water gas, colorless poisonous gas that burns with an intensely hot, bluish (nearly colorless) flame. The gas is a mixture of carbon monoxide and hydrogen with very small amounts of other gases, e.g., carbon dioxide, and is almost entirely combustible as a result. Water gas is so named because of the use of water (steam) in its preparation. This process involves treating white-hot hard coal or coke with a blast of steam; carbon monoxide and hydrogen are formed. The gas is manufactured in vast quantities for commercial use. It is of much importance in the preparation of hydrogen and as a fuel in the making of steel and in other industrial processes, e.g., the Fischer-Tropsch process.

universal gas constant: see gas laws.

tear gas, gas that causes temporary blindness through the excessive flow of tears resulting from irritation of the eyes. The gas is used in chemical warfare and as a means for dispersing mobs. Compounds that cause lacrimation (watering of the eyes) include bromoacetone, benzyl bromide, chloroacetophenone, ethyl iodoacetate, chloropicrin bromobenzyl cyanide, and bromine-substituted xylenes. In warfare a lacrimating compound in liquid form is placed in bombs, shells, or grenades; when the device explodes, the liquid is dispersed as an aerosol.

sneeze gas, poison gas used in riot control and chemical warfare. It is highly irritating to the nose and throat; inhalation of the substance causes sneezing and coughing, but rarely causes permanent injury. Mixed with CN (common tear gas, chloroacetophenone), it is used by police for dispersing crowds. Chemically, it is diphenylamine chloroarsine; sneeze gas is prepared by heating diphenylamine with arsenic trichloride.

riot-control gas: see poison gas; tear gas.

rare gas: see inert gas.

producer gas, fuel gas consisting chiefly of carbon monoxide and nitrogen. It is prepared in a furnace or generator in which air is forced upward through a burning fuel of coal or coke. Although the fuel is introduced through the top, no air is admitted there. The carbon of the fuel is oxidized by the oxygen of the air from below to form the carbon monoxide. The nitrogen of the air, being inert, passes through the fire without change. When steam is introduced with the air, the final gaseous product contains hydrogen also. Producer gas has a low heating value because it is about 60% inert nitrogen. It is widely used in industry because it can be made with cheap fuel. When producer gas contains hydrogen, it is also a source material for the manufacture of synthetic ammonia.

poison gas, any of various gases sometimes used in warfare or riot control because of their poisonous or corrosive nature. These gases may be roughly grouped according to the portal of entry into the body and their physiological effects. Vesicants (blister gases) produce blisters on all body surfaces (see lewisite; mustard gas); lacrimators (tear gas) produce severe eye irritation; sternutators (vomiting gases) cause nausea; nerve gases inhibit proper nerve function; and lung irritants attack the respiratory tract, causing pulmonary edema. By the middle of the 19th cent. the possibility of the use of poison gas as a weapon was already envisaged and was viewed by most people with a peculiar horror—a feeling that has persisted. The first effective use of poison gas came in World War I, when the Germans released (1915) chlorine gas against the Allies in the Ypres sector of the Western Front. The success was immediate, but the attackers, uncertain as to the effect, failed to pursue the retreating French. Shortly afterward protective measures (see gas mask) were introduced as both sides used gas more extensively. The gas shell (much more suitable than wind-blown gas) was introduced by the French. Gas did not have any dominant influence on the course of the war, but it did seem to point toward wide-scale use in the future. However, except for the use of poison gas by the Italians in the war against Ethiopia (1935-36) and by the Japanese against Chinese guerrillas (1937-42), poison gas was not employed in warfare after World War I out of fear of retribution, even though the military powers of the world continued to develop new gases. Poison gas was used in the Iran-Iraq War, and Iraq has used poison gas on its own civilians, in particular the Kurds. In the Persian Gulf War, the UN troops were equipped with antidotes for nerve gas, protective clothing, and gas masks in case Iraq used poison gas. The 1989 treaty between the United States and the USSR provided for an end to production of poison gas and the beginning of destruction of current stockpiles. See also chemical warfare.

oil gas, any of a group of fuel gases produced from oil by exposing it to high temperatures. High-Btu oil gas is so called because of its high heating value; it is often used to supplement natural gas during periods of high demand. Refinery oil gases are produced as byproducts during normal heat treatment in oil refining. Their chief use is in the heating of refinery equipment. Typically, oil gas consists of methane, ethane, propane, butane, and some of their derivatives.

