Sciencemadness Discussion Board - How do you know an element is an element?

How do you know an element is an element?

jgourlay - 9-7-2008 at 06:24

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?

12AX7 - 9-7-2008 at 06:51

"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

blogfast25 - 9-7-2008 at 07:08

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]

ScienceSquirrel - 9-7-2008 at 07:41

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.

Thanks.

jgourlay - 9-7-2008 at 07:49

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.

garage chemist - 9-7-2008 at 08:29

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.

Sauron - 9-7-2008 at 10:24

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.

jgourlay - 9-7-2008 at 10:57

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?

woelen - 9-7-2008 at 12:28

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]

[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: fixed external link(s)]

[Edited on 7.1.14 by bfesser]

Sauron - 9-7-2008 at 14:21

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.

IrC - 9-7-2008 at 15:05

"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]

t1.jpg - 45kB

Sauron - 9-7-2008 at 15:10

Please edit that image, it is way too large for the forum.

As is, it will probably get deleted by the management.

IrC - 9-7-2008 at 15:29

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]

t2.jpg - 57kB

blogfast25 - 10-7-2008 at 03:35

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.

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]

Sauron - 10-7-2008 at 03:56

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.

ScienceSquirrel - 10-7-2008 at 04:06

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.

Sauron - 10-7-2008 at 04:43

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]

jgourlay - 10-7-2008 at 04:47

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?

jgourlay - 10-7-2008 at 04:48

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.

ScienceSquirrel - 10-7-2008 at 05:02

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.

Sauron - 10-7-2008 at 05:19

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.

ScienceSquirrel - 10-7-2008 at 05:23

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! :mad:

[Edited on 10-7-2008 by ScienceSquirrel]

ScienceSquirrel - 10-7-2008 at 05:33

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.

jgourlay - 10-7-2008 at 05:47

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....

jgourlay - 10-7-2008 at 05:50

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.

Sauron - 10-7-2008 at 09:08

Yes to both. The positions of flame, prism and target (photo plate/film holder) would be fixed - I assume.

ScienceSquirrel - 10-7-2008 at 09:26

There are some pretty simple beasts described here.

http://en.wikipedia.org/wiki/Spectroscope

I guess Curie et al would have used something along these lines.

ScienceSquirrel - 10-7-2008 at 09:40

Interestingly there are instructions for building basic spectroscopes on the Internet.

http://sci-toys.com/scitoys/scitoys/light/cd_spectroscope/sp...

One of these pointed at a gas flame with a stick of salt held in it would have no problem picking out the sodium D lines

Sauron - 10-7-2008 at 09:47

In modern instruments liquid samples are more usual and the solution is nebulized (nebularized?) into the flame, sometimes the flame device and nebularizer are combined.

I have seen this technique in some HPLC detectors that use destructive detection (not the usual approach in LC).

I'll have a look at those links now, thanks.

blogfast25 - 11-7-2008 at 03:22

Quote:

Originally posted by jgourlay

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.

Here's an excellent site on CD based homemade spectroscopes, it's remarkably detailed and if you drill down a bit you'll find great information about spectroscopy and the actual spectra (w/ beautiful pix). The simplest versions of these spectroscopes take about 1 - 2 h to build. The author is a professional scientist.

Here's another one I like: solar spectrocope built from a cereal box, a piece of CD ROM and some gaffer tape...

Just comparing the spectrum of an incandescent light bulb (hot) with that of a compact saver bulb (much cooler) is in essence a corroboration of quantum mechanics....

Good luck!

[Edited on 11-7-2008 by blogfast25]

jgourlay - 11-7-2008 at 09:58

Quote:

Originally posted by blogfast25

Quote:

Originally posted by jgourlay

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.

SWEET!!!!

Sauron - 11-7-2008 at 10:11

Amazing what you can do with a CD as a diffraction grating and a cereal box as a camera obscura. These are great for demonstrating the principles involved.

However I would prefer something a wee bit more professional, if I can find one. Some company must have made them for educational labs etc. Or else they may be present in the used market.

