Video: Identifiying a gas using its emission spectrum

A scientist has a sample of an unknown gas. In order to identify the gas, she looks at the spectrum of visible light emitted from when it is heated. This is shown in the figure. Also shown in the figure are the emission spectra of three pure, gaseous elements. Which of the three elements is the unknown gas?

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Video Transcript

A scientist has a sample of an unknown gas. In order to identify the gas, she looks at the spectrum of visible light emitted from when it is heated. This is shown in the figure. Also shown in the figure are the emission spectra of three pure gaseous elements. Which of the three elements is the unknown gas?

Okay, taking a look at our figure, we see a series of emission spectra. The top one is the spectrum of an unknown gas we want to identify. Then below that, we see the emission spectra of hydrogen, helium, and oxygen, respectively. Our question asks, which of these three elements is the unknown gas? Now, the first thing we can realize is that our unknown gas is a pure sample. It’s not a mixture of some number of other elements. In other words, it’s either entirely hydrogen or entirely helium or entirely oxygen. Knowing that, what we want to find is a match between the spectrum of our unknown gas and the spectrum of one of these other gases. And here’s how we can identify a match.

Looking at our unknown gas spectrum, we can see the particular wavelengths corresponding to emission lines of this unknown gas. So, for example, this emission line of our unknown gas is at about 422 nanometers, whereas this one is at about 437 nanometers and so on and so forth for these other emission lines in our spectrum. We want to find which of these three other gases, hydrogen, helium, or oxygen, have emission lines at the same wavelengths as our unknown sample. We can go ahead and make this comparison by eye.

Let’s start with this emission line here at about 422 nanometers. Looking at our spectrum of hydrogen, we don’t see a feature at or near that wave length. This tells us that the two emission spectra do not match. And so are unknown gas can’t be hydrogen. Next, if we consider helium, we see that this gas too has no emission line at or around 422 nanometers. That is, if we were to drop this emission line wavelength down onto our spectrum of helium, we would see that helium has no emission line at that wavelength. This means our unknown gas is not comprised of helium either.

Lastly, we look at oxygen. In this case, note that we do see a corresponding emission line at this wavelength. That’s an encouraging sign. Let’s consider another emission line in our unknown gas. This one right here, at about 437 nanometers, we see also corresponds to an emission line in oxygen. And if we continue looking at other emission lines, we see there continue to be matches between these two spectra. And this is true, we can see, as we go all the way down to the end of the visible spectrum. So, we found a match for the spectrum of our unknown gas, and that match tells us that the unknown gas is oxygen.

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