When zinc oxide (ZnO) is added to burning methanol, we see blue emissions along with sparks and beautiful flashes of red and green. Why? One student had found it so incredulous that they thought our ZnO was contaminated with other substances. It’s unlikely, but you could check the hypothesis by using another source of zinc oxide to compare the results. Years ago, I originally didn’t have the answer to the question, and it encouraged another student, Veronica Chudzinski, to untangle the mystery!
Of course we were already aware that electrons get promoted to different energy levels by the flame’s heat, and then depending on which level they are falling back from, we get different colors. We were also aware that the blue is from methanol’s emissions. After some research, Veronica learned that through electron-emissions, ZnO can produce two distinctive colours, red and green. But why two levels and an ensuing pair of colors? The ZnO produces both colours because it is responding to different temperatures within the flame. To be more precise:
- ZnO leads to red emissions between 568 to 704 °C
- ZnO produces green between 704 to 948 °C
A methanol flame’s maximum temperature is 1152 Kelvin, which is about 880 degrees Celsius so this is consistent with the idea that both colours were produced by the ZnO.
Why the beautiful sparks?
The sparks observed result from ZnO particles that have fallen into the solution; then, as they were lifted with the flame, the methanol evaporated off them and the remaining dust particles produced the linear bursts of light through incandescence.
A word of caution. When using even a 50% solution of methanol in emission demonstrations and experiments, the high temperature of the burning methanol can easily break Pyrex glass. As a crucial precaution, use sand at the bottom of the beaker, which will make the glass more resistant to extreme heat. And do not have students sitting or standing any closer than about 10 to 12 feet from the flame. Equip them with goggles.
If temperatures quoted seem high, they are in fact plausible. See: Flame Temperatures