Question Video: Identifying the Emission Spectrum Corresponding to an Unknown Gas | Nagwa Question Video: Identifying the Emission Spectrum Corresponding to an Unknown Gas | Nagwa

# Question Video: Identifying the Emission Spectrum Corresponding to an Unknown Gas Physics • Third Year of Secondary School

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

Taking a look at this figure that we’ve been given, we can see that we’ve got a series of emission spectra. Up at the top, we’ve got the spectrum of the unknown gas that the scientist wants to identify. Then, below this, we’ve got the emission spectra of helium, oxygen, neon, argon, and xenon, each of which is a pure gaseous element. We’re being asked to identify which of these five elements is the unknown gas. Since we’re asked which one of these five elements the unknown gas is, this means we know that the unknown gas must be a pure sample. That is, it’s not a mixture of some number of different elements. But rather it’s entirely one of these five elements whose emission spectra were shown here.

What we need to do then is to identify a match between this emission spectrum of the unknown gas and one of these five known emission spectra. If we look at our unknown gas spectrum, we can see a load of these bright lines, which are emission lines of this particular gas. At the top of this figure, we’ve got a wavelength scale, which allows us to read off the wavelength of each of these emission lines. So, for example, we can see that this emission line here from the spectrum of the unknown gas has a wavelength of about 454 nanometers. Meanwhile, the wavelength of this line is about 565 nanometers.

Now, since we can see that there’s a whole load of lines in this emission spectrum, we’re not going to individually read off the wavelength of every single one. The important thing to realize is that an emission spectrum kind of acts like a fingerprint for a particular element. What this means is that if we take this helium spectrum, for example, then we know that any helium gas anywhere in the world will produce this exact same emission spectrum with the exact same emission lines at the exact same wavelengths. That is, if we see an emission spectrum anywhere that looks exactly like this one, then we know that we’re looking at an emission spectrum of helium. And of course the same thing holds true for the emission spectra of any other elements, including these ones that we’re also given in this question.

So then, in order for one of these five elements to be a match for the sample of this unknown gas, the element’s emission spectrum must contain all of the emission lines that are present in the spectrum of the unknown gas. And it must also contain no emission lines that are not present in the unknown gas spectrum. Now, we can make these comparisons by eye. And let’s start with the emission spectrum of helium. We can see that there’s a whole load of lines here in the helium emission spectrum that are not present in the spectrum from the unknown gas. We can also see that there’s a whole load of these lines at longer wavelengths in the emission spectrum from the unknown gas that are not present in the helium spectrum. We can say then that the helium spectrum does not match the spectrum from the unknown gas. And so this unknown gas cannot be helium.

Now, let’s have a look at the emission spectrum of oxygen. We can see that this also contains some lines at shorter wavelengths that are not present in the spectrum from the unknown gas. We can also see that there’s all of these lines between about 460 and 520 nanometers in the emission spectrum of the unknown gas, none of which are present in the emission spectrum of oxygen. Since these two spectra don’t match, we can safely say that our unknown gas is not oxygen.

Now, let’s consider the emission spectrum of neon. This one’s looking a lot more promising because for a start in this short wavelength region, there are no emission lines in either the neon spectrum or the spectrum of the unknown gas. Then, where we’ve got a whole load of lines at longer wavelengths in the spectrum from the unknown gas, we’ve also got a whole load of lines at longer wavelengths in the emission spectrum of neon. By tracing vertical lines between a few of the emission lines in the spectrum of neon and those in the unknown gas, we can see that the emission lines in each of these two spectra do appear to occur at the exact same wavelengths.

So it’s looking like this sample of unknown gas could well be neon. Let’s have a quick look at the remaining two spectra though just to be sure. If we look at the emission spectrum of argon, we can see that it’s got all of these lines at longer wavelengths that are not present in the spectrum of the unknown gas. So, we know that this gas is not a sample of argon.

Finally, we’ll look at the spectrum of xenon. We can see, for example, that this leftmost line in the emission spectrum of the unknown gas that we previously identified as having a wavelength of about 454 nanometers is not present in the emission spectrum of xenon. Since in order to have a match between two spectra all of the emission lines need to match up exactly, then this one line alone is enough to say that the unknown gas cannot be xenon.

So we found then that only one of these five emission spectra matches the spectrum of the unknown gas. That’s this spectrum here, which is the emission spectrum of neon. This match between the emission spectra then tells us that the unknown gas is neon.

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