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.
Photo by Robert Burke
A flammable liquid must be at its flash-point temperature before combustion can occur if an ignition source is present.
Flammables can exist as solids, liquids or gases. Some solid materials undergo sublimation - they can go directly from a solid to a vapor or gas without becoming a liquid. These materials can have flash points and other characteristics of combustion. We will, however, limit this discussion to flammable liquids and gases. There are also flammable liquids that are pyrophoric, which means they will spontaneously combust when exposed to air.
For responders, flammable gases are the most difficult to deal with because you don't always know where the gas is going. Flammable liquids are easier to deal with than gases because you can see where the liquid is located in a spill. There are also procedures for stopping the flow of the liquid and keeping it from places you don't want it to go. However, some liquids are volatile, which means they readily produce vapor and can become fire hazards if all of the conditions for combustion are just right. The concept associated with vapor location is vapor density.
Vapor density is to air like specific gravity is to water. Air is given a value of one. Vapors that have a vapor density greater than one will be heavier than air. Those that have a vapor density less than one will be lighter than air. Knowing the vapor density in a spill will help responders find the location of the vapor. Liquids that will burn - or, in reality, liquids that produce vapors that will burn - can be divided into two general groups, flammable and combustible. The temperature that divides flammable and combustible liquids is usually agreed to be 100 degrees Fahrenheit. Those liquids with flash points below 100F are considered flammable and those above 100F are referred to as combustible.
The National Fire Protection Association (NFPA) further subdivides flammable and combustible liquids into subclasses for the purpose of safe storage. When responding to spills of liquids that may burn, all such liquids should be considered to be flammable until more information is gathered about the material.
Under certain conditions, such as increased ambient temperatures that heat road surfaces, combustible liquids can behave much the same as flammable liquids. Other factors such as ignition temperature and temperatures of ignition sources can also play a part and fool responders. Emergency responders should become familiar with the physical characteristics of flammable and combustible liquids and know where to get the necessary information from reference books and other sources.
One very basic characteristic of a flammable liquid or liquefied gas is its boiling point. The boiling point of a liquid is the temperature at which the vapor pressure of the liquid overcomes atmospheric pressure. Atmospheric pressure at sea level is 14.5 psi; it decreases as altitude increases. For example, in New York or Philadelphia, which are near sea level, water boils at 212F. In Denver, where the altitude is one mile above sea level, water boils at about 203F.
As atmospheric pressure is overcome, the vapors from a liquid start to move farther away from a spill; if inside a container, the pressure in the container starts to increase. Several physical characteristics determine whether a flammable liquid or family of liquids have high or low boiling points.
First, liquids with high carbon and hydrogen contents tend to have higher boiling points. They are heavier, which is referred to as the "size" or weight of a compound. For example, propane is a three-carbon alkane hydrocarbon with eight hydrogens, has a boiling point of around -40F, and is a gas at normal temperatures and pressures.
The next characteristic that affects boiling point is polarity. It causes the boiling points of polar liquids to have higher boiling points than non-polar liquids of the same or similar size. For a liquid to boil, not only does it have to overcome atmospheric pressure but you have to overcome polarity that causes the molecules to want to stick together. In order to accomplish this, more energy in the form of heat has to be applied to the liquid to get it to boil, which results in the compound having a higher boiling point.
To compare weights of compounds it is necessary to determine the atomic weights of the elements in the compounds. This information is found on the periodic table of elements. You locate the atomic weight of the element on the table and round it off to the nearest whole number. For example, carbon weighs 12.011 atomic mass units and hydrogen weighs 1.0079 atomic mass units. Carbon would be rounded to 12 and hydrogen to 1. Oxygen weighs 15.9994 atomic mass units and would be rounded to 16. Water is made up of two atoms of hydrogen and 1 atom of oxygen. As a compound, it has an atomic weight of 18 atomic mass units, one from each of the two hydrogens and 16 from the oxygen.
The average molecule of air weighs about 29 atomic mass units. So in effect water is lighter than air! How can this be? Water is a liquid at normal temperatures and pressures. It should be a gas judging by its weight.
The answer is polarity. Water is a polar compound with a boiling point of 212F at sea level. Polarity causes water, which is smaller in weight than air, to have a high boiling point and thus exist as a liquid rather than a gas. It's a good thing water is a liquid and not a gas or life as we know it might not be possible.
Of the families we have discussed in "Street Chemistry," alcohols, ketones, esters, aldehydes and organic acids are polar compounds. Hydrocarbons, aromatics, alkyl halides, nitros, peroxides and ethers are non-polar.
All materials may have some degree of polarity, but for our purposes we can make general statements about groups of hazardous materials. For example, if you compare ethyl alcohol with an atomic weight of 46 and ethyl chloride (an alkyl halide) with an atomic weight of 64, you might expect the ethyl chloride to have a higher boiling point because it weighs more. However, the boiling point of ethyl chloride is 54F and the boiling point of ethyl alcohol is much higher at 173F. The alcohol is polar and the alkyl halide is non-polar; polarity causes the boiling point of the alcohol to be higher.
A third factor that affects boiling point is isomerism or branching. The definition of an isomer is a compound that has the same formula but a different structure. When a compound is in the isomer form, it has the affect of lowering the boiling point of the compound.
One of the most important physical characteristics of a flammable liquid is its flash point. Flash point is the minimum temperature to which a liquid must be heated to produce enough vapor to allow a vapor flash to occur, if an ignition source is present (it is the vapor that burns, not the liquid).
