Photo by Robert Burke The physical state of an element or compound is one of the factors that determines the appropriate level of chemical protective clothing.
Some elements do not exist naturally as single atoms. They chemically bond with another atom of that same element to form "diatomic" molecules. The term "di" simply means two and atomic refers to the atom. Therefore, diatomic means two atoms. The diatomic elements are hydrogen, oxygen, nitrogen, chlorine, bromine, iodine and fluorine.
One way of remembering the diatomic elements is by using the acronym HONClBrIF, pronounced honk-le-brif, which includes the elemental symbol for each of the diatomic elements. Oxygen is commonly referred to as O2 in emergency response. This reference to O2 is primarily because oxygen is a diatomic element. Two oxygen atoms have covalently bonded together and act as one unit. The means of chemical bonding will be discussed later in this article.
Much can be learned about a compound by looking at its elemental composition. For example, generally, chemicals that contain chlorine in their formula may be toxic to some degree because chlorine is toxic. As with many rules of chemistry, there are exceptions, such as sodium chloride (NaCl) table salt. Even if you didn't know that sodium chloride is table salt and treated it as a toxic material, your error would be on the side of safety. If you are going to make errors when dealing with hazardous materials, always make sure you err on the side of safety. You may get chewed out afterwards by your officer or take some ribbing from other responders but no one has ever died from embarrassment! On the other hand, if you are not cautious and your errors are not on the side of safety, your actions could be fatal.
The Atom
An atom is the smallest particle of an element that can be found that retains all of its elemental characteristics. The word atom comes from the Greek, meaning "not cut." For example, take a sheet of paper and tear the paper in half. Keep tearing the paper in half until it becomes so small that it cannot be torn any smaller by hand. Then take a knife and cut the paper into smaller pieces. Eventually, it will not be able to be cut any smaller. The atom is like that last piece of paper.
Photo by Robert Burke Chlorine in its elemental form is toxic, an inhalation hazard and a strong oxidizer.
You cannot have a smaller piece of an element than an atom. A single atom cannot be altered chemically. To create a smaller part of an element would require that the atom be split in a nuclear reaction. Therefore, a single atom is the smallest particle of an element that would normally be encountered. Many elements, both diatomic and regular, are hazardous materials in their elemental state. Examples are chlorine, phosphorus, oxygen, molten sulfur, arsenic and sodium metal. Responders, however, are more likely to encounter combinations of elements that have formed compounds.
The atom is comprised of three major parts: electrons, which have a negative (-) charge; protons, which have a positive (+) charge; and neutrons, which are neutral. The atom is like a miniature solar system with the nucleus in the center and the electrons orbiting around the outside. These parts are referred to as subatomic particles because they are smaller than the atom itself.
The electrons are most important to chemistry and the nucleus is most important to radioactivity. Orbiting in shells or energy levels around the nucleus are varying numbers of electrons. Electrons are very important in discussing chemistry for hazardous materials responders. The outer shell electrons are where the chemical bonding takes place. The elements bond and form compounds to become stable. That is so that each atom of each element will become like the noble gases in family eight of the Periodic Table, which are chemically inert.
Photo by Robert Burke Jet fuel is a mixture of several compounds.
Atoms are electrically neutral so they must have an equal number of protons and electrons so that the positive and negative charges balance each other out. The atom is held together by the strong attraction between the positive protons and the negative electrons. This is an example of the fact that opposite charges attract.
Just as the atom is the smallest part of an element, a single element is the smallest portion of a chemical compound. Chemical compounds are made up of two or more elements covalently or ionicly bonded together. Chemical compounds are represented by formulas much like elements are represented by symbols. The formula for a compound is much like a recipe, it tells how many atoms of each element must be bonded together to make a particular compound.
According to the Condensed Chemical Dictionary, "a formula is a written representation using symbols of a chemical entity or relationship." There are three kinds of chemical formulas: empirical, molecular and structural.
An empirical formula indicates the composition of the relative number and the kind of atoms in a molecule of a compound. For example, CH is the empirical formula for both acetylene and benzene. The molecular formula shows the actual number and kind of atoms in a chemical compound. The molecular formula for hydrochloric acid is HCl, one atom of hydrogen and one atom of chlorine. Anytime there is only one atom of an element, no subscript numbers are used.
Photo by Robert Burke Elemental argon from family eight on the Periodic Table is chemically inert. It is often shipped as a cryogenic liquid which is very cold and when it gives off gases becomes an asphyxiation hazard.
