An oil tanker taking on bunker fuel.

Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking, fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash point of approximately 40 °C (104 °F) and oils burned in cotton or wool-wick burners. In this sense, diesel is a type of fuel oil. Fuel oil is made of long hydrocarbon chains, particularly alkanes, cycloalkanes and aromatics. The term fuel oil is also used in a stricter sense to refer only to the heaviest commercial fuel that can be obtained from crude oil, heavier than gasoline and naphtha.

Classes[link]

The American Society for Testing and Materials (ASTM) has published specifications for six grades of fuel oil, numbered 1 to 6. This classification system was originally based on early refining and combustion engineering practices and nomenclature. Some specifications have changed over the years to reflect new refining practices and refinery by-products. Other organizations have published modified versions of the original six fuel specifications to assist operators of older equipment find appropriate fuels from current refinery products.^[1]

Although the following trends generally hold true, different organizations may have different numerical specifications for the six fuel grades. The boiling point and carbon chain length of the fuel increases with fuel oil number. Viscosity also increases with number, and the heaviest oil has to be heated to get it to flow. Price usually decreases as the fuel number increases.^[2]

Number 1 fuel oil is a volatile distillate oil intended for vaporizing pot-type burners.^[1] It is the kerosene refinery cut that boils off right after the heavy naphtha cut used for gasoline. Older names include coal oil, stove oil and range oil.^[2]

Number 2 fuel oil is a distillate home heating oil.^[1] Trucks and some cars use similar diesel fuel with a cetane number limit describing the ignition quality of the fuel. Both are typically obtained from the light gas oil cut. Gas oil refers to the original use of this fraction in the late 19th and early 20th centuries - the gas oil cut was used as an enriching agent for carburetted water gas manufacture.^[2]

Number 3 fuel oil was a distillate oil for burners requiring low-viscosity fuel. ASTM merged this grade into the number 2 specification, and the term has been rarely used since the mid-20th century.^[1]

Number 4 fuel oil is a commercial heating oil for burner installations not equipped with preheaters.^[1] It may be obtained from the heavy gas oil cut.^[2]

Number 5 fuel oil is a residual-type industrial heating oil requiring preheating to 170 – 220 °F (77 – 104 °C) for proper atomization at the burners.^[1] This fuel is sometimes known as Bunker B. It may be obtained from the heavy gas oil cut,^[2] or it may be a blend of residual oil with enough number 2 oil to adjust viscosity until it can be pumped without preheating.^[1]

Number 6 fuel oil is a high-viscosity residual oil requiring preheating to 220 – 260 °F (104 – 127 °C). Residual means the material remaining after the more valuable cuts of crude oil have boiled off. The residue may contain various undesirable impurities including 2 percent water and one-half percent mineral soil. This fuel may be known as residual fuel oil (RFO), by the Navy specification of Bunker C, or by the Pacific Specification of PS-400.^[1]

Mazut is a residual fuel oil often derived from Russian petroleum sources and is either blended with lighter petroleum fractions or burned directly in specialized boilers and furnaces. It is also used as a petrochemical feedstock.

Bunker fuel[link]

A sample of residual fuel oil.

Small molecules like those in propane gas, naphtha, gasoline for cars, and jet fuel have relatively low boiling points, and they are removed at the start of the fractional distillation process. Heavier petroleum products like diesel and lubricating oil are much less volatile and distill out more slowly, while bunker oil is literally the bottom of the barrel; the only things more dense than bunker fuel are carbon black feedstock and bituminous residue which is used for paving roads (asphalt) and sealing roofs.

Bunker fuel is technically any type of fuel oil used aboard ships. It gets its name from the containers on ships and in ports that it is stored in; in the days of steam they were coal bunkers but now they are bunker fuel tanks. The Australian Customs and the Australian Tax Office define a bunker fuel as the fuel that powers the engine of a ship or aircraft. Bunker A is No. 2 fuel oil, bunker B is No. 4 or No. 5 and bunker C is No. 6. Since No. 6 is the most common, "bunker fuel" is often used as a synonym for No. 6. No. 5 fuel oil is also called navy special fuel oil or just navy special; No. 5 or 6 are also called furnace fuel oil (FFO); the high viscosity requires heating, usually by a recirculated low pressure steam system, before the oil can be pumped from a bunker tank. In the context of shipping, the labeling of bunkers as previously described is rarely used in modern practice.

