HazMat Chemical and Physical Terms

July 23, 2004
Hazardous Materials Response Team personnel need to have a firm grasp of important chemical and physical terms in order to understand how to handle a hazmat incident.
Hazardous Materials Response Team personnel need to have a firm grasp of important chemical and physical terms in order to understand how to handle a hazmat incident. Terms such as flash point, vapor pressure, and boiling point and the data associated with a hazardous material pertaining to these terms can be found in a fine reference book that all responders should have available. The book is called the NIOSH Pocket Guide to Chemical Hazards.

The NIOSH guidebook provides useful data for responders to interpret and which can assist in determining strategy and tactics at hazardous materials releases. Application of the information is the key and that is why knowing the meaning of the following terms from the NIOSH guidebook is so important. Knowledge of these terms also teaches responders "what-to-think" concerning the data. But, equally important, is using the data towards the "how-to-think" intricacies in resolving an incident. This is the connection of critical thinking in the field, a skill that can be developed through practice and experience. In essence, if responders use the "If-Then", cause-effect frame of reference, then they will be better able to use the data from the NIOSH guidebook in developing a game plan to mitigate any hazmat emergency.

Know Thy Enemy

Sun Tzu, an ancient Chinese general, wrote a comprehensive guide to warfare in 600 BC that was called "The Art of War". An important chapter of this book warned all readers that knowledge of the enemy is paramount. Smart leaders, Sun Tzu wrote, would do well to know the traits of their enemies in order to better predict the enemy's threats and then how to counter any attacks. It doesn't take much imagination to realize that these words of wisdom still apply today, not just for warfare but also for emergency response. Indeed, even Frank Brannigan warns that responders should know their enemy which in his context is a building under fire conditions. Obviously, we can also extend this concept to hazardous materials. It behooves all responders to know what the hazardous material can do upon being released. Again, that is why knowledge of chemical and physical terms is important.

Before we cover the terms found in the NIOSH guidebook lets take an opportunity to assess our current knowledge level on these terms. Take a few moments to answer the following questions of this Hazardous Materials Super Quiz. The text that follows will answer the quiz questions but we will also come back to these questions at the end of this article.

Hazardous Materials Super Quiz

Freshman Level ( True or False)

Master's Level

Ph.D. Level

The NIOSH Book

The National Institute of Occupational Safety and Health has published the Pocket Guide to Chemical Hazards since 1978. Many editions have been published since then and the newest edition will be available in Spring 2004. The guidebook contains a great deal of data on 673 chemicals and the alphabetical listing of chemicals is read left to right across two pages when the guide is opened. There is very important information in the guidebook including a cross index with Department of Transportation's Emergency Response Guidebook and also a synonym section. In total there are 13 columns of information in the guidebook.

Column - Topic
1 - Name, formula, ID #'s, Guide #'s
2 - Synonyms
3 - Exposure Limits
4 - IDLH
5 - Physical description
6 - Chemical and Physical Properties
7 - Incompatibilities
8 - Measurement methods
9 - PPE and sanitation
10 - Respirators
11 - Route and symptoms
12 - First Aid
13 - Target Organs

For the purpose of this article we will focus on the terms in Column 6 which are"Chemical and Physical Properties" and include the terms;

Abbreviation of Term - Terms
MW - Molecular Weight
BP - Boiling Point @ 1 atm.
Sol - Solubility (in water)
Sp. Gr. - Specific Gravity
Fl.P. - Flash Point (closed cup F)
IP - Ionization Potential
VP - Vapor Pressure @ 68F
FRZ/MP - Freezing/Melting Point in F
UEL - Upper Explosive Limit
LEL - Lower Explosive Limit

This article will define the terms above, look at some examples of data, reveal some relationships between terms, and discuss the critical thinking approach of "If-Then".

Molecular Weight

MW is simply the mass of the compound and it is derived by adding the mass of each atom in the chemical formula. Responders can calculate a compound's MW with an assist by the Periodic Table where each atom's weight will be listed. MW will technically be in Atomic Mass Units or AMU's. Where MW information can assist responders is in relation to a materials gas or vapor compared to air.

