Final answer:
When a glass rod is heated and a yellow flame is produced, it indicates the presence of sodium or a sodium compound.
Step-by-step explanation:
When a glass rod is heated and a yellow flame is produced, it indicates the presence of sodium. Sodium compounds, when heated, emit a bright yellow color. This is due to the excitation of electrons in sodium atoms, which causes them to move to higher energy levels.
For example, when a wire dipped in a solution of a sodium salt is heated, it emits a bright yellow light characteristic of sodium. This is often used in flame tests to identify the presence of sodium in a compound.
So, the yellow flame produced when a glass rod is heated indicates the presence of sodium or a sodium compound.