The Chinese make a lot of relatively old fashioned optical instruments for labs like polarimeters (I have one) and Abbe refractometers (I am ordering one) Companies in Europe buy these on an OEM basis and mark them up horribly, but the Chinese factories sell them cheap. Maybe they make one of these older spectroscopes.

jgourlay - 11-7-2008 at 10:14

Quote:

Originally posted by Sauron
Amazing what you can do with a CD as a diffraction grating and a cereal box as a camera obscura. These are great for demonstrating the principles involved.

However I would prefer something a wee bit more professional, if I can find one. Some company must have made them for educational labs etc. Or else they may be present in the used market.

The Chinese make a lot of relatively old fashioned optical instruments for labs like polarimeters (I have one) and Abbe refractometers (I am ordering one) Companies in Europe buy these on an OEM basis and mark them up horribly, but the Chinese factories sell them cheap. Maybe they make one of these older spectroscopes.

Actually, the CD version is outstandingly perfect. It will be my son's first self made scientific instrument. However, I have a large equilateral prism on order and I may wait for that because explaining the concept of the rainbow from the prism, which is the necessary precursor of the spectrograph conversation, is easier with a prism.

Actually, I'll admit my ignorance and tell you that it's easier because I think I understand that as opposed to knowing I don't understand how diffraction grating works.

blogfast25 - 11-7-2008 at 11:59

If you can understand how a prism works then spectra by diffraction gratings are also well within your grasp, even though the underlying phenomena are quite different.

I "built" (cut an' glued together is a more accurate description) two Mk II spectroscopes (from the Köppen site), one with a reflective grating, one with a transparent grating (both from pieces of CD ROM, the latter with the foil removed - see Mk IV). I've got all the materials and detailed plan for a Mk III, it's just that damn backyard chemistry keeps getting in the way of putting it all together...

IrC - 11-7-2008 at 18:00

I found this on ebay and for 19 bucks (including S/H) it is hard to beat. Below I pasted some of the text from the auction. I believe this one is right up your alley as far as finding a cheap but very useful instrument in your teaching. While they mention US shipping, I am sure an email to the seller could get you one mailed to any other country very cheaply.

DIFFRACTION GRATING SPECTROMETER SPECTROSCOPE

"Offering for your consideration a new Diffraction Grating Spectrometer/Spectroscope. This spectrometer consists of a black plastic housing with a slit for incoming light and wavelength scale at the wide end, and a small replica diffraction grating at the eye (narrow) end (see photo). The spectrometer breaks down incoming light into color components and displays them against a scale showing wavelengths from 380 to 730 nanometers. The scale is marked every 10 nanometers allowing measurements to +/- 5 nm precision. Various light sources can be identified by the bright emission bands or dark absorption bands in their spectra at specific wavelengths.

This spectrometer is about 7.5" long, 4" wide (wide end) and 1" thick. The wavelength scale is 1.5" long. The spectroscope comes with an information/activity sheet to get you started viewing and measuring various spectra."

Here are two more types:

SPECTROSCOPE

SPECTROSCOPE

Which one of these seems to be the best choice?

----------------------------------------

While I know you guys hate sidetracking threads and this one is primarily talking about the spectroscope, I wanted to touch on the gravity subject mentioned within this same thread. Also, I squished the images for Sauron but the second one did not work well as it made me look like I am standing in Jupiters gravity.

"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..." quote from blogfast25.

I think it is important to be more specific, meaning even in space you are still affected by gravity. If you were floating between the earth and the sun (zero velocity) at the exact distance to balance the gravity between the earth and the sun you would feel weightless, but are still within several gravitational fields. In fact there is nowhere within this galaxy where you would not be subject to gravity, even though you were in effect "weightless". As to being in orbit, your weight is merely being masked by your angular velocity. I know it sounds nitpicky, but I thought it important to mention in regards to teaching young minds insofar as eliminating anything which must be unlearned later, if the child decides someday to go in depth in a given field of study. Having lived many years, I know from experience that it is much easier to learn something complex if there is no "unlearning" to do.

If we were being very picky we could say there is gravity between each particle of our makeup. To me mass is energy, while weight is closer in concept to potential energy relating mass, height, and a specific level of gravitational field. By this I mean the potential energy in terms of momentun unleashed if that weight were suddenly free to fall and then stop when it hit the surface of the attracting mass. I may be explaining this thought poorly but for what it's worth there it is.

