Sciencemadness Discussion Board - Identification of a mystery metal

Identification of a mystery metal

opfromthestart - 30-4-2019 at 14:35

My AP Chemistry teacher recently found a sample of a "silver unknown metal"(written on the container) with a label of 8.19 g/cm^3. I have tested its reactivity with water and dilute sulfuric acid and it seems to be relatively unreactive. Me and one of my classmates think that it may be an alloy of nickel, chromium, cobalt, iron, and beryllium(possibly not all of them, but all could be possibilities). What tests could be easily done in a high school lab that could possibly confirm the identity of the metal as nickel or something else?

On a slightly related note, is there any database or other resource for the rate laws and constants for single and double replacement reactions for metals?

j_sum1 - 30-4-2019 at 15:09

First question: do you know whether it is a relatively pure metal or an alloy?

DraconicAcid - 30-4-2019 at 15:52

Does it react with more concentrated hydrochloric acid? Is it magnetic?

fusso - 30-4-2019 at 16:31

What about conc HNO3? Aqua regia? Bleach? (some PGMs react with bleach)

elementcollector1 - 30-4-2019 at 18:05

Have you verified that density? I don't doubt it, but it's always good to check.

If that density is accurate, the only pure element that would be reasonably close would be terbium (d = 8.23 g/cc). This is disproven in that it does not react with water or dilute sulfuric acid, two things which would easily provoke a rare earth into producing hydrogen. So it must be an alloy.

Magnetism would be an easy check for a ferrous alloy, with the caveat that some stainless steels are nonmagnetic due to Ni content. Nickel would also explain non-reactivity, and the range of density for stainless steel does include the density of your mystery metal (nickel being 8.9 g/cc, iron being 7.87 g/cc and chromium being 7.19 g/cc).

Maybe we could also get a photo of this mysterious sample?

diddi - 30-4-2019 at 18:42

if the density is very accurate, this might help

https://www.engineeringtoolbox.com/metal-alloys-densities-d_...

ScienceHideout - 30-4-2019 at 19:24

Is this an actual case of AP Chem Teacher finds mystery metal, or is he/she fabricating this story to test your practical skills?

Either way, there are a lot of fun ways to find out. Obviously, I would re-check that density just to make sure it is 100% accurate and compare to the table from Diddi above. Excellent preliminary step.

I would avoid destructive chemical techniques at all costs. Corrosive acids are a bitch to use, it creates nasty fumes which are probably a bit too dangerous to make in typical AP chem labs, and you risk destroying your sample... which you especially don't want to do if it is of any value (always possible!). Any professional analytical chemist would just do some XRF analysis to confirm its identity, which is non-destructive, and if the chemist didn't have access to an XRF spectrometer, he or she still likely has the ability to analyze less than a milligram of sample with some ICP instrument.

Since most AP Chem labs are not equipped to do such microscale analysis of such things, you should first rely on cheap and time-tested non-destructive techniques. Once again, test the density over again. Touch a magnet to see if it sticks. Another possibility is calorimetry. Submerge the sample in boiling hot water for several minutes, take it out and drop it into a well-insulated cup of water, and monitor how the temp increases. :cool:

If you've exhausted all options, and you must do destructive analysis, see if you can cut or snap off a tiny piece- half of a gram or so- to dissolve and then do chemical tests!

j_sum1 - 30-4-2019 at 21:29

Further to the magnetism tests:

Test for ferromagnetism by touching a magnet on one side and seeing if you can get coarse iron filings to stick to the other. See if they fall off once the magnet is removed. (Just to eliminate electrostatic and other kinds of attraction.)

Drop it on a piece of polystyrene so that it floats in a dish of water. Even if it is not ferromagnetic you can distinguish whether it is paramagnetic or diamagnetic by putting a strong magnet in close proximity.

Sulaiman - 1-5-2019 at 03:34

A flame test may reveal something https://en.wikipedia.org/wiki/Flame_test

a spark spectrum is probably (I've not personally tried) do-able https://en.wikipedia.org/wiki/Atomic_emission_spectroscopy

opfromthestart - 1-5-2019 at 04:22

Quote: Originally posted by j_sum1

First question: do you know whether it is a relatively pure metal or an alloy?

