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To this point, most of the discussions of "Street Chemistry" have centered on the chemical characteristics of hazardous materials. This column will begin looking at the physical characteristics of some these materials; it could be called "Street Physics" but maybe that would be carrying things a bit far! Actually, we will be looking at the flammable liquid and gas families and the physical characteristics of combustion.
Flammable liquids and gases make up the largest number of hazmat incidents that responders will encounter. Just look at the fuels used to power internal combustion engines and heat our homes; flammable liquids and gases are everywhere. Responders should have at least a basic understanding of the physical characteristics of these materials - flash point, boiling point, ignition temperature, flammable range, vapor pressure, vapor density, volatility, polarity, miscibility, and the effects of temperature on flammable liquids and gases, to name a few.
Flammable liquids can be divided into basic families based on use and physical characteristics. For "Street Chemistry" purposes we will call them the fuel family, hydrocarbon derivatives, and the animal/vegetable oils, that undergo slow oxidation, or spontaneous combustion.
Looking back over the first seven columns in this series, the hydrocarbon derivative families that have flammability as one of their primary hazards include the ethers, amines, alcohols and aldehydes. Alkane, alkene and alkyne hydrocarbons, and the aromatic hydrocarbons (also known as the BTX fraction) make up the fuel families. Many of the compounds in the fuel family are mixtures such as gasoline, diesel fuel, fuel oil, and aviation fuels. These materials are mixtures of two or more of the hydrocarbons with other chemicals as additives. There are also pure compounds among the fuel family members. For example, propane, butane, pentane and octane are pure compounds.
The rule of thumb for determining a mixture or pure compound involves the ability to draw a structural formula for the compound. A structure can be drawn for pure compounds; however, there are no structures for mixtures. Animal/vegetable oils include such compounds as linseed oil, cottonseed oil, corn oil, lard and other cooking oils.
The flammable liquid families can further be divided into those that are polar and those that are non-polar. Polarity not only has an effect on the physical characteristics of combustion, but determines the type of fire extinguishing agent you will need to use to fight fires. Water is a polar compound and generally polar compounds will be miscible or will mix with water. When flammable liquids come in contact with water, it is important to know if they will mix or form a layer with the water. This concept is known as the specific gravity of the compound.
Fuel family compounds are generally lighter than water. Water is given a hypothetical value of one. A chemical with a specific gravity greater than one is heavier than water. If the specific gravity of the compound is less than one, it is said to be lighter than water and will float. Those compounds that float are much easier to clean up and control than those that sink.
Additionally, we are concerned about how easily materials may ignite, how well they burn once ignited and how much heat is produced as they burn. Generally, the higher the carbon and hydrogen content of a flammable liquid compound, the larger or heavier it is, and the more difficult it will be to ignite. When ignited, however, the greater the heat output will be. Compounds that have fewer carbons ignite more easily but do not have the heat output that the heavier compounds do. For example, when burning flammable liquids in a pit to practice extinguishment methods, diesel fuel and gasoline are mixed. Gasoline ignites easier than the diesel fuel and it helps to ignite the mixture with diesel fuel. Once ignited, the diesel fuel puts out more heat than the gasoline and makes the fire more difficult to extinguish.