Hypothetical question

[Terry22]

Hello. I'm afraid I'm not a chemist. Is it possible to identify an anonymous substance in a high school chemistry lab using the equipment available? For example, the students are given an unlabeled blue powder and asked to identify it. How would one go about this, and what would the likely answers be?

 

Is this too complex for a school lab? If so, is there a similar experiment one could ask students to do?

 

Terry

[hermanntrude]

if you are not a chemist, why are you being asked to develop a chemistry lab activity? A non-chemist in a chemistry lab is a dangerous thing indeed

[Terry22]

Ha ha!

 

I'm not being asked to "develop a chemistry-lab activity" but thanks for your concern. This is a "hypothetical question" (well, that's what the title of my post says).

[hermanntrude]

yes, it just seems like a strange hypothetical question to ask if you have no responsibility in chemistry education.

 

A blue powder could be a LOT of things. Sometimes context will give you a clue... for instance if you found it in a chemistry set for kids, it'd probably be a copper salt, most likely copper sulfate or copper chloride, and you could test that using a flame test.

 

However if you have no idea where it came from you'd have to do at least a few tests. Test its properties... things like solubility, melting point, etc. The most likely thing is that it's inorganic, and if that's the case it's most likely a copper containing salt... however, you'd need to be sure of both of those assumptions before even trying to find out which copper salt.

 

Seriously... do you have a blue powder somewhere and you want to find out what it is?

[Terry22]

Yes this must seem strange to a chemist.

 

I must confess, I have no intention of reproducing this experiment. I'm writing a story in which the protagonist (a student) discovers a strange compound (powder or solid or whatever -- the colour isn't so important, neither is the substance, as long as it's not liquid) and he/she must quickly identify it.

 

I'm ignorant when it comes to chemistry, so I was hoping that maybe there are common, simple experiments that students typically attempt at high-school that might make this scenario convincing. I don't want to create a situation that's unbelievable, or wander into the realms of science fiction. I was just wondering whether there are any (probably safe, possibly non-toxic) colourful powders or compounds that are easily identifiable by one or two very simple procedures?

[hermanntrude]

what is your intention for this storyline? is the substance toxic? or is it just a side-line? something they have to do in class but with no consequence? what happens next? is it important to the storyline what conclusion the student comes to?

 

At high-school level, the students would have to be given more information. Either they could be told that the sample is inorganic and (for instance) contains the sulfate ion (or something like that), or they could be told that the sample is one of a number of given substances... for instance, unknown compound A is either copper(II)sulfate, copper (I) sulfate, lithium carbonate, nickel (II) chloride or charcoal.

[CaptainPanic]

To help us, what you can do is take some material, and describe it to us.

 

For example, take some table salt (pretend you don't know what it is... and you refuse to touch/taste it because it might be toxic?). Then describe it to us... the more details, the better. You can then pretend that it was the blue stuff you want to describe... it's actually irrelevant. We need to know what kind of information we're going to start off with.

 

With that, we will have a realistic amount of data, which your fictional character would also be able to get. And then we can see what we can do with that limited amount of data. Perhaps we can design some experiments, perhaps not.

 

The reason why I don't do it myself is: I'm a chemical engineer, and I might look for different things than somebody who doesn't have any background in chemistry.

[Terry22]

Hi,

 

Thanks for your replies,

 

It's a bit of a macguffin in the storyline, so I don't want to turn it into something really major that's going to take a lot of exposition.

 

The substance can be literally anything. I decided on a powder because it's simple, and a coloured powder as opposed to white powder seemed more fun (and less likely to be mistaken as cocaine or other drugs -- this is for kids, y'see) but I guess it doesn't matter too much. It could be a jelly, or ... I dunno ... anything but not a liquid (unless it's quite viscous, perhaps, like treacle).

 

It's not important to the plot exactly WHAT the substance is, it's just important that the student can do a couple of simple tests (either adding water or using a flame) to identify it.

 

As I've never studied chemistry, I naively wondered if there were some simple solutions (pardon the pun) to this. Perhaps I'm making trouble for myself.

 

It's all quite light-hearted but I wanted it to be based on reality rather than make up an imaginary chemical (which I may have to do).

 

Terry

[CaptainPanic]

The reason I ask all this stuff is that the analysis for a powder/solid and a liquid will be completely different... and right now you're asking us to evaluate all types of analysis in the chemistry lab... Click here to get a short introduction into analytical chemistry.

 

There's also a massive difference between a regular school lab and a specialized university lab.

 

And no worries about "turning it into something really major". We're all volunteers here: if we get tired of you, we just click the little cross in the top right corner of the screen.

[Terry22]

OK.

 

I'll give it a go.

 

For the sake of argument it's blue -- not deep blue, but quite a vibrant blue. Very unnatural-looking.

 

It has no odour.

 

It's not as fine as salt, it's quite chalky in appearance.