noble gas: see inert gas.

nerve gas, any of several poison gases intended for military use, e.g., tabun, sarin, soman, and VX. Nerve gases were first developed by Germany during World War II but were not used at that time. These gases generally cause death by asphyxiation, often preceded by such symptoms as blurred vision, excessive salivation, and convulsions. Physiologically, the toxic effect of nerve gases arises because they inactivate the enzyme cholinesterase, which normally controls the transmission of nerve impulses; the impulses continue without control, causing breakdown of respiration and other body functions. Atropine is an effective antidote against most nerve gases. See also chemical warfare.

natural gas, natural mixture of gaseous hydrocarbons found issuing from the ground or obtained from specially driven wells. The composition of natural gas varies in different localities. Its chief component, methane, usually makes up from 80% to 95%, and the balance is composed of varying amounts of ethane, propane, butane, and other hydrocarbon compounds. Some of the hydrocarbons found in gasoline also occur as vapors in natural gas; by liquefying these hydrocarbons, gasoline can be obtained.

Although commonly associated with petroleum deposits it also occurs separately in sand, sandstone, and limestone deposits. Some geologists theorize that natural gas is a byproduct of decaying vegetable matter in underground strata, while others think it may be primordial gases that rise up from the mantle. Because of its flammability and high calorific value, natural gas is used extensively as an illuminant and a fuel.

Natural gas was known to the ancients but was considered by them to be a supernatural phenomenon because, noticed only when ignited, it appeared as a mysterious fire bursting from the ground. One of the earliest attempts to harness it for economic use occurred in the early 19th cent. in Fredonia, N.Y. Toward the latter part of the 19th cent., large industrial cities began to make use of natural gas, and extensive pipeline systems have been constructed to transport gas.

Liquefied natural gas, or LNG, is natural gas that has been pressurized and cooled so as to liquefy it for convenience in shipping and storage. The boiling point of natural gas is extremely low, and only in the 1970s did cryogenic technology (see low-temperature physics) advance enough to make the production and transport of LNG commerically feasible. Some of the natural gas moved to and from the United States is carried as LNG in special tankers.

mustard gas, chemical compound used as a poison gas in World War I. The burning sensation it causes on contact with the skin is similar to that caused by oil from black mustard seeds. The compound is not a gas but a colorless, oily liquid with a somewhat sweet, agreeable odor; it boils at 217°C;. A powerful vesicant, mustard gas causes severe blistering even in small quantities. Highly irritating to the eyes, it quickly causes conjunctivitis and blindness. If inhaled, it attacks the respiratory tract and lungs, causing pulmonary edema. Some effects of exposure to mustard gas are delayed up to 12 hr; death may result several days after exposure. Mustard gas was introduced by the Germans in warfare against the British at Ypres, Belgium, in July, 1917, and took a heavy toll of casualties. It is dispersed as an aerosol by a bursting shell. Chemically, mustard gas is a thioether, 2,2'-dichlorodiethyl sulfide, (ClCH₂CH₂)₂S. It can be prepared by reacting ethylene with sulfur monochloride, S₂Cl₂, or by other methods. Its vesicant property is readily destroyed either by oxidation or by chlorination (e.g., with bleaching powder).

marsh gas: see methane.

liquefied petroleum gas or LPG, mixture of gases, chiefly propane and butane, produced commercially from petroleum and stored under pressure to keep it in a liquid state. The boiling point of liquefied petroleum gas varies from about -44°C; to 0°C; (-47°F; to 32°F;), so that the pressure required to liquefy it is considerable and the containers for it must be of heavy steel. When prepared as fuel, LPG is largely propane; common uses are for powering automotive vehicles, for cooking and heating, and sometimes for lighting in rural areas. LPG is an attractive fuel for internal-combustion engines; because it burns with little air pollution and little solid residue, it does not dilute lubricants, and it has a high octane rating.

liquefied natural gas: see under natural gas.

laughing gas: see nitrous oxide.