Since the 1980s the International Organization for Standardization (ISO) has been the accepted standard for marine fuels (bunkers). The standard is listed under number 8217, with recent updates in 2005 and 2010. They have broken it down to Residual and Distillate fuels. The most common residual fuels in the shipping industry are RMG and RMK.^[3] The differences between the two are mainly the density and viscosity, with RMG generally being delivered at 380 centistokes or less, and RMK at 700 centistokes or less. Ships with more advanced engines can process heavier, more viscous, and thus cheaper, fuel. Governing bodies (i.e. California, European Union) around the world are starting to limit the maximum sulfur of fuels burned in their ports to limit pollution. This is where Marine Distillate Fuels come into play. They have similar properties to Diesel #2 which is used as road Diesel around the world. The most common grades used in shipping are DMA and DMB.^[4]

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Uses[link]

A fuel station in Zigui County on the Yangtze River

Oil has many uses; it heats homes and businesses and fuels trucks, ships and some cars. A small amount of electricity is produced by diesel, but it is more polluting and more expensive than natural gas. It is often used as a backup fuel for peaking power plants in case the supply of natural gas is interrupted or as the main fuel for small electrical generators. In Europe the use of diesel is generally restricted to cars (about 40%), SUVs (about 90%), and trucks and buses (virtually all). The market for home heating using fuel oil, called heating oil, has decreased due to the widespread penetration of natural gas. However, it is very common in some areas, such as the Northeastern United States.

Residual fuel oil is less useful because it is so viscous that it has to be heated with a special heating system before use and it contains relatively high amounts of pollutants, particularly sulfur, which forms sulfur dioxide upon combustion. However, its undesirable properties make it very cheap. In fact, it is the cheapest liquid fuel available. Since it requires heating before use, residual fuel oil cannot be used in road vehicles, boats or small ships, as the heating equipment takes up valuable space and makes the vehicle heavier. Heating the oil is also a delicate procedure, which is inappropriate to do on small, fast moving vehicles. However, power plants and large ships are able to use residual fuel oil.

Residual fuel oil was used more frequently in the past. It powered boilers, railroad steam locomotives and steamships. Locomotives now use diesel; steamships are not as common as they were previously due to their higher operating costs (most LNG carriers use steam plants, as "boil-off" gas emitted from the cargo can be used as a fuel source); and most boilers now use heating oil or natural gas. However, some industrial boilers still use it and so do a few old buildings, including in New York City. The city estimates that the 1% of its buildings that burn fuel oils No. 4 and No. 6 are responsible for 86% of the soot pollution generated by all buildings in the city. New York has made the phase out of these fuel grades part of its environmental plan, PlaNYC, because of concerns for the health effects caused by fine particulates.^[5]

Residual fuel's use in electricity generation has also decreased. In 1973, residual fuel oil produced 16.8% of the electricity in the United States. By 1983, it had fallen to 6.2%, and as of 2005^[update], electricity production from all forms of petroleum, including diesel and residual fuel, is only 3% of total production. The decline is the result of price competition with natural gas and environmental restrictions on emissions. For power plants, the costs of heating the oil, extra pollution control and additional maintenance required after burning it often outweigh the low cost of the fuel. Burning fuel oil, particularly residual fuel oil, also produces much darker smoke and uniformly higher Carbon Dioxide emissions than natural gas,^[6] which affects the community's perception.

Heavy fuel oils continue to be used in the boiler "lighting up" facility in many coal-fired power plants. Although on an enormous scale, this use is analogous to lighting kindling to start a fire; without performing this simple function it is difficult to begin the large-scale combustion process.

The chief drawback to residual fuel oil is its high initial viscosity, particularly in the case of No. 6 oil, which requires a correctly engineered system for storage, pumping, and burning. Though it is still usually lighter than water (with a specific gravity usually ranging from 0.95 to 1.03) it is much heavier and more viscous than No. 2 oil, kerosene, or gasoline. No. 6 oil must, in fact, be stored at around 100 °F (38 °C) heated to 150 °F (66 °C)–250 °F (121 °C) before it can be easily pumped, and in cooler temperatures it can congeal into a tarry semisolid. The flash point of most blends of No. 6 oil is, incidentally, about 150 °F (66 °C). Attempting to pump high-viscosity oil at low temperatures was a frequent cause of damage to fuel lines, furnaces, and related equipment which were often designed with lighter fuels in mind.

(For comparison, BS2869 Class G Heavy Fuel Oil behaves in similar fashion, requiring storage at 104 °F (40 °C), pumping at around 122 °F (50 °C) and finalising for burning at around 194 °F (90 °C) / 248 °F (120 °C).)

Most of the facilities which historically burned No. 6 or other residual oils were industrial plants and similar facilities constructed in the early or mid 20th century, or which had switched from coal to oil fuel during the same time period. In either case, residual oil was seen as a good prospect because it was cheap and readily available, even though it provided less energy per volume-unit than lighter fuels. Most of these facilities have subsequently been closed and demolished, or have replaced their fuel supplies with a simpler one such as gas or No. 2 oil. The high sulfur content of No. 6 oil—up to 3% by weight in some extreme cases—had a corrosive effect on many heating systems (which were usually designed without adequate corrosion protection in mind), shortening their lifespans and increasing the polluting effects. This was particularly the case in furnaces that were regularly shut down and allowed to go cold; the internal condensation produced sulfuric acid.

