Question Video: Descriptions of Flame Emission Spectroscopy Chemistry

Additional data may be obtained by passing the light from a flame test through a spectroscope. Which of the following statements is false? [A] Elements emit light at unique wavelengths and thus are unlikely to be misidentified. [B] Every element emits a single wavelength of light. [C] It is difficult to analyze nonmetallic elements using this method. [D] Each sample measurement can usually be performed in less than a minute. [E] Both the measurement and data analysis can be automated.

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Video Transcript

Additional data may be obtained by passing the light from a flame test through a spectroscope. Which of the following statements is false? (A) Elements emit light at unique wavelengths and thus are unlikely to be misidentified. (B) Every element emits a single wavelength of light. (C) It is difficult to analyze nonmetallic elements using this method. (D) Each sample measurement can usually be performed in less than a minute. (E) Both the measurement and data analysis can be automated.

We conduct flame tests by burning substances, observing the color change in the flame, and using the color to identify metallic ions. As the question indicates, if we want to know what specific wavelengths are being emitted instead of simply seeing their aggregate color in the flame, we can look through a spectroscope. A spectroscope is essentially a prism that refracts light separating the different wavelengths coming from a source of light.

The spectroscope’s reading will look something like this, with bright colored bands scattered across the visible light spectrum. We call the pattern of wavelengths released by an element the element’s emission spectrum. Much like a human’s fingerprint, an element’s emission spectrum is unique and unchanging. So, we can use it to identify the element being burned.

Now, let’s look at the choices to find a false statement. Choice (D) each sample measurement can usually be performed in less than a minute is a true statement. Each measurement simply requires burning the salt and viewing the flame through the handheld spectroscope. Not a very time intensive process. So, we can eliminate (D) as an answer.

Another true statement is choice (A) elements emit light at unique wavelengths and thus are unlikely to be misidentified. Since the wavelengths of light emitted are based on the spacing of the electron energy levels in the atom, as electrons jump between them, each element will release the same predictable pattern of wavelengths or the same emission spectrum each time it is burned. Identifying the element simply involves matching the observed pattern of wavelengths with a reference pattern. So, we can eliminate (A) as an answer.

Choice (C) is also a true statement. It is difficult to analyze nonmetallic elements using this method. While nonmetallic elements do have their own unique emission spectra or patterns of wavelengths emitted, their electrons aren’t as easily excited as the electrons of a metallic element. So, their emission spectra aren’t visible with this method. In addition, nonmetals also often emit wavelengths outside the visible spectrum. As a result of these two reasons, if we burn a salt containing a metal and a nonmetal, only the metal’s emission spectrum will be visible. So, we can eliminate (C) as an answer as well.

The last true statement is (E) both the measurement and the data analysis can be automated. There are indeed computers and machines that can carry out this task. Since the process requires sensing quantifiable wavelengths and comparing them to an index of known emission spectra, this is not a particularly difficult task for a well-designed computer to complete.

The remaining choice, (B) every element emits a single wavelength of light, is a false statement and the correct answer. If we look through a spectroscope at a burning metal ion, we will notice multiple bands of multiple colors representing multiple wavelengths. Multiple wavelengths are present because different electrons in the sample are moving down from various energy levels in the atom, releasing photons of different energy levels with different wavelengths. Even hydrogen, the element with just one electron in its neutral state, has four distinct bands on its emission spectrum. Elements release multiple wavelengths of light when burned. So, the correct answer and false statement is (B) every element emits a single wavelength of light.

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