Flame Test: Cation or Anion?

Most recent answer: 12/15/2015

When a compound is emitted through heat how can you tell if it is a non-metal or metal ion that is responsible for the color change?
- Litzy (age 15)
garland, tx, usa

The origin of colors is the electronic transitions between discretized energy levels of atoms, which is explained here: http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Elements_Organized_by_Block/1_s-Block_Elements/Group__1%3A_The_Alkali_Metals/2Reactions_of_the_Group_1_Elements/Flame_Tests. If you use a pure metal powder and observe a color spectrum, it is clearly due to the metal atoms as there is nothing else.

The question that you are asking comes into play when you use a metallic salt, which upon heating disassociates into metal cations and nonmetalic anion atoms or groups. To be observable, the species of interest needs to have a dominant emission in the visible wavelength range, which is the case for many metals. Most nonmetals tend not to emit in the visible spectrum, or their excitation efficiency is very low. So, color that you observe comes from the metal cations most of the time. You can convince yourself by conducting a simple experiment: simply switch the ion pairs and contrast the outcomes. An easy description of the protocol is here: http://pubs.acs.org/doi/pdf/10.1021/ed081p1776A.


(published on 12/15/2015)