Environmental cleanups at such facilities are frequently complicated by the use of asbestos insulation on the fuel feed lines. No. 6 oil is very persistent, and does not degrade rapidly. Its viscosity and stickiness also make remediation of underground contamination very difficult, since it reduces the effectiveness of methods such as air stripping.

When released into water, such as a river or ocean, residual oil tends to break up into patches or tarballs—mixtures of oil and particulate matter such as silt and floating organic matter- rather than form a single slick. An average of about 5-10% of the material will evaporate within hours of the release, primarily the lighter hydrocarbon fractions. The remainder will then often sink to the bottom of the water column.

Maritime[link]

In the maritime field another type of classification is used for fuel oils:

• MGO (Marine gas oil) - roughly equivalent to No. 2 fuel oil, made from distillate only
• MDO (Marine diesel oil) - A blend of heavy gasoil that may contain very small amounts of black refinery feed stocks, but has a low viscosity up to 12 cSt so it need not be heated for use in internal combustion engines
• IFO (Intermediate fuel oil) A blend of gasoil and heavy fuel oil, with less gasoil than marine diesel oil
• MFO (Marine fuel oil) - same as HFO (just another "naming")
• HFO (Heavy fuel oil) - Pure or nearly pure residual oil, roughly equivalent to No. 6 fuel oil

Marine diesel oil contains some heavy fuel oil, unlike regular diesels. Also, marine fuel oils sometimes contain waste products such as used motor oil.

Standards and classification[link]

CCAI and CII are two indexes which describe the ignition quality of residual fuel oil, and CCAI is especially often calculated for marine fuels. Despite this marine fuels are still quoted on the international bunker markets with their maximum viscosity (which is set by the ISO 8217 standard - see below) due to the fact that marine engines are designed to use different viscosities of fuel.^[7] The unit of viscosity used is the Centistoke and the fuels most frequently quoted are listed below in order of cost, the least expensive first-

• IFO 380 - Intermediate fuel oil with a maximum viscosity of 380 Centistokes (<3.5% sulphur)
• IFO 180 - Intermediate fuel oil with a maximum viscosity of 180 Centistokes (<3.5% sulphur)
• LS 380 - Low-sulphur (<1.5%) intermediate fuel oil with a maximum viscosity of 380 Centistokes
• LS 180 - Low-sulphur (<1.5%) intermediate fuel oil with a maximum viscosity of 180 Centistokes
• MDO - Marine diesel oil.
• MGO - Marine gasoil.
• LSMGO - Marine gasoil.
• LSMGO - Low-sulphur (<0.1%) Marine Gas Oil - The fuel is to be used in EU community Ports and Anchorages. EU Sulphur directive 2005/33/EC

The density is also an important parameter for fuel oils since marine fuels are purified before use to remove water and dirt from the oil. Since the purifiers use centrifugal force, the oil must have a density which is sufficiently different from water. Older purifiers had a maximum of 991 kg/m3; with modern purifiers it is also possible to purify oil with a density of 1010 kg/m3.

The first British standard for fuel oil came in 1982. The latest standard is ISO 8217 from 2005. The ISO standard describe four qualities of distillate fuels and 10 qualities of residual fuels. Over the years the standards have become stricter on environmentally important parameters such as sulfur content. The latest standard also banned the adding of used lubricating oil (ULO).

Some parameters of marine fuel oils according to ISO 8217 (3. ed 2005):

1. Max sulfur content is 1.5% in designated areas. (since 1-07-2010 1% is max).
2. The aluminium+silicon value is used to check for remains of the catalyst after catalytic cracking. Most catalysts contains aluminium or silicon and remains of catalyst can cause damage to the engine.
3. The flash point of all fuels used in the engine room should be at least 60°C (DMX is used for things like emergency generators and not normally used in the engine room).

1. Max sulfur content is 1.5% in designated areas. (since 1-07-2010 1% is max).
2. The aluminium+silicon value is used to check for remains of the catalyst after catalytic cracking. Most catalysts contains aluminium or silicon and remains of catalyst can cause damage to the engine.
3. The flash point of all fuels used in the engine room should be at least 60°C.

Transportation[link]

Fuel oil is transported worldwide by fleets of supertankers making deliveries to suitably sized strategic ports such as Houston, Singapore, Fujairah, Balboa, Cristobal, Gibraltar and Rotterdam. Where a convenient seaport does not exist, inland transport may be achieved with the use of barges. The lighter fuel oils can also be transported through pipelines. The major physical supply chains of Europe are along the Rhine.