From experience I know it is sometimes hard to explain the difference between what is weight and what is mass. While my post may not be very helpful, hopefully it gives a few more insights which may help jgourlay teach.

len1 - 11-7-2008 at 20:27

I had a similar problem when homeschooling my kids (the education system here is tripe). Basically I can find two good answers

The old method: If matter A is a compound A = BC, then reactions of the type are possible

A + D -> DB + C

where A can not be obtained from D and C by any manipulation. This means essentially that something has split-off from A, and it doesnt rely on isolating an element chemically. If A were not a compound then only the following could obtain

A + D -> A_xD_y + A_mD_n

where clealy A can be ontained from C (=A_mD_n) and D.

New method: Mass spectroscopy - not ordinary low-T, where some bond might not be broken and look elementary, but plasma MS. At this temperature, as someone wrote, chemistry does not exist, and all matter becomes a mixture of ionised atoms 'elements'. Each element isotope creates a geometric series of M, M+, M++ mass to charge ratios, and a mixture looks like the direct sum of two such series. In this regard chlorine would look like a mixture of 35 and 37 isotopes.

Sauron - 11-7-2008 at 20:50

Fluorine NOT flourine.

And definitely not flourene.

blogfast25 - 12-7-2008 at 04:55

IrC:

The second one is definitely the most comprehensive one:

• Sturdy brass construction should beat plastic every time
• Adjustable optical path length allows some control over line-fineness, hence resolution. A longer optical path means the light waves will travel more parallel to each other, leading to higher resolution (but with the grating further away from the light source [at longer optical path length], lines will also be fainter at higher resolution)
• Adjustable slit-width allows to adjust resolution to object brightness: where detection is by the naked eye (and not by means of photosensitive pixels or something like that) being able to adjust the amount of incoming light can mean the difference between seeing a line and not seeing it at all

Never stopped to think to look in eBay. Still, I'll be building Mk III before considering any purchase: there's much to be learned from building one's own instrument.

On gravity: agreed; I was being generic as this thread isn't about gravity, mass, weight or inertia...

[Edited on 12-7-2008 by blogfast25]

IrC - 12-7-2008 at 15:00

I agree, if jgourlay had not only started the thread but also asked the gravity question within it I would have felt like it deserved another thread of it's own. Actually, since SCM is about science of many varieties rather than just chemistry the subject may deserve it's own thread seeing as how there is a lot of confusion out there on the subject of gravity. But enough on that.

I thought about the one you mentioned and almost bought it myself but it lacked a scale (and the S/H was high). I don't know but it seems the scale would be so very useful, i.e. we can actually compare the real wavelengths numerically to say a chart of line spectra of the elements or something like that. On another note I saw the bulk of all the nicer looking instruments seemed to be about checking jewels for authenticity. Would this also be just as useful if say I wanted to know which element was involved by comparing real world numbers of various elements with where the lines are on the spectrascope at the time I am actually looking at a spectrum? The problem I have is other than ballpark guesses as to around where the color is in terms of frequency, how can you really tell the element if say there are several elements with lines very close to each other. Not being a master on the subject and having never actually used or owned a spectrometer would it not be better if you had an actual numbered scale to go by? Or is there something I am missing in all of this?

blogfast25 - 13-7-2008 at 04:51

Sure, the absence of a wavelength scale is a bit of a limitation, although it might be possible to add one yourself. It would not stop me from buying the instrument. Calibrating with two really easy lines is certainly possible: assuming the grating is of sufficient quality, the scale is linear and the wavelength of an "unknown" line depends only on the actual distance from the calibration lines. Photography obviously solved that problem, as it records accurately the position of the lines and will allow to determine the wavelength of any line precisely, relative to the position of calibrating lines...

But without a full scale automated ("CSI New York/Miami" style) piece of equipment that can identify lines both by wavelength AND relative intensity, spectroscopy for the identification of chemical elements is a bit of an art form, I guess. While many elements can be identified by means of 1 or 2 prominent lines, to separate the lines from multiple elements probably requires experience, as well as systems both upstream and downstream from the scope.

Upstream is the light generator. For element research typically a hot gas flame into which the sample is gradually fed. At our level that's easier said than done. Anyone with ideas/experience on that one, please do elaborate...

