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# Fuel Study Guide for McGraw-Hill's Firefighter Exams (page 5)

By McGraw-Hill Professional
Updated on Jun 26, 2011

#### Liquids

Liquids make up the stage of matter between solids and gases. A liquid has definite volume but takes the shape of the container it is being stored in. Liquids that produce vapors that burn can be divided into two categories: combustible liquids (kerosene, diesel, heavy fuel oils) and flammable liquids (gasoline, methyl alcohol, acetone). Liquids can present other hazards to firefighters besides fire (corrosiveness and toxicity). In general, liquids that burn are classified as Class B materials; however, vegetable oils used in cooking and the preparation of foods are classified as Class K materials.

Some key characteristics to understand concerning liquids that burn are the flash point, boiling point, specific gravity, solubility, and viscosity.

• Flash point—The flash point is the minimum temperature of a liquid at which it emits vapors to form an ignitable mixture with air. For firefighters, the flash point is the most important property of liquids that burn. The degree of hazard will be determined by the flash point of the liquid because it is the vapors of the liquid that burn, not the liquid itself. Liquids are classified as combustible (flashpoint of 100° F or more) and flammable (flashpoint of less than 100°F).
• Boiling point—The boiling point is the temperature of the liquid at which it will liberate the most vapors. It is the temperature at which the vapor pressure of the liquid equals atmospheric pressure. The normal boiling point of a liquid is the temperature at which it boils at sea level, usually recorded as 14.7 pounds per square inch absolute (psia). It is impossible to raise the temperature of a liquid above its boiling point, except if it is under pressure.
• Specific gravity—The specific gravity of a liquid is the ratio of the weight of the liquid to the weight of an equal volume of water. The specific gravity of water is 1. A liquid (gasoline, 0.8) with a specific gravity less than water will float on water, whereas a liquid with a specific gravity more than 1 (sulfuric acid, 1.8) will sink.
• Solubility—The solubility of a liquid is the percentage by weight of the liquid that will dissolve in water. The solubility of a liquid ranges from negligible (less than one tenth of 1 percent) to complete (100 percent).
• Viscosity—Viscosity is a measure of a liquid's flow (through an opening or into a container) in relation to time. Thick liquids (molasses, asphalt, wax) are on the borderline between liquids and solids and are considered viscous.

#### Gases

Gases are the third stage of matter. The volume of a given amount of gas is dependent on its temperature and the surrounding pressure. An important concept for firefighters to understand regarding gases and vapors being emitted from a liquid is vapor density. Vapor density is the relative density of the gas or vapor as compared to air. The vapor density of air is 1. A gas or vapor with a vapor density more than 1 (butane, 2.1) will be heavier than air and travel along the ground surface in search of an ignition source. A gas or vapor with a vapor density less than 1 (methane, 0.55) will rise and disperse readily into the air. Gases are classified as Class B-type materials.

#### Chemical Properties of Gases

Gases can be classified according to their chemical properties as flammable (burn in air), inert (will not burn in air or in any concentration of oxygen and will not support combustion), oxidizer (will not burn in air or in any concentration of oxygen but will support combustion), toxic (poisonous or irritating when inhaled), and reactive (can rearrange chemically when exposed to heat or shock and explode or can react with other materials and ignite).

• Flammable—A gas that will burn in normal concentrations of oxygen in air is a flammable gas. When discussing flammable gases (or flammable vapors boiling off a liquid) mixing with air, the concept of flammable range must be understood. The flammable range is defined as the ratio of gas or vapor in air that is between the upper and lower flammable limits. The upper flammable limit is the maximum ratio of flammable gases/vapors above which ignition will not occur; it is too rich a mixture. The lower flammable limit is the minimum ratio of flammable gases/vapors in air below which ignition will not occur; it is too lean a mixture. Examples of flammable gases include acetylene, hydrogen, and propane.
• Inert—An inert gas is a nonflammable gas that will not support combustion. Examples include helium, nitrogen, and argon.
• Oxidizer—A nonflammable gas that will support combustion is known as an oxidizer. Examples include oxygen and chlorine.
• Toxic—Gases that cause harm to living tissue via chemical activity are called toxic gases. They can endanger the lives and health of those who inhale or come into skin contact with it. Examples include hydrogen cyanide, carbon monoxide, and ammonia.
• Reactive—Gases that react internally and with other materials are reactive gases. They can be heat sensitive and shock sensitive and also react with organic and inorganic substances to cause combustion. Examples include fluorine and vinyl chloride.