See also[link]

• Diesel fuel
• Gasoline
• Heating oil
• Jet fuel
• Kerosene
• Lubricant
• Naphtha
• Gas Oil Separation Plant
• Fuel management systems
• Fuel price risk management
• Marine fuel management
• Hot bulb engine

References[link]

1. Bunker Pricing Methodologies

External links[link]

• National Park Service - Fuel Oil
• World Bunker Prices
• How Stuff Works - Oil Refining
• Identity and Analysis of Total Petroleum Hydrocarbons
• The International Bunkering Industry Association

An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils.

Etymology[link]

First attested in English 1176, the word oil comes from Old French "oile", from Latin "oleum",^[1] which in turn comes from the Greek "ἔλαιον" (elaion), "olive oil, oil"^[2] and that from "ἐλαία" (elaia), "olive tree".^[3] The earliest attested form of the word is the Mycenaean Greek e-ra-wo, written in Linear B syllabic script.^[4]

Types[link]

This unreferenced section requires citations to ensure verifiability.

Organic oils[link]

Organic oils are produced in remarkable diversity by plants, animals, and other organisms through natural metabolic processes. Lipid is the scientific term for the fatty acids, steroids and similar chemicals often found in the oils produced by living things, while oil refers to an overall mixture of chemicals. Organic oils may also contain chemicals other than lipids, including proteins, waxes and alkaloids.

Lipids can be classified by the way that they are made by an organism, their chemical structure and their limited solubility in water compared to oils. They have a high carbon and hydrogen content and are considerably lacking in oxygen compared to other organic compounds and minerals; they tend to be relatively nonpolar molecules, but may include both polar and nonpolar regions as in the case of phospholipids and steroids.^[5]

Mineral oils[link]

Crude oil, or petroleum, and its refined components, collectively termed petrochemicals, are crucial resources in the modern economy. Crude oil originates from ancient fossilized organic materials, such as zooplankton and algae, which geochemical processes convert into oil.^[6] It is classified as a mineral oil because it does not have an organic origin on human timescales, but is instead obtained from rocks, underground traps, or sands. Mineral oil also refers to several specific distillates of crude oil.

Applications[link]

This unreferenced section requires citations to ensure verifiability.

A bottle of olive oil used in food

Cosmetics[link]

Oils are applied to hair to give it a lustrous look, to prevent tangles and roughness and to stabilize the hair to promote growth.^{[citation needed]} See Hair conditioner.

Religion[link]

Oils are commonly used in ritual anointments. As a particular example, holy anointing oil has been an important ritual liquid for Judaism and Christianity.

Painting[link]

Color pigments are easily suspended in oil, making it suitable as a supporting medium for paints. The oldest known extant oil paintings date from 650 AD.^[7]

Heat transfer[link]

Oils are used as coolants in oil cooling, for instance in electric transformers. Oils are also used to enhance heating in other applications, such as cooking (especially in frying).

Lubrication[link]

Oils are commonly used as lubricants. Mineral oils are more commonly used as machine lubricants than biological oils are.

Fuel[link]

Some oils burn in liquid or aerosol form, generating heat which can be used directly or converted into other forms of energy such as electricity or mechanical work. To obtain many fuel oils, crude oil is pumped from the ground and is shipped via oil tanker to an oil refinery. There, it is converted from crude oil to diesel fuel (petrodiesel), ethane (and other short-chain alkanes), fuel oils (heaviest of commercial fuels, used in ships/furnaces), gasoline (petrol), jet fuel, kerosene, benzene (historically), and liquefied petroleum gas. A 42 gallon barrel (U.S.) of crude oil produces approximately 10 gallons of diesel, 4 gallons of jet fuel, 19 gallons of gasoline, 7 gallons of other products, 3 gallons split between heavy fuel oil and liquified petroleum gases,^[8] and 2 gallons of heating oil. The total production of a barrel of crude into various products results in an increase to 45 gallons.^[8] Not all oils used as fuels are mineral oils, see biodiesel and vegetable oil fuel.

Chemical feedstock[link]

Crude oil can be refined into a wide variety of component hydrocarbons. Petrochemicals are the refined components of crude oil and the chemical products made from them. They are used as detergents, fertilizers, medicines, paints, plastics, synthetic fibers, and synthetic rubber. Organic oils are another important chemical feedstock, especially in green chemistry.

See also[link]

• Cooking oil
• Emulsifier, a chemical which allows oil and water to mix
• Lubrication
• Wax, a class of compounds with oil-like properties that are solid at common temperatures

References[link]

This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations. (October 2009)

External links[link]

Wikimedia Commons has media related to: Oil

• Petroleum Online e-Learning resource from IHRDC

This article reads like a textbook and may need a cleanup. Please help to improve this article to meet Wikipedia's quality standards.

Gas phase particles (atoms, molecules, or ions) move around freely in the absence of an applied electric field.