I do not but since it does not fit the density or reactivity of any pure metals I assume it is an alloy.

Quote: Originally posted by DraconicAcid

Does it react with more concentrated hydrochloric acid? Is it magnetic?

I do not know for either of these, I will try to test magnetism at least fairly soon.

Quote: Originally posted by fusso

What about conc HNO3? Aqua regia? Bleach? (some PGMs react with bleach)

I will try nitric acid but for reasons of safety she is hesitant to use aqua regia. I'll also see if I can try bleach but I doubt it is a PGM, since their densities are significantly higher than the density on the label.

Quote: Originally posted by elementcollector1

Have you verified that density? I don't doubt it, but it's always good to check.

If that density is accurate, the only pure element that would be reasonably close would be terbium (d = 8.23 g/cc). This is disproven in that it does not react with water or dilute sulfuric acid, two things which would easily provoke a rare earth into producing hydrogen. So it must be an alloy.

Magnetism would be an easy check for a ferrous alloy, with the caveat that some stainless steels are nonmagnetic due to Ni content. Nickel would also explain non-reactivity, and the range of density for stainless steel does include the density of your mystery metal (nickel being 8.9 g/cc, iron being 7.87 g/cc and chromium being 7.19 g/cc).

Maybe we could also get a photo of this mysterious sample?

The first metal my chem teacher thought it would be was terbium, hence my tests with water and dilute acid since it would react even with that. I will have to test for stainless steel also but I doubt it because I was able to scrape some of the metal off with a scoopula fairly easily.

I will try to get a photo later today.

Quote: Originally posted by ScienceHideout

Is this an actual case of AP Chem Teacher finds mystery metal, or is he/she fabricating this story to test your practical skills?

Either way, there are a lot of fun ways to find out. Obviously, I would re-check that density just to make sure it is 100% accurate and compare to the table from Diddi above. Excellent preliminary step.

I would avoid destructive chemical techniques at all costs. Corrosive acids are a bitch to use, it creates nasty fumes which are probably a bit too dangerous to make in typical AP chem labs, and you risk destroying your sample... which you especially don't want to do if it is of any value (always possible!). Any professional analytical chemist would just do some XRF analysis to confirm its identity, which is non-destructive, and if the chemist didn't have access to an XRF spectrometer, he or she still likely has the ability to analyze less than a milligram of sample with some ICP instrument.

If you've exhausted all options, and you must do destructive analysis, see if you can cut or snap off a tiny piece- half of a gram or so- to dissolve and then do chemical tests!

I believe that she doesn't actually know what the metal is. She inherited it from a teacher from the 70's and it was labeled as lead. She calculated the density herself so that is confirmed.

I didn't think to test the specific heat but I will attempt to test that and report what I get.

opfromthestart - 1-5-2019 at 04:31

These are pictures of the container and the metal itself. The blue label was put on by my chem teacher.

The damage to the leftmost rod is from a scraping I took to test it against sulfuric acid.

opfromthestart - 1-5-2019 at 04:35

The physics teacher tested it's magnetism with a neodymium magnet and it was not magnetic(or at least ferromagnetic). I will still have to see if it reacts to a magnet when floating on Styrofoam or if iron filings stick to it when it is against a magnet.

Ubya - 1-5-2019 at 06:08

If you were able to scrape it easily it's not SS.
To know exactly what alloy is you must do some wet chemistry (destructive).
Scrape some of it or cut a little piece, you need way less than a gram.
Dissolve it in a few drops of nitric acid (if your teacher thinks you can't do it let her do it), if it doesn't dissolve in cold acid heat it in a water bath (a beaker filled with water on a hot plate).
Notice the color. Any blue or green can indicate copper, nickel, chromium or vanadium, red Cobalt, yellow iron. add HCl to precipitate insoluble AgCl2 and PbCl2(if present), filter or centrifuge. To the supernatant add H2S or Na2S. Bismuth, cadmium, copper and tin sulfides will precipitate. Cadmium sulphide is yellow.
Filter or centrifuge. To the supernatant add ammonia to get pH 10, add again H2S and precipitate nickel, zinc, Cobalt and manganese sulfide. Zinc sulfide is white while manganese sulfide is pinkish.
Then it's a matter of seeing what you get and use other tests to check

fusso - 1-5-2019 at 06:11

Quote: Originally posted by opfromthestart

This confirms the metal is quite soft. Can you bend it easily? Or hammering it to test malleability? Melting point test?