Downstream is the detector: unless you want to be peering into the ocular side of the tube for prolonged periods of time, some piece of photographic equipment and knowledge of required exposure times to bring out faint lines, separate doublets etc, will be a requisite. Digital photography really is the dream ticket here because the images can be inspected almost pixel by pixel for faint lines using some image manipulation software or other.

It makes one wonder what kind of hoops Bunsen and Kirchoff must have been jumping through to spectroscopically discover and later isolate Caesium in mineral water and allegedly following their progress in concentrating the Cs solution by studying the increasing intensity of the prominent blue line. Probably one of the first examples of successful quantitative spectrometry...

Appreciate it gents, thoughts on spectroscopy

jgourlay - 14-7-2008 at 04:36

Gents,

I really appreciate the discussion. I'm adding this board to my list (now expanded to three) of "sites where people know stuff AND are helpful."

Spectroscope: I'm definitely going for build not buy, for a few reasons. First, "build not buy" and "recycle/repurpose before purchase" are two ethics I'm trying to teach the kids. Second, he and I will learn so much more building it ourselves. Remember that the purpose of this is to demonstrate element vs. compound.

For this we'll need a slit, scale, and something to break the light down. THIS provides me the excuse to buy one of those really nice prisms my parents would never let me have!

We'll make a board with depressions for the prism standing on end and for a glass dish. I'll come up with a slit--or maybe pinhole? I can use a laser to rough place the scale on a screen in the projection path of the prism.

In use, we'll put various metal salts in alchohol (or sterno or whatever) and put that sludge into the mortar. THis way the "light source" doesn't change position relative to the prism. As long as in that setup the same wavelength will always fall on the same spot on the scale (and it should), I should be able to prove the point. FURTHER, at least for things that will turn colors when they burn, it should be "analytical" in that I should be able to give him any powder and he should be able to tell me at least some of the constituents.

Moreover, we'll develop the list ourselves. For example, "strontium = 1.5", sodium equal 2.37", sulfur = 4", etc."

IrC - 14-7-2008 at 13:48

Wanting something a little better than what I can build myself I went ahead and bought one to play with. I thought I would post this so I can mention the instructions have one very useful bit of help in calibration. They say to point the thing at a regular florescent light and reading the position of the two bright bands, which are 436 and 546 nanometers.

Polverone - 14-7-2008 at 15:54

You can go a step further by using a DVD instead of a CD, since it is a finer grating. You can go yet a step further by capturing images with a digital camera and analyzing the image with free software to determine numerical values. I don't know if I would suggest starting with this more advanced path, but here are two articles from J. Chem. Ed. about how to do it:

A DVD Spectroscope: A Simple, High-Resolution Classroom Spectroscope
Resolving Spectral Lines with a Periscope-Type DVD Spectroscope

[Edited on 7-14-2008 by Polverone]

blogfast25 - 16-7-2008 at 09:31

Very interesting. Never stopped to think that DVDs have finer gratings than CDs (according to the first *.pdf about twice as fine). That should be something that could be easily verified even without building the actual scope.

-jeffB - 16-7-2008 at 09:47

Quote:

Originally posted by Polverone
You can go a step further by using a DVD instead of a CD, since it is a finer grating. You can go yet a step further by capturing images with a digital camera and analyzing the image with free software to determine numerical values. I don't know if I would suggest starting with this more advanced path, but here are two articles from J. Chem. Ed. about how to do it:

A DVD Spectroscope: A Simple, High-Resolution Classroom Spectroscope
Resolving Spectral Lines with a Periscope-Type DVD Spectroscope

[Edited on 7-14-2008 by Polverone]

Thanks, Polverone!

The first article gives me an error, but the second loaded fine. It mentions using NIH Image as an analysis tool. NIH Image has been largely superseded by ImageJ, a rewrite of NIH Image in Java. ImageJ is completely open-source and free, it runs on a wider variety of platforms, and it's got a very active community developing plugins, macros and other extensions. It's got a very active mailing list for users as well.

I use ImageJ for my day job (processing medical images), and I've even contributed back to the code base a little bit. It's a very capable package, and it can run on just about any machine, even an old and creaky Windows PC.

Polverone - 16-7-2008 at 11:41

Quote:

Originally posted by -jeffB
Thanks, Polverone!

The first article gives me an error, but the second loaded fine.

Oops, sorry about that. Permissions should be ok on the first article now.