NFPA 921 DEFINITIONS STUDY 2025/2026 QUESTIONS
WITH ANSWERS GRADED A+
✔✔Ceiling Layer. - ✔✔A buoyant layer of hot gases and smoke produced by a fire in a
compartment.
✔✔Char. - ✔✔Carbonaceous material that has been burned or pyrolyzed and has a
blackened appearance.
✔✔Char Blisters. - ✔✔Convex segments of carbonized material separated by cracks or
crevasses that form on the surface of char, forming on materials such as wood as the
result of pyrolysis or burning.
✔✔Clean Burn. - ✔✔A distinct and visible fire effect generally apparent on
noncombustible surfaces after combustible layer(s) (such as soot, paint, and paper)
have been burned away. The effect may also appear where soot has failed to be
deposited because of high surface temperatures.
✔✔Combustible. - ✔✔Capable of undergoing combustion.
✔✔Combustible Gas Indicator. - ✔✔An instrument that samples air and indicates
whether there are ignitible vapors or gases present.
✔✔Combustible Liquid. - ✔✔Any liquid that has a closed-cup flash point at or above
37.8°C (100°F). (See also 3.3.79, Flammable Liquid.)
✔✔Combustion. - ✔✔A chemical process of oxidation that occurs at a rate fast enough
to produce heat and usually light in the form of either a glow or flame.
✔✔Combustion Products. - ✔✔The heat, gases, volatilized liquids and solids,
particulate matter, and ash generated by combustion.
✔✔Competent Ignition Source. - ✔✔An ignition source that has sufficient energy and is
capable of transferring that energy to the fuel long enough to raise the fuel to its ignition
temperature. (See 19.4.2.)
✔✔Conduction. - ✔✔Heat transfer to another body or within a body by direct contact.
✔✔Convection. - ✔✔Heat transfer by circulation within a medium such as a gas or a
liquid.
✔✔Creep. - ✔✔The tendency of a material to move or deform permanently to relieve
stresses.
, ✔✔Current. - ✔✔A flow of electric charge.
✔✔Deductive Reasoning. - ✔✔The process by which conclusions are drawn by logical
inference from given premises.
✔✔Deflagration. - ✔✔Propagation of a combustion zone at a velocity that is less than
the speed of sound in the unreacted medium. [68, 2013]
✔✔Density. - ✔✔The mass of a substance per unit volume, usually specified at
standard temperature and pressure. The density of water is approximately one gram per
cubic centimeter. The density of air is approximately 1.275 grams per cubic meter.
✔✔Detection. - ✔✔(1) Sensing the existence of a fire, especially by a detector from one
or more products of the fire, such as smoke, heat, infrared radiation, and the like. (2)
The act or process of discovering and locating a fire.
✔✔Detonation. - ✔✔Propagation of a combustion zone at a velocity greater than the
speed of sound in the unreacted medium. [68, 2013]
✔✔Diffuse Fuel. - ✔✔A gas, vapor, dust, particulate, aerosol, mist, fog, or hybrid
mixture of these, suspended in the atmosphere, which is capable of being ignited and
propagating a flame front.
✔✔Diffusion Flame. - ✔✔A flame in which fuel and air mix or diffuse together at the
region of combustion.
✔✔Drop Down. - ✔✔The spread of fire by the dropping or falling of burning materials.
Synonymous with "fall down."
✔✔Effective Fire Temperatures. - ✔✔Temperatures reached in fires that produce
physical effects that can be related to specific temperature ranges.
✔✔Electric Spark. - ✔✔A small, incandescent particle created by some arcs.
✔✔Empirical Data. - ✔✔Factual data that is based on actual measurement, observation
or direct sensory experience rather than on theory.
✔✔Entrainment. - ✔✔The process of air or gases being drawn into a fire, plume, or jet.
✔✔Explosion. - ✔✔The sudden conversion of potential energy (chemical or mechanical)
into kinetic energy with the production and release of gases under pressure, or the
release of gas under pressure. These high-pressure gases then do mechanical work
such as moving, changing, or shattering nearby materials.