Flash point is a measurement of the liquid temperature. Therefore, even if the ambient temperature is not at the flash point temperature of the liquid, the liquid may have been heated to its flash point by some external heat source. For example, the radiant heat from the sun, heat from a fire or heat from a chemical process may heat the liquid to its flash point. If an ignition source is present, fire can occur, and probably will. If a flammable liquid is not at its flash point temperature, combustion cannot occur. So, when researching chemicals in reference sources, the flash point temperature is the first physical characteristic responders should look for. Reference books used to research chemical characteristics in hazmat emergencies may show different flash point values.
There are different tests used to determine the flash point of a liquid. Tests use open- and closed-cup testing apparatus, which often produce somewhat different temperature values. Open-cup flash point tests try to simulate conditions of a flammable liquid in the open, such as a spill from a container onto the ground. The open-cup test usually results in a higher flash point temperature for the same flammable liquid than the closed-cup method. Responders should use the lowest flash point value given.
Fire point is the temperature to which the liquid is heated that produces enough vapor for ignition to occur after the vapor flash occurs. The fire point temperature is one to three degrees higher than the flash point temperature. Fire point is so close to flash point temperature that it really isn't much of a concern to emergency responders. If a liquid is at its flash point in a spill or leak, then it is very likely to also be at its fire point, and prepare for a fire!
Photo by Robert Burke
This highway tanker is placarded for flammable liquids.
Another term associated with combustion, that is sometimes misunderstood, is ignition temperature, also known as auto-ignition temperature. The ignition temperature is the minimum temperature to which a material must be heated to cause auto-ignition without the need for an ignition source to be present. In other words, a flammable liquid is heated from an outside heat source and auto-ignites when its ignition temperature is reached.
For an ignition source to ignite a material at its flash point temperature, the ignition source must be at the ignition temperature of the liquid. For example, if you placed gasoline in a container and used a lit cigarette to ignite the gasoline, you would be wasting your time. The temperature of a lit cigarette is around 400F and the average ignition temperature of gasoline is around 800F; therefore, the cigarette is not hot enough to ignite the gasoline. On the other hand, if you placed diesel fuel in a container and used a lit cigarette to ignite the diesel fuel, you would have a fire on your hands! The ignition temperature of diesel fuel is about the same as the temperature of the lit cigarette.
Boiling point and flash point have a parallel relationship. When the boiling point of a compound is low, then the flash point is also low. Compounds that have high boiling points also have high flash points. If a flammable liquid that has a low boiling point, and a correspondingly low flash point, the quantity of vapor produced by the compound will be high. If the liquid is spilled on the ground, the vapor will travel farther away from the spill. This is referred to as the vapor content of the spill.
Inside a closed container, the vapor will increase the pressure inside the container. This is the vapor pressure within the container of liquid. If the boiling point and flash point are high, then the vapor content and vapor pressure will be low. Another term associated with combustion is the heat output when the vapor from a liquid burns. Large compounds have high heat output and small compounds have low heat output.
There is a relationship between boiling point, flash point, ignition temperature, vapor content, vapor pressure and heat output. Compounds that have high boiling points tend to have high flash points. Those that have low boiling points tend to have low flash points. The numeric values are different, but the ratio holds true.
Additional terms associated with boiling and flash point include vapor content, vapor pressure, heat output and ignition temperature. Compounds that have high boiling points and high flash points tend to have high heat output, low vapor pressure, low vapor content and low ignition temperatures. The illustration below shows the relationship of combustion characteristics.
If a flammable liquid is at its flash point temperature and the ignition source is at or above the ignition temperature of the liquid, there is still one more physical characteristic that must be correct for combustion to occur. This characteristic is referred to as the flammable range or explosive limits.
The flammable range is expressed on a scale from 0% to 100%. Between 0% and 100%, a flammable liquid vapor will encounter the proper mixture of air and fuel to complete the combustion process. Each vapor has an upper explosive limit (UEL) and lower explosive limit (LEL). Above the UEL there is enough fuel for combustion to occur, but not enough oxygen; it is too rich to burn. Below the LEL the mixture is too lean to burn; there is enough oxygen, but not enough fuel.
Large hydrocarbon compounds such as animal/vegetable oils each have a characteristic double bond somewhere within the structure. This double bond can be attacked by the oxygen in the air and the bond can be broken. When bonds break, it is an exothermic, or heat producing reaction. If the heat is confined, spontaneous ignition can occur.
The One Meridian Plaza building fire in Philadelphia several years ago, that resulted in the deaths of three firefighters, was started by linseed oil-soaked rags that were improperly stored. They spontaneously combusted, starting the fire. I investigated a fire in an aircraft hangar that also resulted from improperly stored rags soaked with linseed oil.
Fuel family mixtures such as gasoline, diesel fuel, fuel oil, aviation fuels, motor oils and grease do not have any double bonds in the compounds. These types of compounds cannot undergo spontaneous combustion and start fires because there is no double bond to break to produce heat required for spontaneous combustion to occur. These materials on rags have been blamed for causing fires in the past when it was not possible for them to be responsible.
Robert Burke will discuss "Hazardous Materials Response: Handling The Incident" at Firehouse Emergency Services Expo '98 in Baltimore July 15-19.
Robert Burke, a Firehouse® contributing editor, is a fire protection/hazardous materials specialist for the University of Maryland and has served on state and county hazmat response teams. Burke is a veteran of over 16 years in career and volunteer fire departments, serving as assistant chief and deputy state fire marshal. He holds an associate's degree in fire protection technology and a bachelor's degree in fire science, and is pursuing a master's degree in public administration. Burke is an adjunct instructor at the National Fire Academy and Maryland Fire and Rescue Institute, and is the author of the textbook Hazardous Materials Chemistry For Emergency Responders. He can be reached on the Internet at firstname.lastname@example.org.