DraconicAcid - 1-5-2019 at 07:51

Quote: Originally posted by Ubya

If you were able to scrape it easily it's not SS.
To know exactly what alloy is you must do some wet chemistry (destructive).
Scrape some of it or cut a little piece, you need way less than a gram.
Dissolve it in a few drops of nitric acid (if your teacher thinks you can't do it let her do it), if it doesn't dissolve in cold acid heat it in a water bath (a beaker filled with water on a hot plate).
Notice the color. Any blue or green can indicate copper, nickel, chromium or vanadium, red Cobalt, yellow iron. add HCl to precipitate insoluble AgCl2 and PbCl2(if present), filter or centrifuge. To the supernatant add H2S or Na2S. Bismuth, cadmium, copper and tin sulfides will precipitate. Cadmium sulphide is yellow.
Filter or centrifuge. To the supernatant add ammonia to get pH 10, add again H2S and precipitate nickel, zinc, Cobalt and manganese sulfide. Zinc sulfide is white while manganese sulfide is pinkish.
Then it's a matter of seeing what you get and use other tests to check

Wet chemistry with sulphides isn't going to happen in a high school lab.

Ubya - 1-5-2019 at 08:29

Thioacetamide could be used to generate S-- in situ, to not have to deal with gaseous H2S or a generator

opfromthestart - 1-5-2019 at 08:30

Quote: Originally posted by DraconicAcid

Wet chemistry with sulphides isn't going to happen in a high school lab.

Why couldn't it? As part of the class we have already used Na₂S in a One Pot Silver demonstration. The lab also has a fume hood so if H₂S fumes are the problem that could be worked around.

[Edited on 5-1-2019 by opfromthestart]

DraconicAcid - 1-5-2019 at 09:02

That surprises me, just from the toxicity of H2S.

At any rate, the colour of the sulphide isn't very characteristic, because small amounts of impurities can turn a white or pink precipitate an ugly brown.

Hmmm...if it's soft enough to be scraped with a spatula, my first guess would be tin, or an alloy of tin (a form of solder?). If it dissolves in HCl to give a colourless solution, that would support that hypothesis. If it gives a white precipitate while doing so, that would probably be some lead chloride (so you had an alloy).

Ubya - 1-5-2019 at 09:26

Most sulphides are black, so even a brown color can be helpful.
I hope the alloy to be a mix of just 2 or 3 metals, one being tin or lead given the softness.

Metacelsus - 1-5-2019 at 09:30

That looks exactly like some Wood's alloy from the lab I worked in back at the University of Minnesota. The type of container is even the same. (I used it as a bath for a high-temperature reaction.)

The density of Wood's alloy is 9.67 g/mL, so it's probably not that. But I would check the melting point to see if it's something similar. Perhaps Field's alloy?

edit: Field's alloy is 7.88 g/mL. Maybe it's a slightly woody field? I'll call it "savanna" alloy

[Edited on 2019-5-1 by Metacelsus]

Sulaiman - 1-5-2019 at 09:34

Tin:Lead solder looks likely, density close to eutectic mixture.

P.S. just got in from my lab - been melting lead and tin so probably biased.

[Edited on 1-5-2019 by Sulaiman]

fusso - 1-5-2019 at 09:42

I now wonder, if it's some kind of fusible alloy...
try putting it in boiling water to see if it melts?

ScienceHideout - 1-5-2019 at 18:44

Visually, I'm inclined to believe it's tin. I know that tin comes in sticks very similar to that. A lot of people have said this, but I'll say it again. Before doing anything else, measure the density again. There is always the possibility that whoever wrote that on the bottle did it wrong.