WITH ANSWERS GRADED A+
✔✔Ceiling Layer. - ✔✔A buoyant layer of hot gases and smoke produced by a fire in a
compartment.
✔✔Char. - ✔✔Carbonaceous material that has been burned or pyrolyzed and has a
blackened appearance.
✔✔Char Blisters. - ✔✔Convex segments of carbonized material separated by cracks or
crevasses that form on the surface of char, forming on materials such as wood as the
result of pyrolysis or burning.
✔✔Clean Burn. - ✔✔A distinct and visible fire effect generally apparent on
noncombustible surfaces after combustible layer(s) (such as soot, paint, and paper)
have been burned away. The effect may also appear where soot has failed to be
deposited because of high surface temperatures.
✔✔Combustible. - ✔✔Capable of undergoing combustion.
✔✔Combustible Gas Indicator. - ✔✔An instrument that samples air and indicates
whether there are ignitible vapors or gases present.
✔✔Combustible Liquid. - ✔✔Any liquid that has a closed-cup flash point at or above
37.8°C (100°F). (See also 3.3.79, Flammable Liquid.)
✔✔Combustion. - ✔✔A chemical process of oxidation that occurs at a rate fast enough
to produce heat and usually light in the form of either a glow or flame.
✔✔Combustion Products. - ✔✔The heat, gases, volatilized liquids and solids,
particulate matter, and ash generated by combustion.
✔✔Competent Ignition Source. - ✔✔An ignition source that has sufficient energy and is
capable of transferring that energy to the fuel long enough to raise the fuel to its ignition
temperature. (See 19.4.2.)
✔✔Conduction. - ✔✔Heat transfer to another body or within a body by direct contact.
✔✔Convection. - ✔✔Heat transfer by circulation within a medium such as a gas or a
liquid.
✔✔Creep. - ✔✔The tendency of a material to move or deform permanently to relieve
stresses.
, ✔✔Current. - ✔✔A flow of electric charge.
✔✔Deductive Reasoning. - ✔✔The process by which conclusions are drawn by logical
inference from given premises.
✔✔Deflagration. - ✔✔Propagation of a combustion zone at a velocity that is less than
the speed of sound in the unreacted medium. [68, 2013]
✔✔Density. - ✔✔The mass of a substance per unit volume, usually specified at
standard temperature and pressure. The density of water is approximately one gram per
cubic centimeter. The density of air is approximately 1.275 grams per cubic meter.
✔✔Detection. - ✔✔(1) Sensing the existence of a fire, especially by a detector from one
or more products of the fire, such as smoke, heat, infrared radiation, and the like. (2)
The act or process of discovering and locating a fire.
✔✔Detonation. - ✔✔Propagation of a combustion zone at a velocity greater than the
speed of sound in the unreacted medium. [68, 2013]
✔✔Diffuse Fuel. - ✔✔A gas, vapor, dust, particulate, aerosol, mist, fog, or hybrid
mixture of these, suspended in the atmosphere, which is capable of being ignited and
propagating a flame front.
✔✔Diffusion Flame. - ✔✔A flame in which fuel and air mix or diffuse together at the
region of combustion.
✔✔Drop Down. - ✔✔The spread of fire by the dropping or falling of burning materials.
Synonymous with "fall down."
✔✔Effective Fire Temperatures. - ✔✔Temperatures reached in fires that produce
physical effects that can be related to specific temperature ranges.
✔✔Electric Spark. - ✔✔A small, incandescent particle created by some arcs.
✔✔Empirical Data. - ✔✔Factual data that is based on actual measurement, observation
or direct sensory experience rather than on theory.
✔✔Entrainment. - ✔✔The process of air or gases being drawn into a fire, plume, or jet.
✔✔Explosion. - ✔✔The sudden conversion of potential energy (chemical or mechanical)
into kinetic energy with the production and release of gases under pressure, or the
release of gas under pressure. These high-pressure gases then do mechanical work
such as moving, changing, or shattering nearby materials.
