Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is hard and ductile. Pure nickel, powdered to maximize the reactive surface area, shows a significant chemical activity, but larger pieces are slow to react with air under standard conditions because an oxide layer forms on the surface and prevents further corrosion. So, pure native nickel is found in Earth's crust only in tiny amounts in ultramafic rocks, in the interiors of larger nickel–iron meteorites that were not exposed to oxygen when outside Earth's atmosphere. Meteoric nickel is found in combination with iron, a reflection of the origin of those elements as major end products of supernova nucleosynthesis. An iron–nickel mixture is thought to compose Earth's outer and inner cores. Use of nickel has been traced as far back as 3500 BCE. Nickel was first isolated and classified as a chemical element in 1751 by Axel Fredrik Cronstedt, who mistook the ore for a copper mineral, in the cobalt mines of Los, Hälsingland, Sweden.
The element's name comes from a mischievous sprite of German miner mythology, who personified the fact that copper-nickel ores resisted refinement into copper. An economically important source of nickel is the iron ore limonite, which contains 1–2% nickel. Nickel's other important ore minerals include pentlandite and a mixture of Ni-rich natural silicates known as garnierite. Major production sites include the Sudbury region in Canada, New Caledonia in the Pacific, Norilsk in Russia. Nickel is oxidized by air at room temperature and is considered corrosion-resistant, it has been used for plating iron and brass, coating chemistry equipment, manufacturing certain alloys that retain a high silvery polish, such as German silver. About 9% of world nickel production is still used for corrosion-resistant nickel plating. Nickel-plated objects sometimes provoke nickel allergy. Nickel has been used in coins, though its rising price has led to some replacement with cheaper metals in recent years. Nickel is one of four elements that are ferromagnetic at room temperature.
Alnico permanent magnets based on nickel are of intermediate strength between iron-based permanent magnets and rare-earth magnets. The metal is valuable in modern times chiefly in alloys. A further 10% is used for nickel-based and copper-based alloys, 7% for alloy steels, 3% in foundries, 9% in plating and 4% in other applications, including the fast-growing battery sector; as a compound, nickel has a number of niche chemical manufacturing uses, such as a catalyst for hydrogenation, cathodes for batteries and metal surface treatments. Nickel is an essential nutrient for some microorganisms and plants that have enzymes with nickel as an active site. Nickel is a silvery-white metal with a slight golden tinge, it is one of only four elements that are magnetic at or near room temperature, the others being iron and gadolinium. Its Curie temperature is 355 °C; the unit cell of nickel is a face-centered cube with the lattice parameter of 0.352 nm, giving an atomic radius of 0.124 nm. This crystal structure is stable to pressures of at least 70 GPa.
Nickel belongs to the transition metals. It is hard and ductile, has a high for transition metals electrical and thermal conductivity; the high compressive strength of 34 GPa, predicted for ideal crystals, is never obtained in the real bulk material due to the formation and movement of dislocations. The nickel atom has two electron configurations, 3d8 4s2 and 3d9 4s1, which are close in energy – the symbol refers to the argon-like core structure. There is some disagreement. Chemistry textbooks quote the electron configuration of nickel as 4s2 3d8, which can be written 3d8 4s2; this configuration agrees with the Madelung energy ordering rule, which predicts that 4s is filled before 3d. It is supported by the experimental fact that the lowest energy state of the nickel atom is a 3d8 4s2 energy level the 3d8 4s2 3F, J = 4 level. However, each of these two configurations splits into several energy levels due to fine structure, the two sets of energy levels overlap; the average energy of states with configuration 3d9 4s1 is lower than the average energy of states with configuration 3d8 4s2.
