Final answer:
A yellow flame observed when heating a glass rod indicates the presence of sodium ions due to the line emission spectrum characteristic of sodium, often a result of sodium compounds used in glass manufacture.
Step-by-step explanation:
When a glass rod is heated and a yellow flame is observed around the point of heating, this indicates the presence of sodium ions. This yellow flame is a result of the line emission spectra characteristic of sodium, which occurs at about 589 nm. In a process similar to a flame test, the heat from the flame excites the electrons of the sodium ions, and as the electrons return to their ground state, they emit energy in the form of yellow light.
The appearance of a yellow flame during the heating of a glass object suggests that the glass contains some amount of sodium impurities. Glass manufacturers often use sodium carbonate or sodium silicate in the glass-making process, which can lead to the presence of sodium in the final product. When heated, the sodium emits the yellow flame as a part of its line emission spectrum, a unique and identifiable property of each element.