Fire, a phenomenon observed for millennia, often causes confusion regarding its classification within the familiar states of matter—solid, liquid, or gas. Understanding fire requires delving into the basic forms matter can take and the processes that create the vibrant flames we see.
Basic States of Matter
Matter exists in three primary states: solid, liquid, or gas. In a solid, particles are tightly packed and vibrate in fixed positions, giving solids a definite shape and volume. Liquids have particles that are more loosely arranged, allowing them to flow and take the shape of their container while maintaining a definite volume. Gases feature particles that are widely dispersed and move freely, expanding to fill any container and lacking a definite shape or volume.
Plasma
Plasma is a fourth state of matter, forming when a gas is heated to such extreme temperatures that its atoms become ionized. This means electrons are stripped from their nuclei, creating a superheated mixture of free electrons and positively charged ions.
What Is Fire?
Fire is the visible manifestation of a rapid chemical process called combustion. This process involves the rapid reaction of a fuel with an oxidizer, typically oxygen from the air, releasing heat and light. For combustion to occur, three components must be present: fuel, oxygen, and heat, often referred to as the “fire triangle.” The heat raises the fuel to its ignition temperature, causing it to release gases. Only these released gases can react with oxygen to sustain the burning process, maintaining the reaction.
Why Fire Is Plasma
The hottest parts of a flame consist of matter in the plasma state. When combustion gases reach extremely high temperatures, the energy breaks apart gas molecules. Atoms within these gases lose their electrons, becoming positively charged ions, with freed electrons moving independently. This electrically charged, ionized gas is plasma, distinguishing it from a neutral gas. While not all parts of a flame are pure plasma, especially cooler regions, the highly energetic core exhibits plasma properties due to ionization.
What You See in a Flame
Flame light and colors result from several factors. Much of the light comes from incandescent soot particles, tiny carbon particles produced by incomplete combustion that glow intensely from extreme heat. These particles emit light across a spectrum, contributing to red, orange, and yellow hues. The color of a flame also depends on the type of fuel burning and the amount of oxygen available. Hotter flames with more complete combustion, often at the base, appear blue due to emission from excited molecules rather than soot.