Forensics C

[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!).

[DarkZephyr]

How can you tell the difference between cow and horse hair under a microscope if the cow hair doesn't have ovoid bodies? Thanks!

[pikachu4919]

How can you tell the difference between cow and horse hair under a microscope if the cow hair doesn't have ovoid bodies? Thanks!

According to the FBI archives (https://archives.fbi.gov/archives/about ... arch02.htm), cow hair does have ovoid bodies, but if those aren't necessarily visible under the microscope way I'd probably go by medullary index (fraction of the hair shaft that the medulla occupies), or cuticle scale pattern. Looking at different pictures, it seems that the cow hair medullary index would GENERALLY (not necessarily all the time) be smaller than that of horse hair, and I'd probably rely more on that since it's usually tougher to spot cuticle scale patterns under a standard school light microscope.

[sciduck]

So a practice test said that the best way to lift a fingerprint from a ransom note is ninhydrin. Can someone explain this to me?

[pikachu4919]

So a practice test said that the best way to lift a fingerprint from a ransom note is ninhydrin. Can someone explain this to me?

I believe that would be because of the use of the fingerprinting method in accordance to the surface it is on. Ninhydrin is effective at lifting fingerprints from paper because it works on porous surfaces (as opposed to a method like cyanoacrylate fuming which is better for nonporous surfaces), which is what paper is. If you want to know the full chemistry behind it I found this: http://scholarlycommons.law.northwester ... ntext=jclc

[sciduck]

So a practice test said that the best way to lift a fingerprint from a ransom note is ninhydrin. Can someone explain this to me?

I believe that would be because of the use of the fingerprinting method in accordance to the surface it is on. Ninhydrin is effective at lifting fingerprints from paper because it works on porous surfaces (as opposed to a method like cyanoacrylate fuming which is better for nonporous surfaces), which is what paper is. If you want to know the full chemistry behind it I found this: http://scholarlycommons.law.northwester ... ntext=jclc

Thanks. But why would this be better than iodine fuming?