inert gas or noble gas, any of the elements in Group 18 of the periodic table. In order of increasing atomic number they are: helium, neon, argon, krypton, xenon, and radon. They are colorless, odorless, tasteless gases and were once believed to be entirely inert, i.e., forming no chemical compounds; however, some compounds of these elements have been produced, i.e., fluorides of krypton, xenon, and radon. The low chemical activity of the inert gases is due to the fact that their outermost, or valence, electron shell is complete, containing two electrons in the case of helium and eight in the remaining cases. The inert gases are sometimes called the rare gases, although argon is not rare (it makes up about 1% of the atmosphere) and helium is commercially extracted from natural gas and the atmosphere.

ideal gas: see gas laws; kinetic-molecular theory of gases.

gas, fuel, gaseous substance that burns in air and releases enough heat to be useful as a fuel, while also remaining sufficiently stable at ordinary temperatures to permit long-term storage without deterioration or undue hazard. It is advantageous if a fuel gas is readily transportable through pipes and is easily liquefied. Practically all fuel gases meet the first condition, and some meet the second as well. Natural gas, which occurs alone and in conjunction with petroleum deposits, is an excellent fuel gas in wide commercial use. Liquefied petroleum gas is a manufactured mixture of flammable gases that is easily stored in its liquefied condition. Oil gas is a type of gas made by applying heat to various petroleum distillates. Its principal use is as a supplement to natural gas during periods of heavy demand. Coal gas may be any of a variety of gases produced by heating coal in the absence of air and driving off the volatile constituents. It is not as high in fuel value as other gases and often contains tars, light oils, ammonia, and hydrogen sulfide. Producer gas is made by forcing a mixture of air and steam through burning coal or coke. Water gas, or blue gas, which burns with a bright blue flame, is made by passing steam over glowing coke.

gas tube: see electron tube.

gas mask, face covering or device used to protect the wearer from injurious gases and other noxious materials by filtering and purifying inhaled air. In addition to military use (see chemical warfare), gas masks are employed in mining, in industrial chemistry, and by firemen and rescue squads. The gas mask consists essentially of a face cover with two eyepieces and a mouthpiece that communicates with a canister containing a filter. The filter absorbs noxious gases as they pass through the canister to the mouth. The face cover also has a one-way outlet valve for exhaled air. See poison gas.

gas laws, physical laws describing the behavior of a gas under various conditions of pressure, volume, and temperature. Experimental results indicate that all real gases behave in approximately the same manner, having their volume reduced by about the same proportion of the original volume for each drop of 1° on the Celsius temperature scale. Graphs drawn to describe this behavior can be extrapolated, and all converge to a point corresponding to about -273°C; (-459°F;)—this point is called absolute zero. A temperature scale defined so that zero degrees corresponds to this zero-volume temperature coordinate is known as an absolute scale. The Kelvin temperature scale begins at this absolute zero and has degrees the same size as those of the Celsius scale.

Gas Laws Relating Two Variables

The simplest gas laws relate pressure, volume, and temperature in pairs. Boyle's law (advanced by Robert Boyle in 1662) states that the pressure and volume of a gas are inversely proportional to one another, or PV = k, where P is pressure, V is volume, and k is a constant of proportionality. Charles's law (published by Jacques A. C. Charles in 1787), sometimes known as Gay-Lussac's law (independently demonstrated by Joseph Gay-Lussac in 1802), states that the volume of an enclosed gas is directly proportional to its temperature, or V = kT. This expression is strictly true only if the temperature is measured on an absolute scale. A third law states that the pressure is directly proportional to the absolute temperature, or P = kT.

Gas Laws Relating Three Variables

The three gas laws relating two variables can be combined into a single law relating pressure, temperature, and volume, which states that the product of pressure and volume is directly proportional to the absolute temperature, or PV = kT. This law describes the behavior of real gases only with a certain range of values for the variables. At temperatures or pressures near those at which the gas condenses to a liquid, the behavior departs from this equation. Nevertheless, it is useful to consider an ideal gas, or perfect gas, an imaginary substance that conforms to this equation for all values of the variables.

The behavior of an ideal gas can be described in terms of the kinetic-molecular theory of gases and leads directly to the relationship PV = kT, which is therefore called the ideal gas law, or general gas law. The constant of proportionality k is usually expressed as the product of the number of moles, n, of the gas and a constant R, known as the universal gas constant. In MKS units, R has the value 8.3149 × 10³ joules/kilogram-mole-degree. The ideal gas law can be further simplified by replacing the ordinary volume V by the specific volume v, which is equal to V/n. The law then has the form Pv = RT. This form has the advantage that all of the variables are intensive; that is, none of the variables depends on the mass of the gas.

