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4.1: Liquid Fuels

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    Specific Gravity

    Specific gravity refers to the ratio of the mass of a given volume of a substance to the mass of an equal volume of water.1 Similar to air when discussing vapor density, water is given a value of 1, therefore, liquids that have a specific gravity of less than one will float on water and liquids that have specific gravity of greater than 1 will sink. This is often observed when you see petroleum products like gas or oil in water, often seen in a rainbow like formation floating on top. This also is visible in some salad dressing like Italian dressing where the primary components of vinegar and oil will separate into layers when stored.

    A vivid iridescent oil spill on rough asphalt surface
    Figure \(\PageIndex{1}\): Oil spill.
    Copyright: "A vivid iridescent oil spill on rough asphalt surface" by Jared Brotman, CC0.

    Boiling Point

    Boiling is a type of vaporization (change of state from liquid to gas) that occurs when the average kinetic energy of the molecules in the liquid becomes large enough for molecules in the liquid separate from each other and convert into the gaseous form. The buoyant force or kinetic energy of the gas bubbles will cause them to rise up and out of the liquid. Boiling happens when these buoyant gasses overcome atmospheric pressure. Boiling will take place at different temperatures with different atmospheric pressures.

    As the liquid heats up, the energy of the bubbles increases until a certain point where the bubbles overcome atmospheric pressure. This is known as the boiling point.2 Water has a boiling point of 212 °F (100°C) however, liquid fuels can have varied boiling points. Many flammable liquids can be above their boiling points at normal environmental temperatures.

    Liquid Fuel Fires

    Ignitable liquids, or liquids that can support combustion, will not do so in the liquid state. These liquids must first be converted to vapor. This conversion requires energy, which can come from ambient temperature, an ignition source, or from the heat generated from the fire itself, known as a heat feedback loop. Gasoline, which can release vapors at temperatures as low as 0°F needs only the temperature of the environment to have sufficient energy to convert to a gas. However, diesel fuel, which does not begin to release sufficient amounts of vapor until around 140° is hard to ignite even with a lit match.

    Flash Point

    The lowest temperature of a liquid, as determined by specific laboratory tests at which the liquid gives off vapors at a sufficient rate to support a momentary flame across the surface is called the flash point.3 The flash point is exactly that, a flash of flame.

    The Flash Point is the primary physical characteristic utilized by NFPA 30 to classify ignitable liquids.  These liquids are primarily broken into two categories, Flammable and Combustible. However, these liquids are further broken down by NFPA 30 into three classes based on flash point and boiling point, with class I considered flammable, and class II & III considered to be combustible liquids.

    • A flammable liquid has a flash point that is below 100° F and a maximum vapor pressure of 40 PSI.
    • A combustible liquid has a flash point at or above 100°F.
    Class Flash Point Boiling Point
    I A Below 73° F Below 100° F
    I B Below 73° F Above 100° F
    I C At or above 73° F and below 100° F  
    II At or above 100° F and below 140° F  
    III A At or above 140° F and below 200° F  
    III B At or above 200° F  

    Fire Point

    The fire point of a liquid is the temperature at which a liquid gives off sufficient vapors to sustain combustion. The fire point of a liquid is typically about 10° to 50° F above the flash point. It is important to keep in mind that a fuel needs not only the energy to heat the fuel to its boiling point, but then will take an additional amount of energy to convert the fuel into a gas. The energy required to make this final conversion to a gas is referred to as the latent heat of vaporization.

    Latent Heat

    Latent heat of vaporization describes the amount of heat required to convert one gram of a substance from a liquid form to a gaseous form.4 Keep in mind that this conversion takes place without an increase in temperature.

    The Latent Heat of Vaporization of Water is 970 BTU or 570 Calories.

    A BTU (British Thermal Unit) is a measurement of the amount of energy required to raise a pound of water 1°F. (approximately 1055 Joules)

    If we put a pint of water or 2 cups, (which is approximately a pound of water) on the stove to boil, and assume it comes out of the tap at 65°F, it would require 147 BTU’s to heat the water to it's boiling point and then another 970 BTU’s to convert it to steam.

    Fuels have varying boiling points and latent heat of vaporization factors; however, they all require energy to become gases.

    Ignition

    Autoignition temperature refers to the lowest temperature at which a combustible material ignites in air without a spark or flame.5 As with Latent Heat of vaporization, different fuels have varied auto ignition temperatures which can vary widely. Below is a list of common liquid fuels an their autoignition temperatures.