The structural formula of a compound indicates the location of the atoms in relation to each other in a molecule as well as the number and location of chemical bonds. The following illustration is an example of a chemical structure for the compound butyric acid which has a molecular formula of C3H7COOH:
The molecular formula is most likely to be encountered by emergency responders. It may be found in reference books, on Material Safety Data Sheet (MSDS) reports and on container labels. By looking at the formulation of the compound, the responder may be able to predict potential hazards of a material. Certain elements present in compounds may represent a particular family of hazardous materials. Families of hazardous materials have particular hazards associated with them. While it is important and necessary for responders to use several reference sources to positively identify the types of compounds and their hazards, "street chemistry" can start you in the right direction.
Chemical Compounds
Two basic groups of chemical compounds are formed from elements. They are salts and non-salts. Salts are made up of a metal and a non-metal. Salts are solids and generally do not burn. There are however, some exceptions. For example, when the non-metal chlorine (Cl) is combined with the metal sodium (Na) the salt compound sodium chloride (table salt) is formed with the molecular formula NaCl. Salt compounds and their families have certain hazards and will be discussed in detail in a later article.
Metals generally do not bond together. Metals that are combined are melted and mixed together to form an alloy, in which the metals do not react chemically. For example, copper and zinc are melted and mixed together to make brass, which is not an element. No chemical bond is involved.
When the outer shell electrons of a metal are given up to a non-metal element, a salt compound is formed through a chemical bond. The outer shell of the metal is now empty so the next shell becomes the outer shell. This shell will have two or eight electrons, which is a stable configuration, just like the noble gases of family eight on the Periodic Table. The metal is then stable and electrically satisfied. The non-metal receives the electrons from the metal and now has eight electrons in its outer shell. The non-metal is now stable and electrically satisfied. The result is that a compound is formed that usually has different characteristics and hazards than the elements used to make it up.
Photo by Robert Burke Phosphorus in its elemental form is air reactive and spontaneously ignites when exposed to air. It is shipped under water.
The second group of compounds is made up totally of non-metal elements. The non-metals are comprised of two or more non-metal elements combining to form a compound. For example, the non-metal carbon combines with the non-metal hydrogen to form a hydrocarbon. A typical hydrocarbon might be methane with the molecular formula CH4. Hydrocarbons will be discussed further in another article.
Non-metals may be solids, liquids or gases. They may burn as well as being toxic, explosive, corrosive and oxidizers. The hazardous materials most frequently encountered by emergency responders are compounds made up of just a few non-metal materials. They are carbon, hydrogen, oxygen, sulfur, nitrogen, phosphorus, fluorine, chlorine, bromine and iodine. In elemental form and in compounds, these elements make up the bulk of hazardous materials encountered by emergency responders.
Chemical bonding, in the case of non-metals, involves electrons that are shared between the non-metal elements. This process of sharing electrons is called covalent bonding. Approximately 90% of covalently bonded hazardous materials are made up of carbon, hydrogen and oxygen. The remaining 10% are composed of chlorine, nitrogen, fluorine, bromine, iodine, sulfur and phosphorus. It is still necessary that each atom of each element have two or eight electrons in the outer shell. However, there is no exchange of electrons. When the bonding takes place, each atom of each element brings along their electrons and shares them with the other elements.
A chemical compound becomes electrically stable through the process of sharing and exchanging electrons. The fact that compounds have become electrically stable does not mean they are no longer hazardous. Quite simply, elements combine and chemical reactions occur so that compounds can become electrically stable. These combinations of elements that form compounds create many new hazardous and non-hazardous chemicals.
Emergency responders may encounter elemental chemicals that are hazardous when released in an accident. However, most of the hazardous chemicals encountered will be in the form of compounds or mixtures. Compounds and mixtures have a broad range of hazards, including explosive, corrosive, flammable, toxic and oxidizers.
Covalently bonded compounds also have specific families based upon the types of elements present. These families have particular hazards associated with them. The covalently bonded hazardous materials families will be discussed in a later article.
Mixtures
Chemical compounds may also exist in the form of mixtures. A mixture is two or more compounds combined together without any chemical bonding taking place. Each of the compounds retains its own characteristic properties.
Photo by Robert Burke Acetylene is highly unstable and is soluble in acetone. When it is dissolved in acetone, it becomes relatively stable.
