Elements can be identified from the color of light they give off when they’re ionized: their emission spectra. Ms. Wilson’s chemistry class today set fire to some metal salts to watch them burn.
A hydrogen atom's electron is bumped up an energy level/shell by ultraviolet light, but releases that light when the electron drops back down to its original shell.
She placed the salt crystals into petri dishes, submerged them in a shallow layer of alcohol, and ignited the alcohol. As traces of the salts were incorporated into the flames, the metal atoms became “excited” as they absorbed some of the energy from the flame by bumping up their electrons into higher electron shells. Since atoms don’t “like” to be excited, their excited electrons quickly dropped back to their stable, ground state, but, in doing so, released the excess energy as light of the characteristic wavelength.
The rate of radioactive decay of Chlorine-36 (blue x's) seems to be related to the distance between the Earth and the Sun (red line). (Image from Dekant, 2012).
In pre-Calculus, we’re figuring out how to match curves to data. The scientists in this study do something similar, trying to see what types of sinusoidal curves will match the data, then seeing what natural phenomena have the same period (the time it takes for one cycle).
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.
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.
A quick and simple experiment that demonstrates endothermic reaction and can include a discussion of ionic and covalent bonds. Mixing baking soda and vinegar together drops the temperature of the liquid by about 4 °C in one minute. (Note that while the temperature drops and the reaction looks endothermic, it’s actually not — other things cause the cooling. However, since it looks like an endothermic reaction I use it as a first approximation of one.)
Ingredients
3 g baking soda – (sodium bicarbonate – NaHCO3)
60 ml vinegar – (acetic acid – CH3COOH)
200 ml styrofoam cup (needs to be big enough to contain the bubbles).
thermometer
Procedure
Add the baking soda to the vinegar in the styrofoam cup. Measure the temperature while stirring for about a minute.
Results
Time (t)
Temperature (°C)
0
25
15
24
30
21
60
21
Discussion
The chemical reaction between baking soda (sodium bicarbonate) and vinegar (acetic acid) can be written:
NaHCO3 + CH3COOH —-> CO2 + H2O + CH3OONa
The products of the reaction are carbon dioxide gas (which gives the bubbles), water, and sodium acetate.
However, a more detailed look shows that for the reaction to work the two chemicals need to be dissolved in water. Dissolving these ionic compounds causes the two ions to separate. Dissolved baking soda dissociates into a sodium and a bicarbonate ion:
sodium bicarbonate —-> sodium ion + bicarbonate ion
NaHCO3 —-> Na+ + HCO3–
Why doesn’t the bicarbonate break into smaller pieces? Because it’s atoms are bonded together more tightly by covalent bonds.
Similarly, the acetic acid in vinegar dissociates into:
It skims over pyrolysis; chemiluminescence, where the chemical reaction (combustion/oxidation) produces excited atoms and molecules that need spit out (emit) blue light to get to their ground state); and the incandescent light emission of microscopic soot particles which produce the yellow parts of the flame.
I’m not sure who the guy chained to the rock is. It might be Prometheus, who stole fire from the gods, but I don’t remember him being sent into hell in the myth.