 

And it's all heaped up in a nice little cone.

 

Put your finger into it and residual powder comes off.

 

Rub your fingers together and it's quite soft and dry -- like talc, staining your fingers slightly, but you can brush it off.

 

If you sniffed it some might go up your nose and make you sneeze.

 

Put water on it and it doesn't soak into it immediately -- kinda beads on the surface, or at least there appears to be some resistance.

 

Drop some into a beaker and it takes a while to disperse and sink -- doesn't really turn the water blue.

 

It's very slightly bitter to taste.

 

No idea what would happen if you burnt it.

 

Terry

[CaptainPanic]

Nice

So, we're dealing with a powder, which is fine enough, which doesn't contain any large chunks. Most likely inorganic.

 

Professional way

My initial guess would be to give it to a lab with an XRD (or more specific: powder diffraction). Modern ones are often coupled to a database which may automatically identify it.

 

High school lab

However, I realize that we're dealing with a high school chemistry lab. That makes it a lot more difficult.

Your first test, I think, should be to see if it dissolves in water. If it does, try to measure how much actually dissolves in water. (Therefore: use small amounts, and also a very small amount of water). Measure the weight and make notes all the time.

 

If it dissolves, you can consider to add some soda. If your blue powder is a water-soluble copper salt, it will precipitate (become solid) when you add soda.

 

You could also see if you can do electrolysis, but that's already more tricky, and can go wrong and hurt you. But if it works, you'll see the characteristic copper (in metal form), which is really easy to identify. Please note that the link I gave was specific for CuSO4.

 

If it does not dissolve, then you can check the density using water. Measure the volume and weight of the water before and after you dump the substance into the water. The difference in weight and volume will give you a density.

If it floats, it might be plastic. Try to melt it, and look up melting points of plastics.

 

And... then I actually don't have inspiration how to continue.

 

With the data you get, you need to hit the books, and look up numbers for copper salts. Hopefully you recognize something.

 

Option 2: it's not inorganic - it's organic

You can first determine the melting point. See if it melts at relatively low temperatures by heating it to less than 100 deg C (you need a fume hood for this!). If it melts at low temperatures, it's usually organic.

 

Second test is easy: try to burn it. If it burns, it's organic. Inorganic compounds almost never have a high vapor pressure / ignition point and generally don't burn.

NOTE: This is not a very smart thing to do actually... do this in a fume hood, with a small sample and adequate protection. Heating up unknown substances usually goes into the "stupid things category"... and the result of this test is not even 100% certain. (i.e. sort of a last hope).

 

If it is organic, you can try to dissolve it into several solvents... but most likely you're screwed because there are just too many options.

 

Final remarks:

Smelling an unknown substance can be dangerous. Minus one point for our fictional character.

The types of analysis you can do in the high school lab are never 100% certain.

And whatever you do in your analysis: never use the entire sample (if you make a mistake, you can lose everything).

I'm a chemical engineer, and it's been a while that I was in a high school lab. They may actually have more sophisticated equipment there

Edited June 11, 2009 by CaptainPanic

[Terry22]

That's fascinating. Thanks for your suggestions.

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Merged post follows:

Consecutive posts merged

Someone suggested Chrome Chloride? Or Copper Chloride?

 

An easy test to determine each?

[John Cuthber]

A solution of copper chloride in water will plate a thin layer of copper onto a clean piece of steel placed in it (the "traditional" item is a knife blade or scalpel blade.

Chromium chloride won't.

 

Long ago the details of a lot of chemistry were worked out with lab equipment not much better than you can find in a school.

[hermanntrude]

don't involve tests like taste and smell and touch. Real chemistry students would be ejected from the lab if they were caught tasting or touching chemicals. Sometimes chemists will cautiously smell something if they're sure it's not toxic by inhalation. there is a technique for that.

 

I'd guess if you want to go for a coloured powder, tell the students that it's inorganic. that way you could probably do a flame test and perhaps one or two more tests to be sure what it is. the flame test would tell you which metal was involved and then perhaps one other test would tell you the other part (sulfate, chloride, etc etc). you could make the test for the other part simpler by telling the students it is one of several choices. or by choosing something insoluble in water (there are less of those and they tend to be more distinctive.

 

My choice would probably be copper (II) hydroxide. The tests could go as follows:

 

flame test: gives a greenish (slightly blue) flame. This indicates the salt is copper(II) and that the other half is not a halide.

 

solubility: forms a gelatinous mixture but doesn't dissolve. A good student might know that copper(II) hydroxide makes a weird gelatinous mixture with water. Any student ought to know that copper (II) hydroxide, and all hydroxides except those of the alkali metals and some of the alkali earths are insoluble.

 

heating a small sample: this would result in decomposition to copper (II) oxide, which is black. This would be pretty much conclusive that the salt is copper (II) hydroxide.

[Terry22]

Thanks,

 

That's very helpful (and informative, too).

 

Terry