Gas is one of the three classical states of matter (the others being liquid and solid). Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point (see phase change), boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons are so energized that they leave their parent atoms from within the gas. A pure gas may be made up of individual atoms (e.g. a noble gas or atomic gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or compound molecules made from a variety of atoms (e.g. carbon dioxide). A gas mixture would contain a variety of pure gases much like the air. What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer. The interaction of gas particles in the presence of electric and gravitational fields are considered negligible as indicated by the constant velocity vectors in the image.

The gaseous state of matter is found between the liquid and plasma states,^[1] the latter of which provides the upper temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases^[2] which are gaining increased attention these days.^[3] High-density atomic gases super cooled to incredibly low temperatures are classified by their statistical behavior as either a Bose gas or a Fermi gas. For a comprehensive listing of these exotic states of matter see list of states of matter.

Etymology[link]

The word gas is a neologism first used by the early 17th century Flemish chemist J.B. Van Helmont. Van Helmont's word appears to have been simply a phonetic transcription of the Greek word Chaos – the g in Dutch being pronounced like the English ch – in which case Van Helmont was simply following the established alchemical usage first attested in the works of Paracelsus. According to Paracelsus's terminology, chaos meant something like "ultra-rarefied water".^[4]

Physical characteristics[link]

Drifting smoke particles provide clues to the movement of the surrounding gas.

As most gases are difficult to observe directly with our senses, they are described through the use of four physical properties or macroscopic characteristics: pressure, volume, number of particles (chemists group them by moles) and temperature. These four characteristics were repeatedly observed by scientists such as Robert Boyle, Jacques Charles, John Dalton, Joseph Gay-Lussac and Amedeo Avogadro for a variety of gases in various settings. Their detailed studies ultimately led to a mathematical relationship among these properties expressed by the ideal gas law (see simplified models section below).

Gas particles are widely separated from one another, and as such are not as strongly intermolecularly bonded to the same degree as liquids or solids. These intermolecular forces result from electrostatic interactions between each gas particle. Like charged areas of different gas particles repel, while oppositely charged regions of different gas particles attract one another; gases that contain permanently charged ions are known as plasmas. Gaseous compounds with polar covalent bonds contain permanent charge imbalances and so experience relatively strong intermolecular forces, although the molecule while the compound's net charge remains neutral. Transient, randomly-induced charges exist across non-polar covalent bonds of molecules and electrostatic interactions caused by them are referred to as Van der Waals forces. The interaction of these intermolecular forces varies within a substance which determines many of the physical properties unique to each gas.^[5][6] A quick comparison of boiling points for compounds formed by ionic and covalent bonds leads us to this conclusion.^[7] The drifting smoke particles in the image provides some insight into low pressure gas behavior.

Compared to the other states of matter, gases have an incredibly low density and viscosity. Pressure and temperature influence the particles within a certain volume. This variation in particle separation and speed is referred to as compressibility. This particle separation and size influences optical properties of gases as can be found in the following list of refractive indices. Finally, gas particles spread apart or diffuse in order to homogeneously distribute themselves throughout any container.

Macroscopic[link]

Shuttle imagery of re-entry phase.

When observing a gas, it is typical to specify a frame of reference or length scale. A larger length scale corresponds to a macroscopic or global point of view of the gas. This region (referred to as a volume) must be sufficient in size to contain a large sampling of gas particles. The resulting statistical analysis of this sample size produces the "average" behavior (i.e. velocity, temperature or pressure) of all the gas particles within the region. By way of contrast, a smaller length scale corresponds to a microscopic or particle point of view.

From this global vantage point, the gas characteristics measured are either in terms of the gas particles themselves (velocity, pressure, or temperature) or their surroundings (volume). By way of example, Robert Boyle studied pneumatic chemistry for a small portion of his career. One of his experiments related the macroscopic properties of pressure and volume of a gas. His experiment used a J-tube manometer which looks like a test tube in the shape of the letter J. Boyle trapped an inert gas in the closed end of the test tube with a column of mercury, thereby locking the number of particles and temperature. He observed that when the pressure was increased on the gas, by adding more mercury to the column, the trapped gas volume decreased. Mathematicians describe this situation as an inverse relationship. Furthermore, when Boyle multiplied the pressure and volume of each observation, the product (math) was always the same, a constant. This relationship held true for every gas that Boyle observed leading to the law, (PV=k), named to honor his work in this field of study.