For this reason, the research literature on atomic calculations quotes the ground state configuration of nickel as 3d9 4s1. The isotopes of nickel range in atomic weight from 48 u to 78 u. Occurring nickel is composed of five stable isotopes. Isotopes heavier than 62Ni cannot be formed by nuclear fusion without losing energy. Nickel-62 has the highest mean nuclear binding energy per nucleon of any nuclide, at 8.7946 MeV/nucleon. Its binding energy is greater than both 56Fe and 58Fe, more abundant elements incorrectly cited as having the most tightly-bound nuclides. Although this would seem to predict nickel-62 as the most abundant heavy element in the universe, the high rate of photodisintegration of nickel in stellar interiors causes iron to be by far the most abundant. Stable isotope nickel-60 is the daughter product of the extinct radionuclide 60Fe, whi
Calcium chloride is an inorganic compound, a salt with the chemical formula CaCl2. It is a white coloured crystalline solid at room temperature soluble in water. Calcium chloride is encountered as a hydrated solid with generic formula CaCl2x, where x = 0, 1, 2, 4, 6; these compounds are used for de-icing and dust control. Because the anhydrous salt is hygroscopic, it is used as a desiccant. By depressing the freezing point of water, calcium chloride is used to prevent ice formation and is used to de-ice; this application consumes the greatest amount of calcium chloride. Calcium chloride is harmless to plants and soil; as a de-icing agent, it is much more effective at lower temperatures than sodium chloride. When distributed for this use, it takes the form of small, white spheres a few millimeters in diameter, called prills. Solutions of calcium chloride can prevent freezing at temperature as low as −52 °C, making it ideal for filling agricultural implement tires as a liquid ballast, aiding traction in cold climates.
It is used in domestic and industrial chemical air dehumidifiers. The second largest application of calcium chloride exploits hygroscopic properties and the tackiness of its hydrates. A concentrated solution keeps a liquid layer on the surface of dirt roads, which suppresses formation of dust, it keeps. If these are allowed to blow away, the large aggregate begins to shift around and the road breaks down. Using calcium chloride reduces the need for grading by as much as 50% and the need for fill-in materials as much as 80%; the average intake of calcium chloride as food additives has been estimated to be 160–345 mg/day. Calcium chloride is permitted as a food additive in the European Union for use as a sequestrant and firming agent with the E number E509, it is considered as recognized as safe by the U. S. Food and Drug Administration, its use in organic crop production is prohibited under the US National Organic Program. In marine aquariums, calcium chloride is one way to introduce bioavailable calcium for calcium carbonate-shelled animals such as mollusks and some cnidarians.
Calcium hydroxide or a calcium reactor can be used. As a firming agent, calcium chloride is used in canned vegetables, in firming soybean curds into tofu and in producing a caviar substitute from vegetable or fruit juices, it is used as an electrolyte in sports drinks and other beverages, including bottled water. The salty taste of calcium chloride is used to flavor pickles without increasing the food's sodium content. Calcium chloride's freezing-point depression properties are used to slow the freezing of the caramel in caramel-filled chocolate bars, it is added to sliced apples to maintain texture. In brewing beer, calcium chloride is sometimes used to correct mineral deficiencies in the brewing water, it affects flavor and chemical reactions during the brewing process, can affect yeast function during fermentation. In cheesemaking, calcium chloride is sometimes added to processed milk to restore the natural balance between calcium and protein in casein, it is added before the coagulant. Calcium chloride is used to prevent cork spot and bitter pit on apples by spraying on the tree during the late growing season.
Drying tubes are packed with calcium chloride. Kelp is dried with calcium chloride for use in producing sodium carbonate. Anhydrous calcium chloride has been approved by the FDA as a packaging aid to ensure dryness; the hydrated salt can be dried for re-use but will dissolve in its own water of hydration if heated and form a hard amalgamated solid when cooled. Calcium chloride is used in concrete mixes to accelerate the initial setting, but chloride ions lead to corrosion of steel rebar, so it should not be used in reinforced concrete; the anhydrous form of calcium chloride may be used for this purpose and can provide a measure of the moisture in concrete. Calcium chloride is included as an additive in plastics and in fire extinguishers, in wastewater treatment as a drainage aid, in blast furnaces as an additive to control scaffolding, in fabric softener as a thinner; the exothermic dissolution of calcium chloride is used in self-heating cans and heating pads. In the oil industry, calcium chloride is used to increase the density of solids-free brines.