EDIT: Also, looking at the bottle, it is probably a single metal and not an alloy. It has an analysis on it that says 100.0%...

[Edited on 2-5-2019 by ScienceHideout]

happyfooddance - 1-5-2019 at 21:30

Quote: Originally posted by Metacelsus

edit: Field's alloy is 7.88 g/mL. Maybe it's a slightly woody field? I'll call it "savanna" alloy

Hehe.

Did no one else think this was funny? Thanks, Mc

Metacelsus - 1-5-2019 at 22:43

Quote: Originally posted by ScienceHideout

Visually, Also, looking at the bottle, it is probably a single metal and not an alloy. It has an analysis on it that says 100.0%...

Good catch! I didn't notice that.

You should definitely check the density again, as other commenters have suggested.

opfromthestart - 2-5-2019 at 03:39

The density was calculated by my teacher when she received the sample. I was incorrect when I said that she recieved it with that label.

opfromthestart - 2-5-2019 at 03:52

Quote: Originally posted by ScienceHideout

Also, looking at the bottle, it is probably a single metal and not an alloy. It has an analysis on it that says 100.0%...

[Edited on 2-5-2019 by ScienceHideout]

I hadn't noticed that. However, as elementcollector1 said, the only pure metal with that density is terbium, which is the first thing I tested it for. Since it did not react I suspect it is not a pure metal.
Also, the container itself may be not specific to the metal and may have been intended for something else, if that is a possibility.

opfromthestart - 2-5-2019 at 06:44

I heated some scrapings of the metal in a crucible with a Bunsen burner. It didn't appear to melt but quickly oxidized when I held the inner blue cone of the flame. It oxidized into a black solid.

I will try dissolving the solid in nitric and hydrochloric acid to see if it has any distinctive metal ion colors.

fusso - 2-5-2019 at 06:51

Small pieces of scrapings would probably completely oxidize before they could melt. Try larger pieces.

[Edited on 190502 by fusso]

elementcollector1 - 2-5-2019 at 17:06

Assuming that black isn't soot, it's a good start. Metals with black oxides, according to the list, are:

-Chromium (II) oxide. Very unlikely owing to the nature of its existence.
-Chromium (IV) oxide. Also unlikely.
-Cobalt oxide. Hardness of the metal is entirely wrong for cobalt.
-Iron (II) oxide. Hardness is wrong, same as cobalt.
-Iron (II, III) oxide - magnetite. See above.
-Copper(II) oxide. Color is wrong.
-Lead (IV) oxide. Now this is a possible candidate.
-Manganese (IV) oxide. See cobalt and iron.
-Silver (I) oxide. I might almost believe this one, honestly. Pure silver is more dense than the sample, but alloyed silver is still on the table.
-Tin (II) oxide. Apparently it's flammable, burning to tin (IV) oxide, so if the black color remained after burning for a while it's probably not this.

Of those, the two best candidates are lead and silver, and my money's definitely on lead, alloyed with something lighter like tin.

[Edited on 5/3/2019 by elementcollector1]

ScienceHideout - 2-5-2019 at 20:22

Quote: Originally posted by opfromthestart

The density was calculated by my teacher when she received the sample. I was incorrect when I said that she recieved it with that label.

I understand, but...

1. We are dealing with a pure metal, according to the analysis.

2. The only metal with a density close to that is terbium.

3. It was determined that said metal is not terbium.

So something is wrong: it is either the label, the density, or the confirmation of not-terbium.

Unless the bottle was recycled (unlikely), the analysis is probably correct. You tested for terbium and got a negative result, which I believe for several reasons.

Density, on the other hand...

We are all human. Your AP chemistry teacher is, too. She may have made a mistake while determining the density. This is why analysts often do measurements in triplicate when applicable. This is not to say she is stupid, just that accidents happen! In situations like this in particular, an experimental blunder could go unnoticed because it wouldn't propagate a big, blatant, questionable error: Something as simple as forgetting to tare the balance, or pressing the wrong button on the calculator could have given her the wrong density, and we have no way of knowing since it was measured only once.