There are many math tools to choose from when analyzing gas properties. As gases are subjected to extreme conditions, the math tools become a bit more complex, from the Euler equations (inviscid flow) to the Navier-Stokes equations^[8] that fully account for viscous effects. These equations are tailored to meet the unique conditions of the gas system in question. Boyle's lab equipment allowed the use of algebra to obtain his analytical results. His results were possible because he was studying gases in relatively low pressure situations where they behaved in an "ideal" manner. These ideal relationships enable safety calculations for a variety of flight conditions on the materials in use. The high technology equipment in use today was designed to help us safely explore the more exotic operating environments where the gases no longer behave in an "ideal" manner. This advanced math, to include statistics and multivariable calculus, makes possible the solution to such complex dynamic situations as space vehicle reentry. One such example might be the analysis of the image depicting space shuttle reentry to ensure the material properties under this loading condition are not exceeded. It is safe to say that in this flight regime, the gas is no longer behaving ideally.

Pressure[link]

Pressurized gases.

The symbol used to represent pressure in equations is "p" or "P" with SI units of pascals.

When describing a container of gas, the term pressure (or absolute pressure) refers to the average force the gas exerts on the surface area of the container. Within this volume, it is sometimes easier to visualize the gas particles moving in straight lines until they collide with the container (see diagram at top of the article). The force imparted by a gas particle into the container during this collision is the change in momentum of the particle. As a reminder from classical mechanics, momentum, by definition, is the product of mass and velocity.^[9] Notice that during a collision only the normal component of velocity changes. A particle traveling parallel to the wall never changes its momentum. So the average force on a surface must be the average change in linear momentum from all of these gas particle collisions. To be more precise, pressure is the sum of all the normal components of force exerted by the particles impacting the walls of the container divided by the surface area of the wall. The image "Pressurized gases" depicts gas pressure and temperature spikes used in the entertainment industry.

Temperature[link]

Air balloon shrinks after submerging into liquid nitrogen

The symbol used to represent temperature in equations is T with SI units of kelvins.

The speed of a gas particle is proportional to its absolute temperature. The volume of the balloon in the video shrinks when the trapped gas particles slow down with the addition of extremely cold nitrogen. The temperature of any physical system is related to the motions of the particles (molecules and atoms) which make up the [gas] system.^[10] In statistical mechanics, temperature is the measure of the average kinetic energy stored in a particle. The methods of storing this energy are dictated by the degrees of freedom of the particle itself (energy modes). Kinetic energy added (endothermic process) to gas particles by way of collisions produces linear, rotational, and vibrational motion as well. By contrast, a molecule in a solid can only increase its vibration modes with the addition of heat as the lattice crystal structure prevents both linear and rotational motions. These heated gas molecules have a greater speed range which constantly varies due to constant collisions with other particles. The speed range can be described by the Maxwell-Boltzmann distribution. Use of this distribution implies ideal gases near thermodynamic equilibrium for the system of particles being considered.

Specific volume[link]

Expanding gases link to changes in specific volume.

The symbol used to represent specific volume in equations is "v" with SI units of cubic meters per kilogram.

The symbol used to represent volume in equations is "V" with SI units of cubic meters.

When performing a thermodynamic analysis, it is typical to speak of intensive and extensive properties. Properties which depend on the amount of gas (either by mass or volume) are called extensive properties, while properties that do not depend on the amount of gas are called intensive properties. Specific volume is an example of an intensive property because it is the ratio of volume occupied by a unit of mass of a gas that is identical throughout a system at equilibrium.^[11] 1000 atoms of protactinium as a gas occupy the same space as any other 1000 atoms for any given temperature and pressure. This concept is easier to visualize for solids such as iron which are incompressible compared to gases. When the seat ejection is initiated in the rocket sled image the specific volume increases with the expanding gases, while mass is conserved. Since a gas fills any container in which it is placed, volume is an extensive property.

Density[link]

The symbol used to represent density in equations is ρ (pronounced rho) with SI units of kilograms per cubic meter. This term is the reciprocal of specific volume.

Since gas molecules can move freely within a container, their mass is normally characterized by density. Density is the mass per volume of a substance or simply, the inverse of specific volume. For gases, the density can vary over a wide range because the particles are free to move closer together when constrained by pressure or volume or both. This variation of density is referred to as compressibility. Like pressure and temperature, density is a state variable of a gas and the change in density during any process is governed by the laws of thermodynamics. For a static gas, the density is the same throughout the entire container. Density is therefore a scalar quantity; it is a simple physical quantity that has a magnitude but no direction associated with it. It can be shown by kinetic theory that the density is inversely proportional to the size of the container in which a fixed mass of gas is confined. In this case of a fixed mass, the density decreases as the volume increases.

Microscopic[link]

If one could observe a gas under a powerful microscope, one would see a collection of particles (molecules, atoms, ions, electrons, etc.) without any definite shape or volume that are in more or less random motion. These neutral gas particles only change direction when they collide with another particle or the sides of the container. By stipulating that these collisions are perfectly elastic, this substance is transformed from a real to an ideal gas. This particle or microscopic view of a gas is described by the Kinetic-molecular theory. All of the assumptions behind this theory can be found in the postulates section of Kinetic Theory.

