by Lee Falin, PhD
One of my favorite TV shows is Survivor Man. If you haven’t seen it, the host, Les Stroud, puts himself into various dangerous situations and then films himself as he shows how to survive those situations. My favorite parts of these shows are watching all the different ways you can start a fire.
Fire has been around since the beginning of time. How long humans have been able to control and use fire is a matter of some debate, but most people believe that it started around a million or so years ago.
Once man had the ability to use fire, he could cook food, better control agriculture, make pottery, forge tools out of metal, and send rockets to space. All thanks to fire.
But what exactly is fire? What makes something burn and why do some fires have different colors?
We Didn’t Start the Fire
If you’re into technical definitions, you could say that fire is the rapid oxidation of some combustible material where various heat, light, radiation, and other materials are given off as byproducts. But where’s the fun in that? Let’s dissect that definition bit by bit in order to get a clearer understanding.
First, oxidation is a type of chemical reaction where electrons are taken from something by some type of oxidizing agent. Usually this oxidizing agent is oxygen itself. You may remember that in an earlier episode called The Basics of pH, I referred to oxygen as a big bully. Its strong electronegativity allows it to easily take electrons from other substance, a process called oxidation.
Oxidation can happen slowly, like when an iron nail slowly rusts, or quickly, like when a piece of paper catches fire. For a fire, we need rapid oxidation.
In order to begin rapid oxidation, some type of heat is usually needed. This can come in some mundane form such as a match or lighter, or something more dramatic such as a bolt of lightning. However, some oxidizers, such as perchloric acid, need almost no heat to start this reaction, and are therefore extremely dangerous to work with.
An Eternal Flame
Once the rapid oxidation has started, energy is released from the reaction in the form of heat, light, and other radiation. This is what normal people would call a flame. The shape, size, color, and makeup of a flame depends upon many complicated factors. For the most part, flame color can be thought of as an indication of flame temperature and fuel composition. For example, red flames are cooler in temperature than blue and white flames.
A fun experiment you can do at home is called a “flame test." A flame test is used to identify the composition of unknown metals. When metal ions get hot, they start to emit light of a predictable color called an emission spectrum. For example, copper makes a blue-green flame whereas calcium is usually dark-red.
An easy way to perform a flame test is to coat a small piece of wire with some metal salts. The most common metal salt in normal people’s homes is Sodium Chloride, or table salt. Hold the coated metal loop over a flame (the flame from a propane torch works best), and you should see it turn bright yellow, indicating that sodium is present. If you have other metal salts, such as calcium chloride or copper chloride, you can see other colors as well.
You may have heard that burning driftwood causes blue and purple flames; this is because of the metal salts absorbed by the wood. As a precaution however, driftwood fires give of large amounts of dioxin, which are carcinogenic. So be careful around driftwood fires. Don’t try to start one up in your kitchen.
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Fire image courtesy of Shutterstock