It doesn't take very long to put a piece on a scale, and then submerge it in water. Sure, you might find that your teacher's analysis was correct, and you still cannot confirm the identity. BUT, there is also a chance that you come up with a different number, and those couple minutes of double checking could provide a definite answer. If you have time to play around with corrosive acids and fire, you have time to do this.

TL;DR- It pays to double check. Take another density measurement to assure that your teacher didn't overlook a simple mistake. It only takes a few seconds.

Fulmen - 3-5-2019 at 05:11

Quote: Originally posted by ScienceHideout

Unless the bottle was recycled (unlikely)

What lead you to that conclusion?

[Edited on 3-5-19 by Fulmen]

opfromthestart - 3-5-2019 at 08:11

She just tested the densities of two of the samples and they came back as 6.407 g/cm^3 and 7.337 g/cm^3. Since this disparity seems too great to be due to measurement error and I did the calculations myself(and checked it), I believe the samples may be different metals or at least different compositions of a similar alloy. One of my classmates will be testing the other 4 samples later today.

OldNubbins - 3-5-2019 at 12:13

Those are either solder bars/sticks for making stained glass windows or babbitt in extruded wire form

ScienceHideout - 3-5-2019 at 13:20

Fulmen, I don't believe that bottle was recycled because I have seen metal sticks packaged in that type of bottle before. So unless someone took some sticks out, and put others in, I doubt it was recycled.

Opfromthestart, That is certainly interesting. I'd be willing to bet that they all are the same metal: I don't see why there would be different ones in the same bottle that look the same, and again, unlikely that there are various alloy compositions due to "100%" being on the bottle. That being said, both 6.407, 7.337, and 8.19 are all very different measurements. I am really looking forward to the results from your classmates- this could provide one single accurate measurement. Out of curiosity, have you guys ever tried either a pycnometric method, or hydrostatic weighing? These could both help improve your density measurements by omitting the need for volume measurement.

OldNubbins, They do certainly look like that! Does stained glass solder normally come in a bottle like that, though? What is the typical melting point of stained glass solder?

[Edited on 3-5-2019 by ScienceHideout]

OldNubbins - 3-5-2019 at 15:30

Regular 50/50 or 60/40 solder (no rosin core) is used, nothing special. Melting point around 400°. Comes in various sizes depending on the application. i have seen bars larger then the OP's. You can tell the OP's material is extruded because of the seams on the sides and tapered ends. There's no way to know if the paper tube in the OP's picture is the original package - wouldn't really help anyways since it can come in anything or nothing.

ScienceHideout - 3-5-2019 at 17:57

Quote: Originally posted by OldNubbins

You can tell the OP's material is extruded because of the seams on the sides and tapered ends.

Is this why you believe it's solder, though? I mean, a lot of metals are extruded...

elementcollector1 - 3-5-2019 at 19:01

Quote: Originally posted by ScienceHideout

Is this why you believe it's solder, though? I mean, a lot of metals are extruded...

True, but that plus other evidence is increasingly leaning towards solder or something similar. For reference, a eutectic lead/tin solder alloy (63% Sn, 37% Pb) has a density of approximately 8.4 g/cc.

Still, chemical tests will likely tell us something more. OP could also differentiate between silver-based and lead-based by checking if the bulk metal melts on stovetop heat (though if it's lead, might want to turn the vent fan up. Or use a hotplate in a fume hood).

OldNubbins - 3-5-2019 at 21:53

Quote: Originally posted by ScienceHideout

Quote: Originally posted by OldNubbins

You can tell the OP's material is extruded because of the seams on the sides and tapered ends.

Is this why you believe it's solder, though? I mean, a lot of metals are extruded...

The extrusion marks are only one of the examples I gave. I am convinced it is solder or some other low melt alloy. You don't have to be, you are welcome to investigate further, unfortunately with the limited amount of information the OP has given, we are at an impasse.

OldNubbins - 3-5-2019 at 21:59

Quote: Originally posted by elementcollector1

Quote: Originally posted by ScienceHideout

Is this why you believe it's solder, though? I mean, a lot of metals are extruded...

