This is an introductory course for students with limited background in chemistry; basic concepts such as atomic and molecular structure, solutions, phases of matter, and quantitative problem solving will be emphasized with the goal of preparing students for further study in chemistry.
[Demo] Colored Flames
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來自 Duke University 的課程
Introduction to Chemistry: Structures and Solutions
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We will cover introduction to light, Bohr model of the hydrogen atom, atomic orbitals, electron configurations, valence versus core electrons. Don’t forget to use this week’s discussion forum for any questions and discussions.
與講師見面
Prof. Dorian A. CanelasAssistant Professor of the Practice
Chemistry
Have you ever wondered how fireworks manufacturers create the different colors
of lights in the sky? The answer, of course, lies in chemistry.
Let's conduct demonstrations in a controlled
environment to look more closely at
differences between the colors of light emitted from the elements as they burn.
Dr.
Lyle has dissolved several ionic compounds in methanol.
First, he's going to show you what the flame for pure
methanol looks like, so you can compare it to the other elements.
Methanol, which is a form of alcohol, burns with a blue flame similar
to one that you might observe if you cook on a natural gas stove.
Next, he has a solution of sodium chloride dissolved in the methanol.
If you're driving down the highway at night and
you see yellow lamps, those are sodium vapor lamps.
The color of flame also depends on temperature, so we might
not see the color right away for some of the samples.
Each flame needs time to get extremely hot
to yield the maximum emission from the metal.
On the camera, this sodium burning looks kind of
pink, but in real life, it was quite yellow.
Since you have plenty of sodium chloride at home, you can check this one
out live, but make sure you have
a controlled environment for containing the flame.
Next, let's look at potassium.
This potassium solution in methanol burns lavender.
Why is each solution burning with different colors?
The colors we see emitted as the elements burn are from atomic emission spectra.
Remember, the wavelength of those observed colors
depend upon the distances between the allowed
electron energy levels and the atoms.
We are promoting the electrons from the ground state
to the excited state, using heat in this experiment.
Then, the atoms emit light as the electrons travel from that
excited state back to the ground state, a process that happens spontaneously.
The next solution he has is of calcium chloride.
It will burn with an orangish red color.
Remember that the blue color is from the methanol solvent.
This one isn't quite as exciting as the
others, and Dr. Lyle hypothesized that it is
because the calcium chloride doesn't dissolve as well
as some of the other salts in methanol.
The next solution is strontium chloride, which emits a
bright red color. Does the emission of red light involve a
higher or lower energy photon than the emission of purple light?
Go ahead and try to answer that question now.
[BLANK_AUDIO]
Hopefully you said that the purple light was a higher energy photon.
The purple light has a smaller wavelength, which corresponds to
a higher frequency and a higher energy than red light.
Next, we have a barium solution.
This barium solution will burn apple green.
It looks like there's a bit of an impurity in this one.
Perhaps it got contaminated with some of the strontium.
[BLANK_AUDIO]
The seventh flame test we'll do is for
a solution of copper two chloride dissolved in methanol.
Remember, even though copper is a cation with a plus two charge here, it still has
27 electrons that can be moved into the excited state by the heat of the flame.
And those can relax back down to lower energies and transitions.
Those transmissions emit a bright green light that we can see with our eyes.
The last sample is boron, which also emits a green light as it burns.
In this case, the boron is in the form
of boric acid, which is dissolved in the methanol.
Which of these colored flame solutions did you enjoy the most?
For the grand finale, we've got all of the solutions burning at the same time.
Dr. Lyle and I lit them as quickly as
we could to make the most exciting campfire ever.
The yellow orange sodium is front and center.
As you have observed in this video, many metals emit a characteristic visible
color as they burn. And this fact can be used by analytical
chemists as they conduct qualitative tests for metals and samples.
This fact is also exploited by fireworks manufacturers, who add particular
elements to their mixtures to give the desired color of light upon explosion.
I hope you've enjoyed
this chemical demonstration of colored flames.
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