    • Gasoline - 477°F
    • Diesel fuel - 422°F
    • Kerosene - 428°F
    • Ethanol - 685°F

    Spontaneous ignition refers to the initiation of combustion of a material by an internal chemical or biological reaction that has produced sufficient heat to ignite the material. If you had a shop class in school, you may have had a specific container for oily rags. This is because these rags can undergo a biological reaction, creating enough heat to ignite spontaneously. Another example of spontaneous ignition is a hay fire, which are caused by the same process. These organic compounds essentially decompose and generate enough heat to ignite the surrounding material. This process can take a few hours or several months to generate the heat needed to ignite.

    Aerosols

    Aerosols are fine particles that are suspended in the air. These particles can be either liquid or solid particles. Aerosols present a more significant hazard because the extremely fine particles essentially absorb energy more quickly and vaporize and ignite more easily. Aerosol particles will propagate rapidly once just a few particles ignite. This same concept is responsible for dust explosions, which will be examined in Unit 5 – Solid Fuel Combustion.

    Wicking

    Also referred to as capillary action, wicking refers to the movement of a liquid within another material caused by the liquid's cohesion and adhesion properties. Wicking can be observed in a towel soaking up water. Wicking can increase the surface of a fuel and reduce the mass of the fuel, which can allow it to vaporize and ignite at lower temperatures. This is why the wax on a candle burns around the wick, but does not burn on the surface of the liquid below the wick.

    A lit pink candle with a steady flame against a dark background.
    Figure \(\PageIndex{2}\): Candle flame.
    Copyright: "Candle flame closeup animation.gif" by Jahobr, licensed CC0.

    Tank Fires

    Tank fires, or fires in liquid fuel storage tanks like the one pictured below, can present unique challenges to firefighters and must be monitored closely as firefighters work to extinguish the fire to prevent extremely dangerous situations.

    Figure \(\PageIndex{3}\): Oil tank fire.
    Copyright: "Gibraltar Explosion" by Josh13770 is licensed under CC BY 2.0.

    When water and foam are used to fight tank fires, the water, having a specific gravity greater than the fuel, will sink below the fuel. As the temperature of the fuel increases, it can heat the water to its boiling point. As the water vapor forms bubbles and rises to the surface, it creates thin layers that ignite as the bubbles break the surface. This is known as frothing. As the fuel continues to heat up, the amount of water vaporizing will increase and can result in burning fuel spilling over the top of the tank, slopover, or an almost explosive reaction known as boilover, where the sudden expansion can push very large amounts of burning fuel out of the tank.

    Boilover

    The wordless video depicts boilover.

    Video: "BoilOver From API 2001" by Bakr Mammar. Standard YouTube License.

    Miscibility

    Miscibility and solubility are very similar concepts, but we will focus on miscibility for this course.

    Miscibility can be defined as the ability of a substance (liquid) to form a homogeneous solution in another substance, or completely dissolve into each other. For example, ethanol, which is found in many gasolines, is miscible in water.

    The miscibility of alcohol-based fuels presents challenges to firefighters trying to utilize water-based foam to separate the fuel from the oxygen. The alcohol combines with the water in the foam and essentially pulls the water out of the foam.

    Miscibility

    Watch the video to learn more about the combination of alcohol and water-based foam.

    Video: "Ethanol's Affect On Fire Foam" by James Cottrell. Standard YouTube License.

    Surface Tension

    Surface tension is an effect caused by an imbalance of forces on the atoms at the surface of a liquid.6 Surface tension can be observed when you spill water on your shirt and the water droplet remains on the surface rather than soak into the clothing. You may also have done the experiment where you float a paperclip on the surface of the water.

    paperclip floating on water
    Figure \(\PageIndex{1}\): Paperclip floating on water due to surface tension.
    Copyright: "Paperclip-floating-on-water" by Wbreslyn, licensed CC0. )

    References

    1. National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024

    2. Pasquale, A. J., Fazzini, D. R., & Bennett, C. (2024). Phase change. In Conceptual Physics. College of DuPage. https://cod.pressbooks.pub/physics1100/chapter/phase-change. Licensed CC BY-NC 4.0.

    3. National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024

    4. Pasquale, A. J., Fazzini, D. R., & Bennett, C. (2024). Phase change. In Conceptual Physics. College of DuPage. https://cod.pressbooks.pub/physics1100/chapter/phase-change. Licensed CC BY-NC 4.0.

    5. National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024

    6. Ball, D. W. (2011). 10.3 Properties of Liquids. In Beginning Chemistry. https://2012books.lardbucket.org/boo...f-liquids.html. Licensed CC BY-NC-SA 3.0.

    This page titled 4.1: Liquid Fuels was last modified on Sun, 13 Sep 2026 15:36:58 GMT and is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Robert Smith.

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