Oxygen
| Infobox on Oxygen | |
|---|---|
| Example of Oxygen |  |
| Facts | |
| Origin | - |
| Stowage factor (in m3/t) | - |
| Humidity / moisture | - |
| Ventilation | - |
| Risk factors | See text |
Oxygen
Contents
Description
Oxygen is a chemical element with symbol O and atomic number 8. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colourless, odourless, tasteless diatomic gas with the formula O2. This substance is an important part of the atmosphere, and is necessary to sustain most terrestrial life.
Oxygen is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element that readily forms compounds (notably oxides) with most elements except the noble gases Helium and Neon. Oxygen is a strong oxidizing agent and only fluorine has greater electronegativity. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium and the most abundant element by mass in the Earth's crust, making up almost half of the crust's mass. Oxygen is too chemically reactive to remain a free element in Earth's atmosphere without being continuously replenished by the photosynthetic action of living organisms, which use the energy of sunlight to produce elemental oxygen from water. Free elemental O2 only began to accumulate in the atmosphere about 2.5 billion years ago. Diatomic oxygen gas constitutes 20.8% of the volume of air.
Oxygen constitutes most of the mass of living organisms, because water is their major constituent (for example, about two-thirds of human body mass. Many major classes of organic molecules in living organisms, such as proteins, nucleic acids, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that are constituents of animal shells, teeth, and bone. Elemental oxygen is produced by cyanobacteria, algae and plants, and is used in cellular respiration for all complex life. Oxygen is toxic to obligately anaerobic organisms, which were the dominant form of early life on Earth until O2 began to accumulate in the atmosphere. Another form (allotrope) of oxygen, ozone (O3), strongly absorbs UVB radiation and consequently the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation, but is a pollutant near the surface where it is a by-product of smog. At even higher low earth orbit altitudes, atomic oxygen is a significant presence and a cause of erosion for spacecraft.
Oxygen is produced industrially by fractional distillation of liquefied air, use of zeolites with pressure-cycling to concentrate oxygen from air, electrolysis of water and other means. Uses of elemental oxygen include the production of steel, plastics and textiles, brazing, welding and cutting of steels and other metals, rocket propellant, oxygen therapy and life support systems in aircraft, submarines, spaceflight and diving.
At standard temperature and pressure, oxygen is a colourless, odourless gas with the molecular formula O2, in which the two oxygen atoms are chemically bonded to each other with a spin triplet electron configuration. This bond has a bond order of two, and is often simplified in description as a double bond or as a combination of one two-electron bond and two three-electron bonds.
Oxygen is more soluble in water than nitrogen is. Water in equilibrium with air contains approximately 1 molecule of dissolved O2 for every 2 molecules of N2, compared to an atmospheric ratio of approximately 1:4. The solubility of oxygen in water is temperature-dependent, and about twice as much dissolves at 0°C than at 20°C. At 25°C and 1 standard atmosphere (101.3 kPa) of air, freshwater contains about 6.04 milliliters (mL) of oxygen per liter, whereas seawater contains about 4.95 mL per liter. At 5°C the solubility increases to 9.0 mL (50% more than at 25°C) per liter for water and 7.2 mL (45% more) per liter for sea water.
Oxygen condenses at 90.20 K (−182.95°C, −297.31°F), and freezes at 54.36 K (−218.79°C, −361.82°F). Both liquid and solid O2 are clear substances with a light sky-blue colour caused by absorption in the red (in contrast with the blue colour of the sky, which is due to Rayleigh scattering of blue light). High-purity liquid O2 is usually obtained by the fractional distillation of liquefied air. Liquid oxygen may also be produced by condensation out of air, using liquid nitrogen as a coolant. It is a highly reactive substance and must be segregated from combustible materials.
Two major methods are employed to produce 100 million tonnes of O2 extracted from air for industrial uses annually. The most common method is to fractionally distill liquefied air into its various components, with N2 distilling as a vapour while O2is left as a liquid. A drawing of three vertical pipes connected at the bottom and filled with oxygen (left pipe), water (middle) and hydrogen (right). Anode and cathode electrodes are inserted into the left and right pipes and externally connected to a battery.
The other major method of producing O2 gas involves passing a stream of clean, dry air through one bed of a pair of identical zeolite molecular sieves, which absorbs the nitrogen and delivers a gas stream that is 90% to 93% O2. Simultaneously, nitrogen gas is released from the other nitrogen-saturated zeolite bed, by reducing the chamber operating pressure and diverting part of the oxygen gas from the producer bed through it, in the reverse direction of flow. After a set cycle time the operation of the two beds is interchanged, thereby allowing for a continuous supply of gaseous oxygen to be pumped through a pipeline. This is known as pressure swing adsorption. Oxygen gas is increasingly obtained by these non-cryogenic technologies.
Oxygen gas can also be produced through electrolysis of water into molecular oxygen and hydrogen. DC electricity must be used: if AC is used, the gases in each limb consist of hydrogen and oxygen in the explosive ratio 2:1. Contrary to popular belief, the 2:1 ratio observed in the DC electrolysis of acidified water does not prove that the empirical formula of water is H2O unless certain assumptions are made about the molecular formulae of hydrogen and oxygen themselves. A similar method is the electrocatalytic O2 evolution from oxides and oxoacids. Chemical catalysts can be used as well, such as in chemical oxygen generators or oxygen candles that are used as part of the life-support equipment on submarines, and are still part of standard equipment on commercial airliners in case of depressurization emergencies. Another air separation technology involves forcing air to dissolve through ceramic membranes based on zirconium dioxide by either high pressure or an electric current, to produce nearly pure O2 gas.
In large quantities, the price of liquid oxygen in 2001 was approximately $0.21/kg. Since the primary cost of production is the energy cost of liquefying the air, the production cost will change as energy cost varies.
Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.
Liquid oxygen has a pale blue colour and is strongly paramagnetic, it can be suspended between the poles of a powerful horseshoe magnet. Liquid oxygen has a density of 1.141 g/cm3 (1.141 kg/L) and is cryogenic with a freezing point of 50.5 K (−368.77°F; −222.65°C) and a boiling point of 90.19 K (−297.33°F, −182.96°C) at 101.325 kPa (760 mmHg). Liquid oxygen has an expansion ratio of 1:861 under 1 standard atmosphere (100 kPa) and 20°C (68°F).
Because of its cryogenic nature, liquid oxygen can cause the materials it touches to become extremely brittle. Liquid oxygen is also a very powerful oxidizing agent: organic materials will burn rapidly and energetically in liquid oxygen. Further, if soaked in liquid oxygen, some materials such as coal briquettes, Carbon Black, etc., can detonate unpredictably from sources of ignition such as flames, sparks or impact from light blows. Petrochemicals often exhibit this behaviour, including asphalt.
Liquid nitrogen has a lower boiling point at −196°C (77 K) than oxygen's −183°C (90 K), and vessels containing liquid nitrogen can condense oxygen from air: when most of the nitrogen has evaporated from such a vessel there is a risk that liquid oxygen remaining can react violently with organic material. Conversely, liquid nitrogen or liquid air can be oxygen-enriched by letting it stand in open air; atmospheric oxygen dissolves in it, while nitrogen evaporates preferentially.