I was wondering -- does anyone have a good way (other than the super-subjective 'which bends easier? which scratches easier?) to differentiate between PC and PMMA? Thanks!
Density. PC is somewhere between 1.2 and 1.6 g/mL, but PMMA is around 1.16 g/mL. If you are incredibly lucky, they will give you corn syrup to test this in. PMMA floats; PC sinks.
25% NaCl solution should also work since its density is ~1.19 g/ml. The Woz (national forensics supervisor but she also supervises in my state) uses this for that reason.
Hey guys, I got put in this event, and I've never done it before. Other than the rules, what should I know about it to start off? How do I prepare for it? Thanks in advance.
Here's a really good post from a past year's forum for this:
What I would recommend is starting right off the bat with learning how to identify the powders since that's a major chunk of the test and subsequently, one of the more important parts of it. For powders, you should have some sort of flowchart in place so as you can sequentially go through different tests (such as solubility, flame colors, HCl reactions, etc.) until you can identify it. For example, to identify boric acid, the only test you need to do for that is to do a flame test because boric acid makes a green flame. LiCl gives off a red flame and KCl gives off a purple flame. Those are the only three powders that will give those distinctive colors. The others will either give you a yellow, orange-ish, or no flame color at all. That's when you need to continue with the next step which would be to determine solubility and so on and so forth.
Then once you have powders down or at least have an idea of how to do them, move to the fibers and plastics and learning to identify those using burn tests (for the fibers in particular since they usually won't allow burn tests for plastics). For plastics, you identify them using densities so you need a flowchart just like for the powders of what to do when one plastic sinks in one solution. So let's say you have a plastic that sinks in water. You know that it must have a higher density so then you test the density using salt water. Different competitions will give different concentrations so you should know the densities of the various concentrations of salt water so that when they give you, let's say, a 10% salt solution and it sinks, you know that it must have a density higher than 1.074 g/mL since that's the density of a 10% salt solution. Then you should test the plastic using corn syrup or some high density liquid to see if it floats or sinks. If it sinks in corn syrup, it has to be PVC (according to the flowchart I've made) and if it floats, it's PETE.
So basically, you need to research flowcharts on powders and plastics as well as learn specific characteristics of fibers such as how they react when burned and how they look. From powders, fibers, and plastics, you move on to everything else. I'd focus less on glass and dirt/tire tracks and such since that's mostly matching. Most tests that I've come across focus mainly on blood, fingerprints, reading mass spec, and the occasional entomology question. When studying those things, you don't need to get too in-depth, but you should go past the surface a little bit. I've had tests where they ask how fingerprints are formed, how many ridges a fingerprint has, things besides matching and identifying fingerprint types. Hope that helps somewhat!
I would add the following to this:
- flame tests can get easily ruined by sodium contamination, so they aren't always 100% reliable unlike solubility, in which it either dissolves or it doesn't and isn't very easily affected by contamination (that's what I recommend should be the first test to use on a flowchart)
- another more feasible difference between PVC and PETE is that PVC gives off a green flame (if provided by the supervisor since burning plastics is not allowed in competition). In fact, if it burns green, it will always be PVC since the other plastics don't give that color.
- glass isn't all matching--you also need to know how to interpret fracture patterns (I've seen questions like that before) aka determining which impacts came first/second, etc.
Be sure to check out the Forensics Wiki
for info about important things to know, and feel free to PM me if you'd like since I've done this event for years.