Category: Natural World

Carbide Cannon

Ms. Wilson’s chemistry class is looking at basic chemical reactions, and today they got to fire an acetylene cannon. When calcium carbide (CaC2) reacts with water (H2O) they produce acetylene (C2H2), which is quite explosive.

CaC2 + 2 H2O → C2H2 + Ca(OH)2

Acetylene is so flammable, because its carbons are held together by a triple bond: when the triple bond breaks it releases a lot of energy (about 839 kJ per mole).

Table 1: Bond strengths of simple hydrocarbons with carbon to carbon bonds

Name Chemical Formula Diagram Carbon to Carbon Bond Strength (kJ/mol)
Acetylene C2H2 839
Ethene C2H4 611
Ethane C2H6 347

The explosion is a result of the combustion of the acetylene:

2C2H2 + 5O2 –> 2H2O + 4CO2

And this whole process — carbide plus water to give acetylene, which is then burned — was used by miners in the early 20th century to make headlamps (among other types of lamps).

A carbide lamp (image via Wikipedia).

The cannon itself is a simple device, made of a 50cm tube of 2-3 inch diameter PVC (sorry about the mixed units), with a screw cap at one end. The carbide grains (about 0.5 g) are placed on the inside of the cap, which is then screwed on to the bottom of the tube. A few drops of water are then added through a small hole in the PVC using a plastic dropper — you can listen for the sizzling to tell if the carbide decomposition reaction is happening. Finally a flame is applied to the same hole as the water. The sock, by the way, is just lightly tucked in near the top of the PVC tube, about 5 cm in.

The explosion was loud, and Ms. Wilson’s sock traveled about 10 meters. It was suitably impressive. I think the student who was the most impressed was the one who had weighed out the calcium carbide, becaues 0.5 grams is really only four or five grains.

Ms. Wilson demonstrates the carbide sock cannon.

On the Origin of Species

Perhaps the key reason for the profound influence of Darwin’s “On the Origin of Species” is that it’s such a well written and well reasoned argument based on years of study. It is a wonderful example of how science should be done, and how it should be presented. In the past I’ve had my middle schoolers try to translate sections of Darwin’s writing into plainer, more modern English, with some very good results. They pick up a lot of vocabulary, and are introduced to longer, more complex sentences that are, however, clearly written.

Diagram and notes on the bird species P.Nanus from The Zoology of the Voyage of H.M.S. Beagle, Part 3: Birds by J. Gould and G.R. Gray (edited by C.Darwin). Image via Darwin Online.

The text of “On the Origin of Species” is available for free from the Gutenberg library. Images of the original document can be found (also for free) at the UK website, Darwin Online (which also includes the Darwin’s annotated copy). Darwin Online also hosts lot of Darwin’s other works, as well as notes of the other scientists on The Beagle, among which is included some wonderful scientific diagrams.

This year, I’m going to have the middle schoolers read the introduction, while the honors environmental science students will read selected chapters and present to the class — this will be their off-block assignment.

Diagram of the fish Cofsyphus Darwini by L. Jenyns in The Zoology of the Voyage of H.M.S. Beagle, Part 4: Fish (edited by C.Darwin). Image via Darwin Online.. .

A Plant Responds to Water

Pepper plant responding to being watered. These images were taken over the course of two hours.

During class on Friday, I watered my Chinese five-color hot pepper plant for the first time in three days. It responded quite well, helping to illustrate one reason (to maintain their rigidity/prevent wilting) why plants need water. I did this because I was curious about how fast plants respond to water, and with the data from the images I should be able to demonstrate what a scientific report should look like.

The full plant’s response:

Notes

The original camera images were cropped for the gif-animation using Imagemagick’s convert

convert $i -crop 500x400+1550+1100 crop-$i

The image file sequences were converted to mp4 video using ffmpeg (instructions here):

ffmpeg -r 5 -b 64k -i crop-image00%02d.jpg watering64k-1000.mp4

where:

  • frame rate = r = 5 frames/second
  • bitrate = b = 64k

And the Snakes Ate the Birds who Ate the Spiders

The introduction of snakes to Guam has reverberated through the ecosystem.

