Public Lab Research note

Flame testing metal salts

by kgradow1 | | 4,789 views | 2 comments |

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What I want to do

This demo was conducted as part of a fire science class at Parts and Crafts with elementary and middle school-aged students. The goal was three-fold:

  1. Use the spectrometer to illustrate the difference between blackbody radiation and emissions spectrum (the lightbulb demo under "Things to Do with your Spectrometer")

  2. Capture emissions spectra of different metal salts

  3. Do both of these things kitchen-counter-style, in the absence of any formal lab setup, in a way that passes my "Yep, I'll do this with kids!" threshold. This is my first time running a fire science class so navigating this threshold is an ongoing consideration.

Setup and materials

PLOTS spectrometer: We used an earlier version and didn't modify the arm at all. In future, we'll set up a better rig for more flexibility.


Butane torch: We used this version but any hand-held torch would do. Must produce a steady blue flame and be small enough to easily hold.


Metal salts: All chemicals were purchased from the Science Company. Preparation involved dipping popsicle sticks in water and then into their respective salts. We tested the following chemicals:

Wires for testing: Copper, iron, zinc, and aluminum

My attempt and results

Flame tests

We tested strontium, copper, sodium, and potassium.





Good to know you can do this method and not burn the house down. Popsicle sticks + torch is quick and dirty but works in a pinch, especially if you don't have a bunsen burner handy.

copper.png Copper chloride

potassium_chloride.png Potassium chloride

strontium.png Strontium chloride

PA190044.jpg Sodium chloride (we got a great emission line for sodium but weren't on the ball capturing the spectrum)

Blackbody observations

Following the flame tests we did a very quick demo of blackbody radiation, comparing iron wire, copper wire, zinc wire, and aluminum foil according to a temperature gradient.

unnamed_(1).jpg Zinc wire

2015-10-19_16_20_43-HEAT-COLORS-for-Blacksmiths.png (from

The goal was to identify which heat transfer (which wire heats faster) vs. temperature (which ultimately gets hotter) based on the color of the wire.

Questions and next steps

Swan bands Why are blue flames blue? (great explanation here) Given that "playing with matches" (and candles) is the most common interaction kids this age are going to have with fire, it seems useful to understand a little bit about what's going on here. Unfortunately the butane torch itself was not bright enough to get an emission spectrum for the free carbon.

swan_bands.png This is what they're supposed to look like. Sadly, none spotted.

Blackbody radiation When we heated the zinc, it sparked bright blue, which matches its emission spectrum. This was a) AWESOME and b) completely unexpected (sorry no photos). Is this actually the emission spectrum that we're seeing?

We did have a pretty great comparison for copper and iron.
unnamed_(1).jpg We also saw this distinctive orange color when we burned the iron wire. Emission line?

Capturing blackbody on the spectrometer

Lightbulb test: When you raise the temperature of an incandescent bulb, it looks like the spectrum shifts toward the blue as you "turn up" the lightbulb. This corresponds to what you would expect, right? (no pics, sorry)

Capturing blackbody radiation: We used popsicle sticks to test the metal salts --- which meant that inevitably, five or six seconds in, the popsicle stick would catch fire and start burning independently. This meant that we had a number of samples where we captured both the emission line and the full spectrum of the incandescent flame, which was a cool and unexpected result.

Why I'm interested

Explaining the relationship between heat and light in exothermic reactions.

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Thank you for this incredible documentation! Do you have a list of what you purchased from the Science Company? I bet other people would like to replicate this demonstration!

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