People have been coloring flames pretty much since the days of Prometheus. Even before chemistry was recognized as a science, colored fire was a useful chemistry demonstration. A person might wonder why a fire made with driftwood would burn in a different color from an ordinary wood fire. Why would adding minerals or salts to a fire change its flames?
Colored fire raises questions and piques scientific inquiry. As a modern chemistry demonstration, colored flames are used to evoke wonder in students and also to illustrate the emission spectra of metal salts (the flame test). Yet, the demonstration has come under fire because… well… fire is fire. It can burn you. It can spread. Like any lab hazard, fire needs to be treated with respect and handled using proper safety precautions. Numerous accidents have occurred with the colored fire demonstration. Yet, other instructors have performed the demonstration hundreds or possibly thousands of times without incident. Should the demonstration be banned because it’s potentially dangerous? Instead, why don’t we keep it in the repertoire, but perform it more safely. It’s really pretty simple. Don’t use liquid fuel. Don’t use toxic chemicals.
Here is how I make colored flames. If you’re not a chemist or you just want to minimize risk while performing the demonstration, give it a try and see if it feels safer to you. In addition to being safer than the tradition demo, you don’t need any exotic chemicals to produce the colors.
- hand sanitizer (around 65% alcohol)
- red – strontium salt (take from the inside of an emergency flare)
- orange – mix red and tiny amount of yellow chemicals
- yellow – table salt – sodium chloride
- green – boric acid or borax or copper sulfate
- blue – natural color of the flame
- violet – salt substitute – potassium chloride
Set Up the Demo
- I went low-tech here, setting a potholder on my kitchen counter, with a cookie sheet for the demonstration. You can use any fire-safe surface. It’s nice to use a container with a lip because the liquid (water) will run and can make a mess.
- Sprinkle a small amount of each chemical in a line running across your dish. You can mix the colors, but be aware some emission spectra are much stronger than others. For example, any contamination of the other colors with the sodium from table salt will turn your flame mostly yellow, regardless of the other salts that are present.
- When you are ready to perform the demonstration, pump a layer of hand sanitizer across the salts.
- Dim the lights.
- Use a long match or long handled lighter to ignite one end of the gel. The flame will spread across, displaying all the colors.
- Now, the nice thing about hand sanitizer is it’s mostly water. The flame goes out as soon as enough alcohol burns off to make the gel watery enough to extinguish the fire. It doesn’t take long.
- If you want to re-start the demonstration, wait until the flame is out. This is the key mistake people make with the traditional colored fire project, too. Don’t add fuel to a burning fire and expect good things to happen. Once the flame is out, you can apply more alcohol gel and light it. You do not need to add more salts.
Do you have any additional tips or advice for people wishing to perform this demonstration? Feel free to comment.
Here’s the video made when the photograph was taken. Easy. Safe.