One way I check for lead is to melt the alloy (solder, babbitt, other low-melts) in question, throw some KNO3 into the melt and give it a mix. Yellowish litharge should form (PbO), Bob's your uncle!

Fulmen - 4-5-2019 at 00:19

Quote: Originally posted by OldNubbins

You can tell the OP's material is extruded because of the seams on the sides and tapered ends

I disagree. The seams looks more like mould lines, extruded bars will typically have fine axial lines around the entire perimeter. The surface also has wrinkles that looks more like cast (or perhaps even stamped) metal.
And just to be clear, not only have I seen cast and extruded lead before, I've even made it and the tooling myself.

Most of the ends aren't really tapered, they look more like they've been cut with bolt cutters. The tapering could be from use if it's solder.

phlogiston - 4-5-2019 at 12:26

I remember a thread where someone found a mystery metal rod, which turned out to be actual platinum. (185 grams of it!)

Found it: https://www.sciencemadness.org/whisper/viewthread.php?tid=20...

[Edited on 4-5-2019 by phlogiston]

Heptylene - 4-5-2019 at 13:08

Quote: Originally posted by phlogiston

I remember a thread where someone found a mystery metal rod, which turned out to be actual platinum. (185 grams of it!)

Found it: https://www.sciencemadness.org/whisper/viewthread.php?tid=20...

[Edited on 4-5-2019 by phlogiston]

I read the thread, apparently there wasn't any real conclusion was there?

Regarding the mystery metal in this thread, those look like extruded metal rods, cut with a hydraulic shear. When you cut thick wire you the same pattern on the cut end. That narrows it to softer metals that can be extruded that way. I have a bar of cadmium that looks similar, different density though.

Sulaiman - 4-5-2019 at 13:51

Quote: Originally posted by opfromthestart

She just tested the densities of two of the samples and they came back as 6.407 g/cm^3 and 7.337 g/cm^3.

ALARM !
... those are very precise numbers,
and different from each other and the original sample.

The test of density is surprisingly quick, easy and accurate.
It does not rely on the accuracy of the only instrument required, the scales,
it only relies on their linearity - for a weight ratio.

The crud will need to be mechanically removed from each stick
(keep the crud from each, separately, until you KNOW that it is worthless)
and, any tiny bubbles on the submerged surface will give significant errors.
I have used this method many times, and cross-checked by other means, 1% accuracy is easily attainable using cheap scales.
(Correction must of course be made for the variation of the density of water with temperature if accurate results are required.)

If there are three of you then each of you should measure all samples - then compare notes.
(using your teacher's results) as a reference - with an open objective mind)
Simple techniques are worth mastering - for confidence in your own abilities.
PLUS
it would enable more concise and precise discussions with other members.

P.S. I was refering to this type of measurement https://www.youtube.com/watch?v=xYdSEAm-7uI
https://www.youtube.com/watch?v=N1HCrNMnb9I

[Edited on 4-5-2019 by Sulaiman]

opfromthestart - 6-5-2019 at 05:12

The samples were tested with an analytical balance(the fancy one with doors, i think about .001g accuracy).

The samples tested around 7.3 but some were as high as 7.6(each was tested 3 times by both the teacher and my classmate, I wasnt there).

When my teacher removed most of the oxidation, the density dropped to 7.06. This sets the pure metal possibilities to Mn, Cr, and In, and possibly a few I overlooked.

fusso - 6-5-2019 at 05:20

The few metal oxides that are denser than the metal are some alkali, earth & RE metals...

[Edited on 190506 by fusso]

opfromthestart - 6-5-2019 at 06:55

Quote: Originally posted by fusso

The few metal oxides that are denser than the metal are some alkali, earth & RE metals...

[Edited on 190506 by fusso]

Well, it doesn't react with acid or water so i doubt its any of those.

fusso - 6-5-2019 at 07:52

Most earth sulphates are insoluble so it may form some protective layer in H2SO4?

opfromthestart - 6-5-2019 at 08:00

If it would be a RE metal, it would probably have to be Ce, Pr, or Yb based on the density.

opfromthestart - 6-5-2019 at 08:15

Cerium and Ytterbium oxide are not black, and Praseodymium is paramagnetic, which the rods are not. So... still probably an alloy.