The two types of mixtures are homogeneous and heterogeneous. Homogeneous means "the same kind" in Latin. In a homogenous mixture, every part is exactly like every other part. For example, water has a molecular formula of H2O. Pure water is homogeneous; it contains no substances other then hydrogen and oxygen. Loosely translated to include mixtures, homogeneous refers to two or more compounds or elements that are uniformly dispersed in each other. A solution is another example of a homogeneous mixture.
Heterogeneous means "different kinds" in Latin. In a heterogeneous mixture, the different parts of the mixture have different properties. The components in a heterogeneous mixture can be separated mechanically into their component parts. Examples of heterogeneous mixtures are gasoline, the air we breathe, blood and mayonnaise.
Solubility is another term associated with mixing two or more compounds together. The definition of solubility from the Condensed Chemical Dictionary is "the ability or tendency of one substance to blend uniformly with another, e.g., solid in liquid, liquid in liquid, gas in liquid, and gas in gas." The solubility may have varying degrees from one substance to another.
When researching chemicals in reference sources, relative solubility terms may include very soluble, slightly soluble, moderately soluble and insoluble. Generally, nothing is absolutely insoluble. Insoluble actually means "very sparingly soluble"; that is, only trace amounts dissolve.
Compounds of the alkali metals of family one on the Periodic Table are all soluble. Salts containing the ammonium ion (NH4), nitrates (NO3), perchlorates (ClO4), chlorates (ClO3), and organic peroxides containing carbon, hydrogen and two oxygen's bonded together are also soluble. All binary salt chlorides (Cl-), bromides (Br-) and iodides (I-) are soluble, except those containing silver (Ag), lead (Pb2) and mercury (Hg2). All sulfates are soluble except for those with the metals lead (Pb2), calcium (Ca2) strontium (Sr2), mercury (Hg2) and barium (Ba2). All hydroxides (OH) and metal oxides (containing O) are insoluble, except those of family one on the Periodic Table and Ca2, Sr2 and Ba2. All compounds that contain phosphate (PO4) carbonate (CO3), sulfate (SO3) and sulfur (S) are insoluble, except for those containing metals in family one on the Periodic Table and the ammonium ion (NH4). Most hydrocarbon compounds are mixtures and are not soluble, such as gasoline, diesel fuel and fuel oil.
Photo by Robert Burke Molten sulfur is an element that presents inhalation and thermal hazards.
Compounds that are polar such as the alcohols, ketones, aldehydes, esters and organic acids are soluble in water, because water is also a polar compound. This concept is important to understand when selecting firefighting foams. Foam when applied to a flammable liquid is largely water. Polar liquids will take the water from the foam and the foam blanket will break down. Therefore, fighting fires involving polar liquids will require alcohol type or polar solvent foams.
There are some factors that effect the solubility of a material. One is particle size. The smaller the particle, the more surface area that is exposed to the solvent; therefore, more dissolving takes place over a shorter period of time. Higher temperatures usually increase the rate of dissolving. The term miscibility is often used synonymously with the term solubility. If a compound, for example, is miscible in water, it is water soluble. If it is immiscible, then it is insoluble in water.
Acetylene gas is produced when calcium carbide, a salt, is mixed with water. Acetylene gas is very flammable and unstable. When placed in a container, acetylene gas is dissolved in acetone to stabilize it. Many inorganic acids are made when a gas is dissolved in water to form the liquid acid. Anhydrous ammonia is very soluble in water. Vapor clouds of anhydrous ammonia can be knocked down with fog patterns, dissolving the ammonia in water and forming ammonium hydroxide.
Robert Burke will present "Hazardous Materials: Responding To Chemical & Biological Terrorism" at Firehouse Emergency Services Expo '97 in Baltimore July 24-27.
Robert Burke, a Firehouse® contributing editor, is a Fire Protection Specialist with the University of Maryland at Baltimore. He is a certified Hazardous Materials Specialist and has served on state and county hazardous materials response teams. Burke is a member of the Earleigh Heights, MD, Volunteer Fire Company and has 16 years' experience in career and volunteer fire departments, attaining the rank of assistant chief, as well as serving as a deputy state fire marshal. He holds a bachelor's degree in fire science and is an adjunct instructor at the National Fire Academy, Maryland Fire and Rescue Institute and Delaware County, PA, Fire Academy. Burke's book, Hazardous Materials Chemistry For Emergency Responders, was published in January 1997. He can be reached on the Internet at [email protected], Part 1 of this report was published in May 1997.
