Video Transcript
When an alkali metal is dissolved in hydrochloric acid and analyzed using a flame test, an intense orange flame is observed. Why might more information be needed to confirm the identity of the metal in the sample? (A) The orange color is difficult to differentiate from the color of the Bunsen flame. (B) The orange color of the flame is produced by chloride irons. (C) The alkali metal producing the orange flame may be present as a contaminant. (D) The orange color of the flame is produced by the solvent. (E) More than one alkali metal produces an orange flame in a flame test.
This question concerns the behavior of alkali metals during a simple lab test known as a flame test. The alkali metals are found in group one of the periodic table, and those commonly encountered are lithium, sodium, potassium, rubidium, and cesium. During a flame test, a small sample of a metal or metal compound is dissolved into some hydrochloric acid. All of the alkali metals will react with hydrochloric acid. In fact, alkali metals and alkali metal salts will react with hydrochloric acid, and they’ll be converted into metal chloride salts. The purpose of converting a sample into a metal chloride salt solution is that metal chloride salts are easily vaporized in the hot Bunsen burner flame during a flame test.
The next part of the flame test involves dipping a clean platinum or nichrome wire loop into the metal chloride sample solution. The wire loop is then introduced into a hot or nonluminous Bunsen burner flame. If the sample produces a distinct color in the flame, it’s recorded. It’s important to use a nonluminous Bunsen flame because it’s got a very pale color to start with. The nonluminous flame is produced by having the air supply hole on the Bunsen burner fully open. A very-pale-blue flame is produced.
In our question here, we can see that an intense orange flame has been observed in this flame test. Statement (A) suggests that the orange color is difficult to differentiate from the color of the Bunsen flame. In this context, the word “differentiate” means to distinguish from or to tell apart from. Since the nonluminous Bunsen flame is so pale, it’s actually very easy to see the color produced by the metal salt in the flame. Response (A) is therefore incorrect.
The reason for doing a flame test is that it can be used to identify a metal from its emission spectrum. In this context, the color of the flame produced can be used to identify the metal. It’s described as a qualitative test as it doesn’t tell us how much metal is present. It just suggests the possible identity of the metal. All of the alkali metals in group one give characteristic flame colors in a flame test. Lithium produces a red to crimson flame. Sodium produces an intense orange to yellow or golden-yellow flame. Potassium produces a lilac or pink flame, whilst rubidium is seen as a red to violet flame. And cesium compounds produce a blue to violet flame. We can see that all the alkali metals produce distinctly different flame colors. Since we see an intense orange flame in our question here, it’s likely it’s derived from a sodium compound.
Statement (E) suggests that more than one alkali metal produces an orange flame in a flame test. This is clearly an incorrect statement as the only alkali metal that produces an intense orange flame is sodium. The orange color that is observed here is produced when electrons in low-energy orbitals or ground-state orbitals are excited by the hot Bunsen flame. The electrons absorb energy from the flame, and they’re promoted to higher energy orbitals. When the electrons fall back to lower energy orbitals, energy may be released as visible light if it falls within the visible part of the electromagnetic spectrum.
Statement (B) suggests the orange color of the flame is produced by chloride ions, and statement (D) suggests that the orange color of the flame is produced by the solvent. The orange color is produced by sodium atoms in the flame. It’s caused by excited electrons in the sodium atoms falling back to lower energy levels and releasing orange visible light. No visible light is emitted or seen when excited electrons in chloride ions fall back to their ground state. The chloride ions in the metal chloride that was prepared from the sample in this question will not provide any color to the flame at all. Statement (B) is not the correct answer.
Statement (D) suggests the orange color is produced by the solvent. This is the substance used to dissolve the sample. The solvent is in fact water here, and water contains no excitable electrons that produce a flame color at all. Statement (D) is not a correct answer either.
Statement (C) suggests that the alkali metal producing the orange flame may be present as a contaminant. We’ve seen that sodium is the alkali metal responsible for the orange flame. However, if it’s present as a contaminant, there will be other metals present. It’s just that sodium is there as well. Even at very low concentrations, sodium atoms produce a highly intense orange-yellow flame in a flame test. This can easily mask or obscure any other colors produced by other atoms that may also be present. Because of the masking ability of sodium atoms in a flame test, if an orange flame is observed, it might be necessary to run other tests to confirm if other metals might be present. Statement (C) is therefore the correct answer.
So, to answer the question “When an alkali metal is dissolved in hydrochloric acid and analyzed using a flame test, an intense orange flame is observed. Why might more information be needed to confirm the identity of the metal in the sample?,” the alkali metal producing the orange flame may be present as a contaminant is the correct answer.