MrHomeScientist - 6-5-2019 at 11:32

So you measured the weight with an analytical balance; good! That should be pretty accurate (assuming it's calibrated; it couldn't hurt to double check that against calibration weights).

Now, how was the volume measured?

I know it seems like we are harassing you about the density, but you have 3 very different measurements there and we are trying to find where the discrepancy is. If your density measurements and calculations are accurate, then these rods might not all be the same alloy. It's an interesting problem, and we just want to be thorough.

Sulaiman - 6-5-2019 at 16:04

Using the method that I pointed to it does not matter if the scales are neither accurate nor calibrated ,
- it only relies upon zero and the linearity of the scales, and care of the operator.
(and that the sample does not react with air or water)

MrHomeScientist - 7-5-2019 at 08:20

Yep, I definitely recommend the specific gravity method. It's worked perfectly for me, and only requires one instrument so you avoid a lot of potential measurement error.

stamasd - 7-5-2019 at 15:39

The most accurate method to quickly determine the composition of a metal piece is by spectroscopy. Spark or plasma spectroscopy more precisely.
Essentially, a spark or arc is created by passing electricity between an electrode of a known composition (graphite usually) and the metal to be analyzed. The light emitted by the arc is passed through a prism, and the spectrum is photographed. Each metal produces a very characteristic pattern of emission lines, and based on the position of the emission lines in the spectrum you can determine which metals are present. Also the relative intensity of the lines belonging to each metal gives you an estimate of the relative proportions.
If your teacher wants to go this route, the chemical mystery could turn into a nice combined physics+chemistry experiment.

Ozone - 7-5-2019 at 16:03

I have a can of Cadmium metal sticks that look a lot like these (likewise, the can). However, the absence of yellow CdS which is evident (sparse patches) makes this less likely. The density of Cd is about 8.65 g/cc, which seems pretty close.

Cheers,

O3

opfromthestart - 11-6-2019 at 13:04

More on tests:
It reacts very quickly with nitric acid to form a white precipitate, which seems poorly soluble.
It reacts very slowly with hydrochloric acid, slowly bubbling after a few minutes.
It probably does not contain any lead or silver as no precipitate formed when sodium chloride was added to a solution of the precipitate from the nitric acid reaction.

fusso - 11-6-2019 at 22:48

"solution of the precipitate"? if it's insoluble how did you dissolve it?
One of the metal which form a white ppt when reacted with HNO3 is Sn.

Cryolite. - 11-6-2019 at 23:31

I'm thinking the mystery metal is tin as well. To test for tin, dissolve shavings of the metal in hydrochloric acid and add zinc granules. After the zinc has fully reacted, dip a water-filled test tube into the mixture and then hold it in the flame of a bunsen burner. An electric blue glow on the test tube indicates the presence of Sn ions.

Here's a video demonstration of this test: https://www.youtube.com/watch?v=rY9LcPj_eU8

AJKOER - 18-6-2019 at 16:18

A test that may be of value, especially if it is an alloy, is place a small piece of the unknown metal in a microwave safe vessel with vinegar, may be some 3 % H2O2 and a must, a little NaCl. Now take a small piece of stainless steel (as one possible choice, or copper, or zinc, or....) as the second electrode. Ideally, try to keep the approximate electrode surface area about the same (you can use multiple pieces of the stainless steel, if needed, to achieve this).

Microwave for 2 minutes and determine which electrode is being attacked. If it is unclear, think of swapping out the vinegar for another acid.

The metal/alloy that is less noble will NOT be attacked. Lookup the anodic index of various alloys and choose another until you witness an alternation between experiments of the metal moving from noble to more anodic (or the other way around). This may can you an idea of the anodic index of the possible alloys/metals consistent with its electrochemical behavior.

You can also freeze the product solution after the test and try to recover any crystals for added clues using the crystals color, shape and size if it was subject to anodic corrosion.

[Edited on 19-6-2019 by AJKOER]