Fire is something that captures the imagination. Some believe that fire is matter, since it occupies space and contains mass, but, technically, fire itself is not the matter that we see. Fire is the result of exothermic chemical reactions, or those that give off heat. On Earth, combustion reactions, exothermic chemical reactions in which an element such as oxygen is involved, causes materials to burn. The heat energy created from this burning is absorbed by surrounding gases, which, in an effort to reduce energy, begin to glow and give off light. This light, which follows the wispy and fast-moving gases, is what we call a flame.
But let’s deviate for a second and explore the combustion that makes fire possible. Fire requires three things: a fuel source, heat and an oxidizer. Oxidizers, like oxygen, are characterized by their high energy levels. Since the objective of atoms is to reduce their energy level, these oxidizers are readily reactive, and will form bonds without much, if any, additional energy input into the reaction (DAC 2). The substances that oxidizers bond with can be anything that will give up their electrons. However, an oxidizer and an electron-donating substance by themselves can’t just start a fire. If that were the case, the Earth would actually be on fire.
It needs a spark; something needs to accelerate the combustion. That something is heat. Although oxidizers do not need anything to bond, adding energy to the system increases the speed at which the molecules involved gives off energy. Remember, if heat energy is given off, it’s an exothermic chemical reaction. That initial energy that kicks off chemical reactions is referred to as activation energy. When this external energy is added to a system where an oxidizer exists, the molecules or atoms that are giving up their electrons become fuel sources with which the oxidizer reacts, generating heat with each reaction. If the surroundings become hot enough, in an instant, a flame appears.
Fire is just a beautiful display of glowing gas. When I was thinking of this, I had a question. Why is “water” the first answer given when someone asks, “how does one put out a fire?” After all, it is composed of hydrogen, which is extremely flammable, making it a fuel source, and oxygen, an oxidizer. Why isn’t combustion made worse by adding water? In order to figure out if it is or isn’t, we have to look at the interaction between water, which exists as a liquid in natural conditions, and fire.
Generally, when liquid water contacts fire, it absorbs the heat energy emitted by the combustion reaction, and turns into vapor, which is heavier than air. The heavier vapor sinks below the air, separating the fuel from the oxygen. Furthermore, without heat, the flame cannot sustain itself and extinguishes.
But this only happens in some cases. After all, the fuel sources can vary. As such, here’s something that many people don’t know: fire is defined by classes according to its fuel source. The class of fire demonstrated above, which can be extinguished by water, is what occurs when the fuel source is a combustible solid, such as wood, paper, coal or trash. This class of fire is designated Class A.
The next class of fires — Class B — are created when burning flammable gases or liquids, such as gasoline, oil, and propane. So, what happens when you throw water on a Class B fire? Water can’t smother these fires for a few reasons.
First, the density of water is greater than that of most Class B fuel sources, meaning when it’s thrown onto the fuel source, it will sink below it. If it’s not on top of the source, then it can’t prevent the heat energy from interacting with the environment.
Secondly, and perhaps more importantly, liquids and gases tend to have lower thermal conductivity than solids. In other words, the amount of time that it takes a container of a gas or liquid to transfer heat is far longer than the amount of time that it would take a solid to. This is due to the structure of the atoms in each state; the more tightly packed the atoms, the easier heat can be transferred.
Why does that matter? Because the amount of heat that it takes liquids and gases to actually catch fire is larger compared to ordinary combustible solids. For example, your common gas stove produces a flame (after the gas is stoked by an electric ignition switch or a pilot light — both forms of activation energy) that can reach up to 1500 degrees Celsius. These fires are so hot that the surrounding vapors give off a blue color, indicating that the atoms are vibrating at a very fast frequency. Surrounding that blue flame would be yellow and red flames, which are progressively cooler with a lower frequency. In other words, liquid and gas fires are far hotter than solid fires.
What happens when liquid water contacts something of that temperature? The liquid water transforms into superheated steam, rather than ordinary water vapor. The difference between the two is that water vapor is far closer to the liquid phase than superheated steam is. And you’ve probably seen that difference for yourself in the form of fog over a lake. The fog, which is water vapor, is a testament to the transition between the liquid layer and the gas above it. As such, water vapor contains visible water droplets, which can condense into liquid water if it’s cold enough. Superheated steam has no visible water droplets within it. As such, if the steam was cooled, there would be no condensation. Since steam is far more of a pure gas than water vapor is (even though they are both gaseous water), it expands far faster. But this vigorous form of water can actually splatter the fuel source around, carrying the still-reacting fuel and the fire with it. This, without fail, causes Class B fires to spread, making liquid water a counterproductive option. To fight these, you would want to smother the gas with something light, like a foam.
It’s just as fruitless to use water the farther up in class you go. Class C fires are electrical fires, caused by electrical equipment like motors or computers. Water doesn’t work on these because it’s conductive to electricity — it would actually do nothing to quell the fuel source. Fortunately, this is solved by cutting the power, leading to a lower class fire, if any fire, to deal with. Class D fires are those that are the result of a combustible metal like magnesium or sodium, which burn even hotter than flammable gases or liquids, clocking in at anywhere around 2000 degrees. They input so much energy into the water molecule that the individual atoms that compose vibrate differently and eventually splits from each other into individual hydrogen and oxygen. You can probably reason what happens after that.
Make sure you check your fire extinguishers to ensure that it is the appropriate class according to the possible fuel sources nearby. And give a word of thanks to the fire fighters when you can.