd" suggestions? I'll check instructables and google, but I figure you might know a
better source.
I'm thinking a good place to dig may be setting various metal salts on fire in alchohol and then explaining the colors. For example, NaCl, NaOH,
NaCO2 and the KCL, KOH, two or three strontium salts etc. Then I could make the link that, for example, these three sodium salts have very different
properties: here is how we know sodium is in each one. Then, maybe, slide on over to United Nuclear, get a little sodium and show the sodium colored
flames when it hits water.
-----different subject
I've been surfacing around the forum a lot, and there a few pictures. Many of these pictures lead me to believe a lot of you are not doing your work
in a lab, but on work bench in the garage. Is this....kosher? Is it conducive to safety, etc?
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jgourlay
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| Quote: | Originally posted by Sauron
I'd be curious to find a description and sketches or photos of a 19th century spectroscope such as the Curies used. I winder if a project existed in
The Amateur Scientist to build such a spectroscope, it sounds like a Stang-ish idea to me. |
Sauron: don't know if specifically what you want is here, but if you haven't heard of it, I highly recommend you seek out a copy of the Lindsay out
of print books catalog.
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ScienceSquirrel
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I have actually used an optical spectrometer to measure absorption lines but it was a long time ago so I really can't remember much about it.
The instrument was quite small and the lines were measured by turning a knob until a hair lined up with the line you wanted to measure and the
wavelength was read off.
We measured lines for hydrogen and iodine.
I think a small halogen bulb such as is used in Maglite torches would make a good light source.
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Sauron
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I own a UV-Vis spectrometer. Sequoia-Turner 340. Can read absorbance or transmission. Conveniently dual voltage 110/220 VAC 50/60 Hz.
What I am talking about, though, is a classical, flame (or arc) atomic emission spectroscope. Burn a sample, pass the light from flame/arc through a
prism, observe/record the lines.
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ScienceSquirrel
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Quite a few of the first elements to be recognised as such was on the basis of the fact that they could undergo a 'loop' of reactions eg;
copper -> copper sulphate -> copper oxide -> copper
When the idea of atomic mass came along an element could be recognised as such by the fact that quantitative analysis of a pure compound containing
the element always gave a constant result. Some elements were known and had their atomic masses determined before they were isolated as pure
materials.
Flourine for instance killed a lot of chemists before it was eventually isolated.
Damn, damn, damn etc it's FLUORINE! 
[Edited on 10-7-2008 by ScienceSquirrel]
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ScienceSquirrel
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| Quote: | Originally posted by Sauron
I own a UV-Vis spectrometer. Sequoia-Turner 340. Can read absorbance or transmission. Conveniently dual voltage 110/220 VAC 50/60 Hz.
What I am talking about, though, is a classical, flame (or arc) atomic emission spectroscope. Burn a sample, pass the light from flame/arc through a
prism, observe/record the lines. |
This was a lot more basic than that.
It had a source of illumination, the light passed through a tube containing hydrogen gas or iodine vapour etc and then through a prism and you were
looking directly at the spectrum. Lines were read off by peering through a lense at a hair that could be moved up and down the spectrum.
I have seen an emission spectroscope that worked the same way. The sample was placed in a small depression ground into the tip of one of the
electrodes making up the carbon arc.
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jgourlay
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| Quote: | Originally posted by ScienceSquirrel
Flourine for instance killed a lot of chemists before it was eventually isolated. |
Note to self---ixnay on the attempt to sublimate flourite....
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jgourlay
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| Quote: | Originally posted by Sauron
What I am talking about, though, is a classical, flame (or arc) atomic emission spectroscope. Burn a sample, pass the light from flame/arc through a
prism, observe/record the lines. |
I assume it's important to have the sample in the same position relative to the prism everytime? And you are measuring position of the line on a
scale?
If so, that's a fantabulous method because it also brings in the entire subject of refraction.
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