Accidentally introduced to the island in the 1940s, the snake decimated the island’s native bird species in one of the most infamous ecological disasters from an invasive species.

By the 1980s, 10 of 12 native bird species had been wiped out.

Since many birds consume spiders, compete with spiders for insect prey and utilize spider webs in their nests, their loss has led to a spider explosion on the island, researchers said.

UPI (2012): Bird loss has island overrun with spiders

Note (for the Algebra students): The scientific article includes a nice box and whisker plot showing how many more spiderwebs there are on Guam compared to other islands.

Number of spider webs on different islands. Guam is the only island shown that has had a severe reduction in birds. Image from Rogers et al., 2012.

Blowing Bubbles to Acidify Water

Changing colors of universal indicator show how blowing bubbles acidifies water (light green-second beaker) from neutral pH (dark green-third beaker) standard. For comparison, the first beaker (red) is acidified while the last beaker (blue) is made alkaline.

CO2 + H2O —-> H2CO3

This useful little reaction, where carbon dioxide reacts with water to produce carbonic acid, came up in my middle school class when we talked about respiration, it’ll come up soon in environmental science with the effects of carbon dioxide on the oceans (acidification), and it offers the opportunity to discuss pH and balancing chemical reactions in chemistry.

The middle school class did the neat little experiment where students blow bubbles in water (through a straw), and the carbon dioxide in their breath reacts with the water to slightly acidify it. A little universal pH indicator in the water (or even cabbage juice indicator) shows the acidification pretty well if you make sure to keep a standard nearby so students can see the change in color.

The fact that the CO2 in your breath is enough to acidify water begs the question — which was asked — how much of the air you exhale is carbon dioxide? According to the Oak Ridge Carbon Dioxide Information Analysis Center’s FAQ page, it’s concentration is about 3.7% by volume. Which is a lot more than the 0.04% average of the atmosphere.

Of course if you really want to talk about the pH you need to get into the acid equilibrium and the dissociation of the carbonic acid to produce H+ ions; you can get the these details here.

Flowers are “Creepy”

My students are researching the organisms they collected from the creek, and I was outlining the types of information I wanted them to find. We were talking about how many animals have seasonal reproductive cycles, and I pointed out that plants flower seasonally as well. One of my students put two and two together and came up with something close to a whole number: “You mean to say that flowers are … some sort of … creepy … sexual things?”

Microbe from the Creek

Microbe collected from the TFS Creek on 9/10/2012. Possibly a species of desmid.

The TFS campus has an excellent ecological gradient. It starts at the hydrologic base-level, with the small, usually permanent, creek in the valley. Then the landscape ranges up, past a narrow but dense riparian zone to the anthropomorphic campus, then up a shrub-covered hillslope that transitions abruptly into the advancing, mature, forest of the hill-top nature reserve. My environmental science class is taking advantage of our geographic proximity by doing a year-long ecological survey project.

We’ve just started, this fall, on the stream and riparian zone. I asked each of them to identify and do some research on a single organism. They all chose some type of macro-organism: spiders, crayfish, flowering herbs (note: just because it’s called an herb does not mean it’s edible), mushrooms, and more. There’s quite a bit of biodiversity down there, although, with the creek just now coming back from our particularly dry summer, the fish are few and far between.

Close-up view of the micro-organism under 1000x magnification (oil immersion lens).

Since no-one chose to look for micro-organisms — even though I did suggest they were an important part of the ecology — I decided do so myself.

I found a loosely held together patch of algae, which I collected with the hope that it would harbor its own little microscopic ecological system. And it did. There were amoebas zipping around, the filamentous algae itself, and these little organisms that I can’t quite identify yet. T

hey may be desimids, but I’m not sure. They look slightly green, but I can’t see any clear chloroplasts (like these). I’ll try staining them tomorrow to see if I can identify any organelles.

A terrible picture "showing" the patch of fillamentous algae I collected from the creek.