The van der Waals equation (for the Dutch physicist Johannes van der Waals) is another gas law involving pressure, temperature, and volume. It takes into account the variations in behavior of different real gases from that of an ideal gas. The van der Waals equation is usually given as (P + a/v²)(v - b) = RT, where a and b are constants that have different particular values for different real gases. Other, more complicated equations exist that describe the behavior of real gases over an even wider range of values for pressure, temperature, and volume.

See also thermodynamics.

gas, in physics, one of the three commonly recognized states of matter, the other two being solid and liquid. A substance in the gaseous state has neither definite shape nor definite volume. Like liquids, gases are fluids and assume the shape of their containers. Unlike liquids, they will expand to fill any container, regardless of its size. All gases condense into liquids or solids when sufficiently cooled or compressed (see compression; condensation; liquefaction). Most gases first liquefy, but some pass directly into the solid state (see sublimation); carbon dioxide, for example, can condense into dry ice. Some gases are extremely soluble in certain liquids, the liquid absorbing many times its own volume of gas. Some solids, by a process called adsorption, can take up many times their own volume of certain gases. The behavior of gases under various conditions of pressure, temperature, and volume is described by the various gas laws. Many of the properties of gases can be understood by considering the fact that only a small part of the volume of a gas is occupied by its atoms or molecules, which are in rapid, random motion. See kinetic-molecular theory of gases.

coal gas, gas obtained in the destructive distillation of soft coal, as a byproduct in the preparation of coke. Its composition varies, but in general it is made up largely of hydrogen and methane with small amounts of other hydrocarbons, carbon monoxide (a poisonous gas), carbon dioxide, and nitrogen. It is used as a fuel and illuminant.

blister gas: see poison gas.

Any of a group of substances, most often synthetic organic halogen compounds, that irritate the mucous membranes of the eyes, causing a stinging sensation and tears. They may also irritate the upper respiratory tract, causing coughing, choking, and general debility. Tear gas was first used in warfare in World War I, but since its effects are short-lasting and rarely disabling, it came into use by law-enforcement agencies as a means of dispersing mobs, disabling rioters, and flushing out armed suspects without the use of deadly force.

Learn more about tear gas with a free trial on Britannica.com.

or marsh gas

Organic compound, chemical formula CH₄, colourless, odourless gas that occurs in natural gas (called firedamp in coal mines) and from bacterial decomposition of vegetation in the absence of oxygen (including in the rumens of cattle and other ruminants and in the gut of termites). The simplest member of the paraffin hydrocarbons, methane burns readily, forming carbon dioxide and water if supplied with enough oxygen for complete combustion or carbon monoxide if the oxygen is insufficient. Mixtures of 5–14percnt methane in air are explosive and have caused many mine disasters. The chief source of methane is natural gas, but it can also be produced from coal. Abundant, cheap, and clean, methane is used widely as a fuel in homes, commercial establishments, and factories; as a safety measure, it is mixed with trace amounts of an odorant to allow its detection. It is also a raw material for many industrial materials, including fertilizers, explosives, chloroform, carbon tetrachloride, and carbon black, and is the principal source of methanol.

Learn more about methane with a free trial on Britannica.com.

or inert gas

Any of the seven chemical elements that make up the rightmost group of the periodic table as usually arranged: helium, neon, argon, krypton, xenon, radon, and element 118. All are colourless, odourless, and nonflammable and, except for element 118, occur in tiny amounts in the atmosphere (though helium is the most plentiful element in the universe after hydrogen). Their stable electronic configurations, with no unpaired electrons to share, make them extremely unreactive—hence “noble” (i.e., aloof) or inert—though krypton, xenon, and radon, with outer electrons held less firmly, can form compounds (mainly with fluorine). These gases absorb and give off electromagnetic radiation in a much less complex way than other substances, a property exploited in their use in fluorescent lighting devices and discharge lamps. They glow with a characteristic colour when confined in a transparent container at low pressure with an electric current passing through it. Their very low boiling and melting points make them useful as refrigerants for low-temperature research (see cryogenics).

