[Tekker]

Hi all,

I have a gen chem lab final coming up on Thursday and we are going to be given 10 unlabeled test tubes where we have to identify the substances in each one. So I’m looking for tips, tricks, and basically anything to add that will help this process go as smoothly as possible.

The substances we are going to be given are:

Aluminum nitrate - Al(NO3)3
Ammonia - NH3
Hydrochloric acid - HCl
Magnesium nitrate – Mg(NO3)2
Nitric acid – HNO3
Silver nitrate – AgNO3
Sodium carbonate – NaCO3
Sodium chloride - NaCl
Sodium hydroxide - NaOH
Zinc nitrate – Zn(NO3)2

Our teacher told us that 3 of them are acidic and 3 are basic. However, there’s only two that jump out as being acidic (HCl and HNO3), there’s only one that jumps out as being basic (NaOH). Searching on google for pH levels I found two other possible basic substances (NH3 and NaCO3). The pH levels that I found to be more neutral were NaCl, Al(NO3)3, and Mg(NO3)2. Do these seem right? The two I couldn’t find pH levels for are AgNO3 and Zn(NO3)2, so I have no idea what categories these would fall into.

We are going to be given time to experiment just before we take the final with each substance labeled so we can see what kind of reactions they undergo and what products are formed. We can also use our lab notebook and any notes we have on the final. So I’m trying to think ahead now and figure out a good plan of action to get through it. Here are the steps I’ve come up with thus far:

1. Obviously look for any color differences in each substance. I’m assuming they’ll all be clear colored to keep us from getting anything right off the bat, but one can hope.

2. Test the pH of each substance and set them into groups for acidic, basic, and neutral.

2.5 See if I can identify any substances right off the bat based on the pH levels. That would be very nice if one or more of them give unique pH levels.

3. Then I guess it’s just a matter of picking one of the acids and testing that one against all the others to figure out what the acid is. Maybe that’ll help identify others along the way. I don't know which one would be a good one to start with, so I guess just pick any one and go with it?

The main thing is to make sure that I can narrow it down enough so I don’t end up thinking one substance is something else because they gave a few similar results. Our teacher told us is that HCl + AgNO3 will result in a light purplish color, so if Mg(NO3)2 also gives a purple reaction I need to have some other tests to run so I won’t confuse the two.

Another thing I’ve looked into is the solubility table to see which ones will form solids and only Na2CO3 and NaOH will form solids. Since they are both bases I’ll have to figure out what else they react with to distinguish them apart.

That’s pretty much where I’m at for the moment. If anyone has any helpful advice on how to approach this thing, whether it’s adding on to what I have or completely different, I’d really appreciate it.

Thanks
-tkr

[billnotgatez]

I am curious. What other things are you allowed to use other than the 10 test tubes of unknowns? For instance, can we assume you can use a pH meter?  Are you allowed to use known chemicals?

[Tekker]

I am curious. What other things are you allowed to use other than the 10 test tubes of unknowns? For instance, can we assume you can use a pH meter?  Are you allowed to use known chemicals?

We can only use the chemicals we are given (no known chemicals) and I'm not sure what all we will have, but pH paper I know is one of the things we'll have available. I imagine we'll have plenty of test tubes for mixing chemicals. But other than that, they haven't told us.

-tkr

[mike]

Silver nitrate and sodium chloride should give a precipitate (greenish-yellow) of silver chloride.

Ammonia you should be able to smell (smells a bit like a cleaning product).

Sodium carbonate and an acid should produce carbon dioxide gas (bubbles).

[Borek]

Silver nitrate and sodium chloride should give a precipitate (greenish-yellow) of silver chloride.

White, getting gray if exposed to light!

[mike]

White, getting gray if exposed to light!

Yes Borek is correct, sorry about that

[Tekker]

Excellent! Thanks guys, that's the kind of stuff I was hoping for!

So I should be looking for a white precipitata then for silver nitrate and sodium chloride instead of greenish-yellow? Since there is going to be light in the room.... (hopefully )

-tkr

[Tekker]

Well, it's over.... LOL

It was a little different than I was expecting. We actually made a grid for all of the chemicals on our lab desks using chalk, then put Saran Wrap over it and performed our reactions right over the grid. That way we could get a visual on what each substance did.

Unfortunately I got some very different results when it came time to perform the final. I don't know how, but I certainly did. I heard the same thing from others who had the lab before me and they basically said that you'll think the teacher had sabotaged your chemicals. LOL So at least I know it wasn't just me.

I think I managed to get them all, the pH levels really helped to verify whether they were the right chemicals or not. We also had one "magic row" that changed all kinds of crazy colors, from a kind of cream color, to brown, and a couple purple-ish colors. That was a very helpful row. The two I had problems with were magnesium nitrate and zinc nitrate. The reactions and the pH levels were incredibly similiar. Luckily I rememberd our teacher had mentioned that zinc nitrate with a base will form a "skin" on the surface.... So armed with that bit of info I was able to get it. The teacher's verification made me feel better also.

I'm feeling confident, but I've felt that way before and been let down.. So we'll see how it goes tomorrow.

Oh I almost forgot, another guy in our lab stapled a dollar to his paper when it turned it in. Our teacher thought that was hilarious and was showing it to everyone else in the class and all the other teachers down the hall in their offices.

-tkr