Forensics C

[daydreamer0023]

Just wondering, is salt the only powder with a cubic crystal? Silly question, I know.

I don't think crystal shape works well for IDing powders, although I haven't tried much. I think they're extremely variable, and lack of consistency makes it hard to ID. I would think that at least KCl would also have a cubic structure, since it's very similar chemically to NaCl.

Yeah I mean considering the national supervisor's description of sodium acetate is "very fine white powder" and the sample of sodium acetate we had at our school was medium-sized clear crystals, I'd say really don't rely on the physical appearance of the powder as a key identifier. There are plenty of other ones you can use that are far more robust.

I haven't looked at crystals structure a lot, but I knew from CB that one of the key features of salt was cubic crystals, so I was kind of going to use it as more of a verification of the correct powder than an identifier.

[moumantai]

Hi guys! Just wanted to check in. This is my first year doing Forensics. I'm in first-year chemistry and my partner is in AP Chemistry. I'm really excited to give this event a shot; Is there any rule-of-thumb advice we can get?

[Panda Weasley]

Hi guys! Just wanted to check in. This is my first year doing Forensics. I'm in first-year chemistry and my partner is in AP Chemistry. I'm really excited to give this event a shot; Is there any rule-of-thumb advice we can get?

Not really "rule-of-thumb" worthy, but my advice to you is to repeatedly practice IDing everything. Once you feel familiar with everything give each other unknowns to identify. If you don't have a lot of time to prepare, focus on the powders as they are worth the most points.
I hope this was helpful, good luck!

[pikachu4919]

Hi guys! Just wanted to check in. This is my first year doing Forensics. I'm in first-year chemistry and my partner is in AP Chemistry. I'm really excited to give this event a shot; Is there any rule-of-thumb advice we can get?

Not really "rule-of-thumb" worthy, but my advice to you is to repeatedly practice IDing everything. Once you feel familiar with everything give each other unknowns to identify. If you don't have a lot of time to prepare, focus on the powders as they are worth the most points.
I hope this was helpful, good luck!

Panda Weasley is spot on that lots and lots of practice on the lab practical is the best way to prepare for Forensics. But I'd also say to try to practice polymer ID just as much as powder ID because it can also count for a lot of points. And I say "try" because it's definitely slightly more difficult to access practice versions of the polymers than the powders. I mean powder ID is hard because messing up by contamination is pretty darn easy to do but polymer ID is also hard because while the indicator tests are shorter and not as easy to mess up, you still have three different types of polymers that you have to handle, each with its own set of unique ways to ID each. (When I competed, my partner did powders and I did polymers and my partner found polymer ID hard for her and I found powder ID hard for myself so...yeah) For example, the most recent national exam, there were 14 powders to ID worth 3 pts each but there were also 18 total polymers to ID also worth 3 pts each. Not every test is structured exactly like that but the fact that there is a broader range of possible polymers than possible powders also means there might be more polymers than powders on the exams. Also, never forget the trivia/essay. Those are (somewhat) easy points (most of the time, unless it's nationals when you have a boatload of them that you have to juggle along with the ID's and chromatography and all that jazz) that can help your score out a lot in the case that your ID's go awry.

The TL;DR version:
Powder ID is important, and you should practice it a ton. But don't focus on it so much that you sacrifice all the other parts of the exam, especially polymer ID.

[Lumitailz]

For Powder ID, do supervisors usually provide anhydrous or hydrated powders? (Normally I ID magnesium sulfate just by its appearance, but if they give me anhydrous then I would be stumped )

[pikachu4919]

For Powder ID, do supervisors usually provide anhydrous or hydrated powders? (Normally I ID magnesium sulfate just by its appearance, but if they give me anhydrous then I would be stumped )

Depends, but I got a lot of anhydrous ones back in my day. There are some powders such as LiCl have to be anhydrous anyways. I'd also like to say that it's recommended to avoid using a powder's appearance as a primary indicator test bc of the fact that there are so many different forms that they can come in (i.e. anhydrous/hydrous). For MgSO4, a sure way to indicate it is adding NaOH to a solution of it because it will form an insoluble Mg(OH)2 precipitate that will be visible.

[Lumitailz]

Depends, but I got a lot of anhydrous ones back in my day. There are some powders such as LiCl have to be anhydrous anyways. I'd also like to say that it's recommended to avoid using a powder's appearance as a primary indicator test bc of the fact that there are so many different forms that they can come in (i.e. anhydrous/hydrous). For MgSO4, a sure way to indicate it is adding NaOH to a solution of it because it will form an insoluble Mg(OH)2 precipitate that will be visible.

Alright, thanks! I think I will make some changes to my flowchart. Also, I was double checking the event supervisor powders chart with all the descriptions, and I noticed that calcium sulfate was marked as very soluble in water. Is that true? When I search it up on Google I get mixed results- saying that its slightly soluble, etc. And if it does have low solubility, shouldn't it have low conductivity as well?

[Panda Weasley]

Depends, but I got a lot of anhydrous ones back in my day. There are some powders such as LiCl have to be anhydrous anyways. I'd also like to say that it's recommended to avoid using a powder's appearance as a primary indicator test bc of the fact that there are so many different forms that they can come in (i.e. anhydrous/hydrous). For MgSO4, a sure way to indicate it is adding NaOH to a solution of it because it will form an insoluble Mg(OH)2 precipitate that will be visible.

Alright, thanks! I think I will make some changes to my flowchart. Also, I was double checking the event supervisor powders chart with all the descriptions, and I noticed that calcium sulfate was marked as very soluble in water. Is that true? When I search it up on Google I get mixed results- saying that its slightly soluble, etc. And if it does have low solubility, shouldn't it have low conductivity as well?

I ran into this problem as well when trying to confirm my results. Everyone describes things differently, so it's best to trust your results. As for your second question I believe the answer is yes, but someone should confirm that since I never really payed attention to conductivity.

[Lumitailz]

Another question - What determines whether a salt would dissolve in NaOH? I'll definitely do some lab tests in the future, but I want to know the chemistry behind it. For example, I get that calcium salts would all precipitate due to Ca(OH)2, but I don't understand why NaCl dissolves and Na2CO3 doesn't (unless the chart is messed up again).

[pikachu4919]

Another question - What determines whether a salt would dissolve in NaOH? I'll definitely do some lab tests in the future, but I want to know the chemistry behind it. For example, I get that calcium salts would all precipitate due to Ca(OH)2, but I don't understand why NaCl dissolves and Na2CO3 doesn't (unless the chart is messed up again).

Which chart are you using? Anyways, determining solubility is mainly based on understanding of chemical reactions (specifically double replacement/precipitation), ions, and periodic trends (often, using periodic trends is how to predict ion behavior in reactions, especially cations).

So, when you put a salt in a solution, think about the cation and the anion of the salt. In this case, for sodium carbonate, the cation is Na+ and the anion is (CO3)2-. Since Na+ is a highly reactive alkali metal (same with the others in its group), it will dissolve pretty much always dissolve, and NaOH itself is no exception. Then, you would have to look at which ions are present: Na+, OH-, and (CO3)2-. None of those can combine to make a solid, insoluble precipitate, so Na2CO3 really should be able to dissolve in NaOH (check in lab). Usually when I use NaOH to do a precipitate test I dissolve the powder in water first and then add NaOH so you can see it better, plus you can check its solubility in water first (that can tell a lot!).