Learn more about noble gas with a free trial on Britannica.com.

or ideal gas

Gas whose physical behaviour conforms to the general gas law, which states that for a given quantity of gas, the product of the volume math.V and pressure math.P is proportional to the absolute temperature math.T, or math.Pmath.V = math.kmath.T, where math.k is a constant. A perfect gas is assumed to consist of a large number of molecules in random motion, which obey Newton's laws of motion. Their volume is assumed to be negligibly small, and no forces are presumed to act on the molecules except during momentary collisions. Though no gas has these properties, real gases at sufficiently high temperatures and low pressures can be described this way.

Learn more about perfect gas with a free trial on Britannica.com.

Colourless, highly flammable gaseous hydrocarbon consisting primarily of methane and ethane. It may also contain heavier hydrocarbons, carbon dioxide, hydrogen, hydrogen sulfide, nitrogen, helium, and argon. It commonly occurs in association with crude oil (see petroleum). Natural gas is extracted from wells drilled into the Earth. Some natural gas can be used as it comes from the well, without any refining, but most requires processing. It is transported either in its natural gaseous state by pipeline or, after liquefaction by cooling, by tankers. Liquefied natural gas occupies only about 1/600 of the volume of the gas. It has grown steadily as a source of energy since the 1930s.

Learn more about natural gas with a free trial on Britannica.com.

or laughing gas

Inorganic compound, one of the oxides of nitrogen. A colourless gas with a pleasantly sweetish odour and taste, it has an analgesic effect when inhaled; it is used as an anesthetic (often called just “gas”) in dentistry and surgery. This effect is preceded by mild hysteria, sometimes with laughter, hence the name laughing gas. It is also used as a propellant in food aerosols and as a leak detector.

Learn more about nitrous oxide with a free trial on Britannica.com.

Volatile material (mostly swallowed air, partly digestive by-products) in the digestive tract, which normally contains 150–500 cc of gas. Air in the stomach is either belched out or passed to the intestines. Some of its oxygen is absorbed into the blood along the way. Carbon dioxide produced by digestion is added. Nitrogen, the major component, is inert and usually passed on. Obstructions in the small intestine can trap gas in distended pockets, causing severe pain. In the large intestine, bacterial fermentation products are added—mostly hydrogen but also methane, hydrogen sulfide, ammonia, and sulfur-containing mercaptans. Excess gas in the colon is eventually expelled from the body.

Learn more about intestinal gas with a free trial on Britannica.com.

or vapor lamp

Lighting device consisting of a transparent container within which a gas is energized by an applied voltage and made to glow. After practical generators were devised in the 19th century, many experimenters applied electric power to tubes of gas. From circa 1900, electric discharge lamps were in use in Europe and the U.S. Fluorescent, neon, mercury, sodium, and metal-halide lamps are of the electric discharge variety.

Learn more about electric discharge lamp with a free trial on Britannica.com.

Type of chromatography with a gas mixture as the mobile phase. In a packed column, the packing or solid support (held in a tube) serves as the stationary phase (vapour-phase chromatography, or VPC) or is coated with a liquid stationary phase (gas-liquid chromatography, or GLC). In capillary columns, the stationary phase coats the walls of small-diameter tubes. The sample of gas or volatile liquid to be analyzed is injected into the inlet; its components move through with a carrier gas (usually hydrogen, helium, or argon) at rates influenced by their degree of interaction with the stationary phase. The temperature, nature of the stationary phase, and column length can be varied to improve separation. The gas stream issuing from the column's end may pass through a thermal conductivity detector or a flame ionization detector, where its properties are compared with those of known reference substances. GC is used to measure air pollutants, essential oils, gases or alcohol in blood, and composition of industrial process streams.

Learn more about gas chromatography (GC) with a free trial on Britannica.com.

One of the three fundamental states of matter, in which matter has no definite shape, is very fluid, and has a density about 0.1percnt that of liquids. Gas is very compressible but tends to expand indefinitely, and it fills any container. A small change in temperature or pressure produces a substantial change in its volume; these relationships are expressed as equations in the gas laws. The kinetic theory of gases, developed in the 19th century, describes gases as assemblages of tiny particles (atoms or molecules) in constant motion and contributed much to an understanding of their behaviour. The term gas can also mean gasoline, natural gas, or the anesthetic nitrous oxide. Seealso solid.

Learn more about gas with a free trial on Britannica.com.