Critical thinking involving MW;

  • If the compound weight is > 29 AMU's,
  • Then the gas/vapor will be heavier than air.
  • If the compound weight is < 29 AMU's,
  • Then the gas/vapor will be lighter than air.

This concept is called vapor density and it is a measurement of the density of a gas compared to the density of the ambient atmosphere or air. In other words it is the weight of a gas/vapor compared to air. If the MW of a compound is divided by 29, the MW of a sample of air, then a ratio will result. Since the ratio of air compared to air is 1.0, then gases with ratios that are greater than 1.0 will sink when mixed in air. Gases with vapor densities less than 1.0 will generally rise in air. The NIOSH guidebook does not include this ratio but it can be calculated with the data provided. (For a small number of gases the abbreviation "RGasD" may appear and denote the vapor density ratio. Example; Ethylene Oxide is 1.49)

Which gases are lighter than air?

Most gases and vapors are heavier than air including the vapors from all liquids at room temperature. It would be extremely difficult to remember all of these gases and vapors. An easier way to remember the gas to air relationship is to simply remember the short list. The short list is the gases that are lighter than air. A very old acronym to recall these gases was "HA HA MICE" which stood for;

  • Hydrogen
  • Ammonia
  • Helium
  • Acetylene
  • Methane
  • Illuminating Gases (natural gas with a methane/ethane mixture)
  • Carbon Monoxide
  • Ethylene

A new acronym has been developed to update the old list of lighter than air gases and it is "4H MEDIC ANNA". This acronym includes the same gases as above (underlined) and an additional five gases. All of these gases have vapor densities of 1.0 or less.

  • Hydrogen, Helium
  • Hydrogen Fluoride
  • Hydrogen Cyanide
  • Methane
  • Ethylene
  • Diborane
  • Illuminating Gases
  • Carbon Monoxide
  • Ammonia
  • Neon
  • Nitrogen
  • Acetylene

Boiling Point

BP is the temperature at which a liquid readily changes to a gas. It can also be thought of as the temperature at which the vapor pressure of a liquid is equal to the atmospheric pressure at its surface. Since atmospheric pressure at sea level is equal to 14.7 psi then this means that a material's vapor pressure will be slightly greater than 14.7 psi at the material's BP.

Critical thinking involving BP;

  • If a material has a low BP,
  • Then the material will readily evaporate and large quantities of gas or vapor will be in the environment or the enclosed area.
  • If a material has a low BP,
  • Then it will also have a low FP.
  • If a material has a BP below the freezing point of water,
  • Then water soaked rags could possibly be used to seal a container or valve leak. (The wetted rag would literally freeze and plug the opening. This is especially true of compressed and liquefied gases.)

Examples and comparisons;

  • Acrylonitrile 171F
  • Acetone 133F
  • Ethylene Oxide 51F
  • Methane -250F
  • Sulfuric Acid 554F


Solubility (Sol.) refers to the degree in which a substance will dissolve in water. Miscibility is the ability of a liquid to dissolve in water and "miscible" means that it will totally dissolve in water. Polar compounds, which have slight electrical charges, dissolve in water and non-polar compounds do not mix with water.

Critical thinking involving solubility;

  • If a product has a very low solubility,
  • Then it will float or sink in water.
  • If a product is miscible,
  • Then it mixes totally in water-it does not sink or float.

Examples and comparisons;

  • Acrylonitrile - 7%
  • Acetone - miscible
  • Ethylene Oxide - miscible
  • Sulfuric Acid - miscible

Specific Gravity

The weight or density of a liquid or solid compared to water is the specific gravity (Sp.Gr. or SG) of the material. Floaters and sinkers are very immiscible with water and will have specific gravities less than 1.0 or greater than 1.0, respectively. All hydrocarbons float so therefore their specific gravities are less than 1.0. Water has a value of 1.0 for comparison purposes. All chlorinated hydrocarbons, such as methylene chloride, have specific gravities greater than 1.0. Specific gravity is listed for many materials in Column 6 of the NIOSH guidebook.

