Definition of liquid. It is a state of matter that is not solid. Liquid commonly refers to substances, such as water, oil and alcohol, and the like, that are neither solids nor gases. Water ceases to be a liquid when it is frozen or turned to steam. Liquids are somewhat easier than gases to deal with because you can at least see where the liquid is located in a spill, and based upon terrain you will know where it will likely go (Figure 3.15). Procedures can be deployed for stopping the flow of a liquid keeping it from places you don't want it to go, such as sewers and water supplies. Damming, diking and collection are examples of actions used to control the movement of liquids.
Figure 3.15 Liquids are somewhat easier than gases to deal with because you can at least see where the liquid is located in a spill and based upon terrain you will know where it will likely go. (Courtesy Gwinnett County, GA Fire Department).
Definition of gas. It is a state of matter that has no definite shape or volume. It flows freely when released into the environment depending on its physical characteristics such as vapor density, vapor pressure and temperature (Figure 3.16). Gas will conform to a container and remain in a properly designed container, unless the container is damaged or breached in an accident. In spite of its lack of organization, a released gas may be controlled or contained if you know the physical characteristics of the gas. For responders, gases are the most difficult to deal with because you don't always know where the gas is or where it is going, and may not be able to stop it. Gases and vapors may be invisible or colorless. Locating them will likely require monitoring instruments.
Temperature and Pressure
Temperature and pressure have a great deal to do with the physical state of a chemical; however, they do not change the chemical properties. Elevated temperatures can be your worst enemy on the scene of a chemical release. Ambient temperature and radiated heat from a fire can change the physical state of a liquid chemical depending on the boiling point of the liquid. Water is a liquid at normal temperatures and pressures. If water exposed to temperatures below 32°F (0°C), it becomes a solid. There are some exceptions: water with varying levels of salt may take a lower temperature to freeze. The solid form of water still maintains all of the chemical properties of liquid water.
If water is heated above 212°F (100°C), it becomes a gas. However, the gas still maintains all of the chemical properties of liquid water.
Figure 3.16 Gas is the most difficult physical state for responders to deal with.
Figure 3.17 Comparison of water boiling points at various elevations.
As temperature increases, more particles have enough energy to escape to the gas phase. This increases the vapor pressure. When vapor pressure equals atmospheric pressure, the liquid boils. Altitude also affects the boiling point of water. At the lowest point in Death Valley, the boiling point would be about 212.0oF/100.3°C, and in deep mines, the boiling point could be even higher. As elevation increases, atmospheric pressure decreases because air is less dense at higher altitudes. Therefore, less heat is required to make the vapor pressure equal to the atmospheric pressure. The boiling point is lower at higher altitudes (Figure 3.17).
Physical State and Chemical Hazards
The hazard presented by a chemical may be affected by the physical state of the material when it is encountered. For example, only gases burn, solids and liquids do not burn, even though they may be listed as flammable. A solid or liquid must be heated until it produces enough vapor to burn. Water has a cooling effect on the skin as a liquid. When water is turned into a gas (steam), it causes burns to the skin. When water is in the solid state, it can freeze the skin and cause thermal burns. It is important to determine the physical state of a hazardous material early in the incident in order to formulate mitigation goals.