Science Crime Busters B

[SciBomb97]

can someone tell me the difference between zinc and aluminum?

17 protons.

The flow chart and tables say that one will react quickly with HCl while the other is delayed. I can't tell you wha that looks like in practice because I don't have these samples, but I'd try it at least once if you can.

thanks!

Yeah, we tried this one so many times at practice (the 6th graders...they never get used to it).
For aluminum, there is definitely a delay between contact and reaction. You have to wait like a whole minute to about 2 minutes before something happens. When it does start to react, there will be a slight fizz followed up by a pretty violent bubbling that lasts for about 10-20 seconds.
Zinc, on the other hand, will react upon contact with HCl by fizzing at a realtively constant magnitude. It won't be anything very violent, just some moderate fizzing.
Also, there may be some confusion between magnesium and zinc. While zinc has a moderate fizz on contact, magnesium will practically evaporate on contact and bubble violently if a piece is dropped into a fair amount of HCl. You'll see what I mean if you ever get to try it.
Hope that helps.

[Cheese_Muffin_Man]

Thanks! i think it will help us out a lot

[Cheese_Muffin_Man]

how can u test for gypsum?

[Skink]

The best approach is NOT to rule out things (ie "Test for Gypsum"/"Test for Calcium Carbonate") unless you already have a good guess as to what something is, but the best approach is to simply follow the flow chart, which is page 2 of that link. If you have a hunch you know what it is, you can skip steps.

[EpicFailure]

Has anyone tested Ascorbic Acid and got a pH other than 2? Same with Alka-Seltzer. Has anyone gotten a pH other than 6 for that?

[LoopQuantumGravity]

To answer your question, I always get those pH values when I do the tests.

Could some one help me on this:
What is the best way to tell the difference between NaCl and sugar if no microscope is provided? They didn't have one at state.

[SciBomb97]

You should have a handlens/magnifying glass in your crime kit (you're allowed to take one). Even at low magnification, it's apparent whether the crystals are all cubic or just similar to cubic. You should look at a sample of NaCl and a sample of sugar with a magnifying glass before going to competition, the difference is very apparent once you see them both.

[ThornStone]

which of the liquids would be best used to clean a cut? and is rubbing alcohol more flammable than hydrogen peroxide?

[LoopQuantumGravity]

which of the liquids would be best used to clean a cut? and is rubbing alcohol more flammable than hydrogen peroxide?

Hydrogen peroxide is commonly used to clean cuts.
Rubbing (isopropyl) alcohol is flammable, while hydrogen peroxide is not.

[ThornStone]

which of the liquids would be best used to clean a cut? and is rubbing alcohol more flammable than hydrogen peroxide?

Hydrogen peroxide is commonly used to clean cuts.
Rubbing (isopropyl) alcohol is flammable, while hydrogen peroxide is not.

we were looking through old tests to prepare for nationals and we found a question that asked "which liquid fizzes when used to clean a cut?" and the answer is vinegar, so we weren't sure about the answer. and http://www.dailymail.co.uk/news/article ... leach.html says that hydrogen peroxide is highly flammable.

[SciBomb97]

which of the liquids would be best used to clean a cut? and is rubbing alcohol more flammable than hydrogen peroxide?

Hydrogen peroxide is commonly used to clean cuts.
Rubbing (isopropyl) alcohol is flammable, while hydrogen peroxide is not.

we were looking through old tests to prepare for nationals and we found a question that asked "which liquid fizzes when used to clean a cut?" and the answer is vinegar, so we weren't sure about the answer. and http://www.dailymail.co.uk/news/article ... leach.html says that hydrogen peroxide is highly flammable.

Well, I'd think it's hydrogen peroxide. Have you ever had a cut and used hydrogen peroxide on it? You should try it sometime if you haven't, the first time I tried, it started fizzing so I was like uhh.... I personally have no idea why you'd clean out a cut with vinegar other than the fact that the acidity may prevent infection, but whatevs.
And the thing with the flammability is that hydrogen peroxide is only flammable at concentrations greater than about 70% (flash point of 70 degrees C), which may have been the case with the bleach in the car.
Hope that clears things up.

[bam]

Does anybody know if the same person will be writing the Nationals test this year??

[outofsight13]

When testing pH in mixtures, will the result be sort of a mix of the components? For example, if the mixture is calcium carbonate (pH of 7) and baking soda (pH of 9), would the result of the test show ~8? Thanks!

[labchick]

When testing pH in mixtures, will the result be sort of a mix of the components? For example, if the mixture is calcium carbonate (pH of 7) and baking soda (pH of 9), would the result of the test show ~8? Thanks!

From what I've seen, it tends to be a tie-dye-like blend of the two colors, e.g for the above mixture it would be green on some of the paper and yellow on the rest. But, it varies a lot depending on the mixture.

[LoopQuantumGravity]

When testing pH in mixtures, will the result be sort of a mix of the components? For example, if the mixture is calcium carbonate (pH of 7) and baking soda (pH of 9), would the result of the test show ~8? Thanks!

Actually, no. Even though the concentrations of H+ ions should average out, the pH scale is logarithmic.
For example, if a powder with a pH of 6 and one with a pH of 8 were in a 1:1 mixture, the pH would be -log((10^(-6)+10^(-8))/2), or roughly 6.3.