Forensics C

[slytherin-at-heart]

Does anyone know of any good resources for analyzing glass? In my state, we have a lot less to study this year than usual, so I have a feeling that something about glass will show up on the test.

[BoldlyGoingNowhere]

What would you recommend studying most if we have one and a half days for last minute cramming?

[personasaurus rex]

What would you recommend studying most if we have one and a half days for last minute cramming?

how to do polymers and powders. That is the bulk of the points and if you get that you're good to go. And learn how to write an effective analysis - that's the most important part!

[BoldlyGoingNowhere]

What are the differences between polystyrene and polycarbonate?
Sorry for all the questions, I'm not quite sure what I'm doing...

[NinjaChicken]

Tips for writing a conclusive yet concise analysis?

[BoldlyGoingNowhere]

Does anyone know of any good resources for analyzing glass? In my state, we have a lot less to study this year than usual, so I have a feeling that something about glass will show up on the test.

There's a quick intro to glass on this website: http://www.sciencenc.com/event-help/forensics.php
For a more in-depth analysis on determining angle of refraction, use this site: http://www.brooklyn.cuny.edu/bc/ahp/Cel ... .Main.html
There's an index of refraction on this site: http://hyperphysics.phy-astr.gsu.edu/hb ... indrf.html It's not just glass, but it works.
Hope this helps!

[salcedam]

What are the differences between polystyrene and polycarbonate?
Sorry for all the questions, I'm not quite sure what I'm doing...

Don't be sorry for asking a lot of questions. That's what these forums are for.

Polystyrene (PS) is what is usually styrofoam, egg cartons, and packing pellets. It has a density of about 1.07 g/mL. If you burn it, there's a yellow flame and it burns pretty quickly. It's formed from the addition polymerization of styrene.

Polycarbonate (PC) is what's like bullet-proof glass and sunglasses and it has a density of about 1.22 g/mL.

It's kinda tough to tell the difference between the two of these plastics since their densities are pretty close so hopefully, if there are these two plastics on a test, they'll have different concentrations of NaCl solutions because PS floats in 10% while PC doesn't start floating until it's in a 20% NaCl solution.

Tips for writing a conclusive yet concise analysis?

What my partner or I usually do for analysis is to write the name of each suspect down and then to write down everything that matches. Then after that, we write any reasoning towards excluding certain pieces of evidence if there is such a case. (i.e. In one test, all the suspects that were Chemistry teachers had sodium carbonate powder found on them so we wrote that despite having a powder sample that matched, it should be discounted because it doesn't specifically incriminate anyone.) And then whoever has the most matches is the suspect and if we have time, we go over the story and talk about the motives of the suspect. The most important part of the analysis is making sure that you write all the matches. That should, at least, give you some points and it lays the groundwork for the rest of the analysis as to who did it and why.

[willowmist]

My partner and I are having trouble with the plastics with a density greater than water. Is the only way to distinguish them by density with the NaCl solutions? If we aren't given these, then how can we tell the differences between PS, PMMA, PC, PETE, and PVC? If I'm not mistaken, I think all of them except PVC are transparent? Also, in the rules, when it says that burn test results may be provided, what kind of results do they mean? All I could find was observations about the burn tests during the testing, but not anything about how the plastics look/are afterwards, if that's what the rules mean.

Also, can anyone help with how to analyze mass spectra?

And for spatters, where it says that we should be able to determine angle and velocity of the spatter given just the photograph, how would you do this?

We have an invitationals tomorrow and we're kind of desperately cramming. >___< Thanks.

[AlphaTauri]

I notice you're from NJ...would the invitational tomorrow be Athens or Stoga by any chance? (If it's Stoga, well dang, I'm helping the competition. :P)

Plastics: If they're going to give you plastics, they have to give you solutions for you to determine density. I interpreted this to mean that they have to give you enough solutions for you to narrow the plastics to 1 or 2 options, although I'm not 100% certain because I've never competed in 4n6 before. Burn test results will likely be things like "self-extinguishing, does not drip, smells of hydrochloric acid, white smoke" (which is PVC, fyi), but again, don't quote me on that.

Mass spectra: Well, I'd like a bit of help on that too. Sorry.

Spatters: If you poke around on the internet a little bit, you should be able to find the equations and info you need. I personally found the Wikipedia article really helpful.

[personasaurus rex]

How do you tell the difference between PVC/PETE and PMMA/PC? I know that between PVC and PETE, they both sink with saturated NaCl so is the only distinguishing factor by their transparencies? Also between PMMA and PC, they're both transparent and float in NaCl, so is there some other distinguishing factor if they do not provide burn test results/uses?