Kinetic theory[link]

Kinetic theory provides insight into the macroscopic properties of gases by considering their molecular composition and motion. Starting with the definitions of momentum and kinetic energy,^[12] one can use the conservation of momentum and geometric relationships of a cube to relate macro system properties of temperature and pressure to the microscopic property of kinetic energy per molecule. The theory provides averaged values for these two properties.

The theory also explains how the gas system responds to change. For example, as a gas is heated from absolute zero, when it is (in theory) perfectly still, its internal energy (temperature) is increased. As a gas is heated, the particles speed up and its temperature rise. This results in greater numbers of collisions with the container sides each second due to the higher particle speeds associated with elevated temperatures. As the number of collisions (per unit time) increase on the surface area of the container, the pressure increases in a proportional manner.

Brownian motion[link]

Brownian motion is the mathematical model used to describe the random movement of particles suspended in a fluid. The gas particle animation, using pink and green particles, illustrates how this behavior results in the spreading out of gases (entropy). These events are also described by particle theory.

Since it is at the limit of (or beyond) current technology to observe individual gas particles (atoms or molecules), only theoretical calculations give suggestions as to how they move, but their motion is different from Brownian Motion. The reason is that Brownian Motion involves a smooth drag due to the frictional force of many gas molecules, punctuated by violent collisions of an individual (or several) gas molecule(s) with the particle. The particle (generally consisting of millions or billions of atoms) thus moves in a jagged course, yet not so jagged as would be expected if an individual gas molecule was examined.

Intermolecular forces[link]

When gases are compressed, intermolecular forces like those shown here start to play a more active role.

As discussed earlier, momentary attractions (or repulsions) between particles have an effect on gas dynamics. In physical chemistry, the name given to these intermolecular forces is van der Waals force. These forces play a key role in determining physical properties of a gas such as viscosity and flow rate (see physical characteristics section). Ignoring these forces in certain conditions (see Kinetic-molecular theory) allows a real gas to be treated like an ideal gas. This assumption allows the use of ideal gas laws which greatly simplifies the path to a solution.

Proper use of these gas relationships requires us to take one more look at the Kinetic-molecular theory (KMT). When these gas particles possess a magnetic charge or Intermolecular force they gradually influence one another as the spacing between them is reduced (the hydrogen bond model illustrates one example). In the absence of any charge, at some point when the spacing between gas particles is greatly reduced they can no longer avoid collisions between themselves at normal gas temperatures found in a lab. Another case for increased collisions among gas particles would include a fixed volume of gas, which upon heating would contain very fast particles. What this means to us is that these ideal equations provide reasonable results except for extremely high pressure [compressible] or high temperature [ionized] conditions. Notice that all of these excepted conditions allow energy transfer to take place within the gas system. The absence of these internal transfers is what is referred to as ideal conditions (perfect – or well behaved) in which the energy exchange occurs only at the boundaries of the system. Real gases experience some of these collisions and intermolecular forces. When these collisions are statistically negligible [incompressible], results from these ideal equations are still valid. At the other end of the spectrum, when the gas particles are compressed into close proximity they behave more like a liquid, and hence another connection to fluid dynamics.

Simplified models[link]

An equation of state (for gases) is a mathematical model used to roughly describe or predict the state properties of a gas. At present, there is no single equation of state that accurately predicts the properties of all gases under all conditions. Therefore, a number of much more accurate equations of state have been developed for gases in specific temperature and pressure ranges. The "gas models" that are most widely discussed are "perfect gas", "ideal gas" and "real gas". Each of these models has its own set of assumptions to facilitate the analysis of a given thermodynamic system.^[13] Each successive model expands the temperature range of coverage to which it applies. The image of first powered flight at Kitty Hawk, North Carolina illustrates one example on the successful application of these relationships in 1903. More recent examples include the 2009 maiden flights of the first solar powered aircraft, the Solar Impulse, and the first commercial airliner to be constructed primarily from composite materials, the Dreamliner.

First flight at Kitty Hawk, NC.

Ideal and perfect gas models[link]

The equation of state for an ideal or perfect gas is the ideal gas law and reads

Failed to parse (Missing texvc executable; please see math/README to configure.): PV=nRT,

where P is the pressure, V is the volume, n is amount of gas (in mol units), R is the universal gas constant, 8.314 J/(mol K), and T is the temperature. Written this way, it is sometimes called the "chemist's version", since it emphasizes the number of molecules n. It can also be written as

Failed to parse (Missing texvc executable; please see math/README to configure.): PV=\rho R_s T,

where Failed to parse (Missing texvc executable; please see math/README to configure.): R_s

is the specific gas constant for a particular gas, in units J/(kg K). This notation is the "gas dynamicist's" version, which is more practical in modeling of gas flows involving acceleration without chemical reactions.