Critical thinking involving SG;

  • If a material has a SG less than 1.0,
  • Then the material may be absorbed or vacuumed from the surface of a body of water (provided the material is immiscible).
  • If a material that has a SG less than 1.0 is leaking from the bottom of a container,
  • Then it may be possible to "float" the material on a water layer that only allows water to escape through the leak. (Obviously, enough water would have to occupy the lower level of the container that meets or slightly exceeds the leak rate.)
  • If the material has a SG greater than 1.0,
  • Then the material may be hard to recover on the bottom of a body of water. (provided the material is immiscible)

Flash Point

Fl.P., or FP, is the temperature at which a liquid generates enough vapors to create an ignitable mixture at or near the surface of the liquid. It is also the minimum temperature a liquid must reach to produce enough vapor to allow for combustion in the presence of an ignition source momentarily (flash). Fire Point is where combustion is sustained but is not listed in the NIOSH guidebook. Flash Point is perhaps the most important physical trait of a material because it is the temperature at which combustion begins. In the presence of an ignition source and adequate oxygen then a fire can be produced.

Critical thinking involving FP;

  • If a material has a low FP,
  • Then it can combust a low temperatures.
  • If a material has a FP,
  • Then it will only combust above the FP.
  • If a material has a low BP,
  • Then it will also have a low FP.

Examples and comparisons;

  • Acrylonitrile - 30F
  • Acetone - 0F
  • Ethylene Oxide - -20F
  • Sulfuric Acid - not flammable

Ionization Potential

This information assists responders in selecting the correct lamp for photoionization detectors (PID's). The lamp energy needs to exceed a material's IP in order to ionize outer electrons so the monitor can read concentrations.

Critical thinking involving IP;

  • If a material has a high IP
  • Then it will not readily be ionized

Vapor Pressure

VP is the measure of a liquids ability to evaporate or readily change from a liquid to a gas. VP can also be defined as the pressure exerted by the escaping vapor against the sides or top of the container at equilibrium, (the state at which the vapor pressure has stabilized and is no longer rising or falling). VP is often measured in mm/Hg but can also be recorded in the following units with the following comparisons;

Generally, the higher the VP, the quicker the evaporation of the material. Some comparisons (approximations for comparison purposes);

  • Sarin nerve agent 2.9 mm/Hg @68 F
  • Water 25 mm/Hg @ 68 F
  • Acetone 250 mm/Hg @ 68 F
  • Acetylene 2,500 mm/Hg @ 68 F

It is very important to realize that VP is very temperature dependent! In other words, for each material its VP will proportionately increase as the temperature increases. For this reason, a hot day will produce a great deal more vapor of a material compared to the same material release in the cold of winter.

Critical thinking involving VP;

  • If a material has a high vapor pressure at a given temperature,
  • Then the concentration of the material in the air will be large.
  • *Converse is also true!
  • If a material has a high VP,
  • Then it will evaporate quickly.
  • If a material has a low BP,
  • Then it will have a high VP.

Examples and comparisons (all data is at 68F);

  • Acrylonitrile - 83 mm/Hg
  • Acetone - 180 mm/Hg
  • Ethylene Oxide - 1.46 atm
  • Sulfuric Acid - 0.001 mm/Hg

Case Study

On October 21, 1999 in a small Minnesota town a delivery person inadvertently filled a half-full propane storage tank with anhydrous ammonia. A few days later this mistake was discovered by a facility person who noticed the ammonia odor as he fueled his propane powered delivery truck. Shortly thereafter it was determined that the tank contents would have to be off-loaded and burned in a controlled fashion at a nearby gravel pit. To leave the mixture of the gases in the tank could have proved to be disastrous. According to Dalton's Law of Partial Pressure when more than one gas is contained in a tank the resulting vapor pressure for the tank will be equal to the sum of the separate vapor pressures of each gas in the tank. Adding the vapor pressures for both the propane and the anhydrous ammonia at a specific temperature could exceed the pressure that would activate the pressure relief valve (PRV). If the PRV was activated a toxic, flammable cloud could develop. Quick action based on the material's chemical and physical hazards mitigated this incident safely.

