Video Transcript
Which s-block metal does not produce a colored flame in a flame test? (A) Calcium, (B) magnesium, (C) rubidium, (D) barium, and (E) cesium.
A flame test is a simple chemical test that can be performed in a lab to identify a metal in a metal salt. When a flame test is performed, a metal salt is introduced into a hot Bunsen burner flame. A small amount of the sample is introduced into the flame on a platinum or nichrome wire loop. The color produced by the compound in the flame is then recorded. Only a few metal atoms produce colored flames in this way. The color is produced when electrons in low-energy orbitals or ground-state orbitals in the metal atoms become excited when they’re introduced into the flame.
These electrons absorb energy from the flame, and they’re promoted to higher-energy orbitals. These higher-energy electrons can be described as being in an excited state. When these electrons fall back to their original ground state, energy can be released in the form of visible light. In this question, we’re being asked about s-block metals that produce colored flames in a flame test. The s-block metals are located on the left-hand side of the periodic table in group one and group two. Hydrogen is often shown as being located in group one. But remember that it’s not a metal. In s-block elements, we find that the highest energy orbitals that are occupied by electrons are s orbitals.
In our question, we can see that rubidium and cesium are metals that are located in group one of the periodic table. Calcium, magnesium, and barium are located in group two of the periodic table. Firstly, we’ll look at the group one metals in the question here to see what color their salts produce in separate flame tests. Rubidium salts produce a red-to-violet flame in a flame test, while cesium salts produce a blue-to-violet flame in a flame test. Since both of these metals produce colored flames, they’re not the correct answer. We’re being asked to identify a metal that does not produce a colored flame in a flame test.
Next, we’ll look at the group two metals in our list to see what color their salts produce in their respective flame tests. Calcium salts produce a characteristic brick-red flame, whilst barium salts produce a pale-green flame. Since both of these metals produce colored flames, we can eliminate them from our list of possible answers. When magnesium salts are introduced into a hot Bunsen burner flame, no obvious flame color is observed. Magnesium is the s-block metal from our list that does not produce a colored flame in a flame test. The introduction of magnesium salts into a hot Bunsen burner flame in a flame test is not to be confused with the burning of magnesium ribbon in a similar flame.
When magnesium metal is burnt in a hot Bunsen burner flame, a dazzling bright-white light is seen, and also white smoke is observed. This is, in fact, a very exothermic chemical reaction where magnesium metal reacts with oxygen in the air to produce magnesium oxide. The reaction must not be confused with a flame test involving magnesium salts. Magnesium salts don’t give colored flames in a flame test. The correct answer is magnesium.