The ideal gas law does not make an assumption about the specific heat of a gas. In the most general case, the specific heat is a function of both temperature and pressure. If the pressure-dependence is neglected (and possibly the temperature-dependence as well) in a particular application, sometimes the gas is said to be a perfect gas, although the exact assumptions may vary depending on the author and/or field of science.

For an ideal gas, the ideal gas law applies without restrictions on the specific heat. An ideal gas is a simplified "real gas" with the assumption that the compressibility factor Z is set to 1 meaning that this pneumatic ratio remains constant. A compressibility factor of one also requires the four state variables to follow the ideal gas law.

This approximation is more suitable for applications in engineering although simpler models can be used to produce a "ball-park" range as to where the real solution should lie. An example where the "ideal gas approximation" would be suitable would be inside a combustion chamber of a jet engine.^[14] It may also be useful to keep the elementary reactions and chemical dissociations for calculating emissions.

Real gas[link]

21 April 1990 eruption of Mount Redoubt, Alaska, illustrating real gases not in thermodynamic equilibrium.

Each one of the assumptions listed below adds to the complexity of the problem's solution. As the density of a gas increases with pressure rises, the intermolecular forces play a more substantial role in gas behavior which results in the ideal gas law no longer providing "reasonable" results. At the upper end of the engine temperature ranges (e.g. combustor sections – 1300 K), the complex fuel particles absorb internal energy by means of rotations and vibrations that cause their specific heats to vary from those of diatomic molecules and noble gases. At more than double that temperature, electronic excitation and dissociation of the gas particles begins to occur causing the pressure to adjust to a greater number of particles (transition from gas to plasma).^[15] Finally, all of the thermodynamic processes were presumed to describe uniform gases whose velocities varied according to a fixed distribution. Using a non-equilibrium situation implies the flow field must be characterized in some manner to enable a solution. One of the first attempts to expand the boundaries of the ideal gas law was to include coverage for different thermodynamic processes by adjusting the equation to read pVⁿ = constant and then varying the n through different values such as the specific heat ratio, γ.

Real gas effects include those adjustments made to account for a greater range of gas behavior:

• Compressibility effects (Z allowed to vary from 1.0)
• Variable heat capacity (specific heats vary with temperature)
• Van der Waals forces (related to compressibility, can substitute other equations of state)
• Non-equilibrium thermodynamic effects
• Issues with molecular dissociation and elementary reactions with variable composition.

For most applications, such a detailed analysis is excessive. Examples where "Real Gas effects" would have a significant impact would be on the Space Shuttle re-entry where extremely high temperatures and pressures are present or the gases produced during geological events as in the image of the 1990 eruption of Mount Redoubt.

Historical synthesis[link]

Boyle's law[link]

Boyle's equipment.

Boyle's Law was perhaps the first expression of an equation of state. In 1662 Robert Boyle performed a series of experiments employing a J-shaped glass tube, which was sealed on one end. Mercury was added to the tube, trapping a fixed quantity of air in the short, sealed end of the tube. Then the volume of gas was carefully measured as additional mercury was added to the tube. The pressure of the gas could be determined by the difference between the mercury level in the short end of the tube and that in the long, open end. Through these experiments, Boyle noted that the gas volume varied inversely with the pressure.^[16] The image of Boyle's Equipment shows some of the exotic tools used by Boyle during his study of gases.

• Boyle's Law – describes a gas in which the number of particles and Temperature are constant.
• PV = constant in this situation constant = nRT from the ideal gas law.

Law of volumes[link]

In 1787, the French physicist and balloon pioneer, Jacques Charles, found that oxygen, nitrogen, hydrogen, carbon dioxide, and air expand to the same extent over the same 80 kelvin interval.

In 1802, Joseph Louis Gay-Lussac published results of similar, though more extensive experiments, indicating a linear relationship between volume and temperature. Gay-Lussac credited Charle's earlier work by naming the law in his honor. In the absence of this linkage, Dalton could have been in contention for this honor for his previously published work on partial pressures.

• Law of Volumes – Both Charles and Gay-Lussac played a role in developing this relationship.^[17]
• V/T = constant – notice that constant = nR/P from the ideal gas law.

Avogadro's law[link]

Dalton's notation.

In 1811, Amedeo Avogadro verified that equal volumes of pure gases contain the same number of particles. His theory was not generally accepted until 1858 when another Italian chemist Stanislao Cannizzaro was able to explain non-ideal exceptions. For his work with gases a century prior, the number that bears his name Avogadro's constant represents the number of atoms found in 12 grams of elemental carbon-12 (6.022×10²³ mol⁻¹). This specific number of gas particles, at standard temperature and pressure (ideal gas law) occupies 22.40 liters, which is referred to as the molar volume.