Melting Point/Freezing Point

MP is a specific temperature at which a solid turns into a liquid. It is also the same temperature at which a liquid will freeze. So, therefore, Melting Point = Freezing Point.

Critical thinking involving MP/FZP;

  • If a material has a lower FZP than water,
  • Then water soaked rags could be used as plugging devices. (Provided the ambient temperature is below the FZP of water and there are no compatibility concerns).

Examples and comparisons;

  • Acrylonitrile -116F
  • Acetone -140F
  • Ethylene Oxide -171F
  • Sulfuric Acid 51F

Flammable Limits

Remember it is the gas that burns! Flammable limits refer to the mixture of a gas in air in which combustion will occur. This can also be called flammable range or FR. The FR is the Lower Explosive Limit (LEL) to Upper Explosive Limit (UEL) for a gas. A unique way to remember this concept is to think of FR as "The Window of Opportunity". In other words, when a flammable gas is mixed correctly with air the "opportunity" for combustion exists is an ignition source enters the picture.

FR can also be thought of as football goal posts. Obviously, it would be much easier to kick a field goal if the posts were very wide. For this reason gases with wide FR's are much more dangerous to respond to than gases with narrow FR's. Some wide range FR gases are;

  • Ethylene Oxide 3.0 to 100%
  • Hydrogen 4.0 to 75%
  • Hydrazine 2.9 to 98%
  • Carbon Monoxide 12.5 to 74%

Some specific groups of chemicals that also have wide FR's are: Aldehydes, Alcohols, Ethers. This is because they are oxygenated fuels. Examples;

  • Acetaldehyde 4.0 to 60%
  • Methanol 6.0 to 36%
  • Diethyl Ether 1.9 to 36%

Critical thinking involving FR;

  • If a material has a wide flammable range,
  • Then it presents a high hazard to personnel because of a potential gas/vapor ignition.
  • If a FR exists for a material,
  • Then it will only combust within those lower and upper flammable limits.

Examples and comparisons;

  • Acrylonitrile 3.0 to 17%
  • Acetone 2.5 to 12.8%
  • Ethylene Oxide 3 to 100%
  • Sulfuric Acid not flammable

Ignition Temperature

IT is the temperature at which the material will self-ignite. This is also called the Autoignition Temperature. IT is the temperature that an ignition source must achieve in order to ignite a material. Unfortunately, IT is not referenced in the NIOSH guidebook.

Critical thinking involving IT;

  • If a material has a low BP,
  • Then it will have a high IT.

At this point let's return to the initial quiz and see how you did.

Hazardous Materials Super Quiz Answers

Freshman Level ( True or False)

1. False The NIOSH guidebook lists a chemical's vapor density. (True for a very small amount of the listed chemicals)2. True The NIOSH guidebook can assist in determining a chemical's physical state. (Found in Column 5)3. True Vapor pressure is temperature dependent.4. True A chemical's specific gravity will not matter if it is miscible.5. False The NIOSH guidebook lists a chemical's ignition temperature.

Master's Level

6. False Melting point is not the same as freezing point.7. True Boiling point is where the atmospheric pressure equals the material's vapor pressure.8. False A product will only burn below the flash point.

Ph.D. Level

9. False A product will only burn above the UEL.10. False Vapor pressure is the most important quality of a chemical.11. True If a material has a low boiling point it will also have a low flash point.12. True If two different gases exist in the same container the vapor pressure will be the sum of the two gases individual vapor pressures.

Well, how did you do? Did you learn a few things?


The NIOSH Pocket Guide to Chemical Hazards provides a great deal of data on 673 chemicals. Even though this article and quiz only focused on the Column 6 terms, which contained approximately 17 terms, there are many more terms that would be beneficial for hazmat responders to know. However, of the terms in Column 6 and the data associated with a referenced chemical, there is enough data to identify the characteristics of the chemical-our enemy. Based on this profile of the chemical, responders can then also devise the strategy and tactics that will be needed to safely mitigate the release.

Drill idea; Copy the quiz portion of this article and paste into a Word program and print them out for your hazmat personnel to complete. Hold an answer/discussion period to cover the terms in the NIOSH guidebook.

As usual, email the author at [email protected] with any comments or questions.

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