It is used to provide inhibition of swelling clays in the water phase of invert emulsion drilling fluids. CaCl2 acts as flux material in the Davy process for the industrial production of sodium metal, through the electrolysis of molten NaCl. CaCl2 is used as a flux and electrolyte in the FFC Cambridge process for titanium production, where it ensures the proper exchange of calcium and oxygen ions between the electrodes. Calcium chloride is used in the production of activated charcoal. Calcium chloride is an ingredient used in ceramic slipware, it suspends clay particles so that they float within the solution making it easier to use in a variety of slipcasting techniques. Calcium chloride dihydrate dissolved in ethanol has been used as a sterilant for male animals; the solution is injected into the testes of the animal. Within 1 month, necrosis of testicular tissue results in sterilization. Calcium chloride can act as an irritant by desiccating moist skin. Solid calcium chloride dissolves exothermically, burns can result in the mouth and esophagus if it is ingested.
Ingestion of concentrated solutions or solid products may cause gastrointestinal irritation or ulceration. Consumption of calcium
In chemistry, the valence or valency of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules. The concept of valence developed in the second half of the 19th century and helped explain the molecular structure of inorganic and organic compounds; the quest for the underlying causes of valence led to the modern theories of chemical bonding, including the cubical atom, Lewis structures, valence bond theory, molecular orbitals, valence shell electron pair repulsion theory, all of the advanced methods of quantum chemistry. The combining power, or affinity of an atom of a given element is determined by the number of hydrogen atoms that it combines with. In methane, carbon has a valence of 4. Chlorine, as it has a valence of one, can be substituted for hydrogen, so phosphorus has a valence of 5 in phosphorus pentachloride, PCl5. Valence diagrams of a compound represent the connectivity of the elements, with lines drawn between two elements, sometimes called bonds, representing a saturated valency for each element.
The two tables below show some examples of different compounds, their valence diagrams, the valences for each element of the compound. Valence only describes connectivity. A line between atoms does not represent a pair of electrons. Valence is defined by the IUPAC as: The maximum number of univalent atoms that may combine with an atom of the element under consideration, or with a fragment, or for which an atom of this element can be substituted. An alternative modern description is: The number of hydrogen atoms that can combine with an element in a binary hydride or twice the number of oxygen atoms combining with an element in its oxide or oxides; this definition differs from the IUPAC definition as an element can be said to have more than one valence. The etymology of the words valence and valency traces back to 1425, meaning "extract, preparation", from Latin valentia "strength, capacity", from the earlier valor "worth, value", the chemical meaning referring to the "combining power of an element" is recorded from 1884, from German Valenz.
In 1789, William Higgins published views on what he called combinations of "ultimate" particles, which foreshadowed the concept of valency bonds. If, for example, according to Higgins, the force between the ultimate particle of oxygen and the ultimate particle of nitrogen were 6 the strength of the force would be divided accordingly, for the other combinations of ultimate particles; the exact inception, however, of the theory of chemical valencies can be traced to an 1852 paper by Edward Frankland, in which he combined the older theories of free radicals with thoughts on chemical affinity to show that certain elements have the tendency to combine with other elements to form compounds containing 3, i.e. in the 3-atom groups or 5, i.e. in the 5-atom groups, equivalents of the attached elements. According to him, this is the manner in which their affinities are best satisfied, by following these examples and postulates, he declares how obvious it is that This “combining power” was afterwards called quantivalence or valency.
In 1857 August Kekulé proposed fixed valences for many elements, such as 4 for carbon, used them to propose structural formulas for many organic molecules, which are still accepted today. Most 19th-century chemists defined the valence of an element as the number of its bonds without distinguishing different types of valence or of bond. However, in 1893 Alfred Werner described transition metal coordination complexes such as Cl3, in which he distinguished principal and subsidiary valences, corresponding to the modern concepts of oxidation state and coordination number respectively. For main-group elements, in 1904 Richard Abegg considered positive and negative valences, proposed Abegg's rule to the effect that their difference is 8; the Rutherford model of the nuclear atom showed that the exterior of an atom is occupied by electrons, which suggests that electrons are responsible for the interaction of atoms and the formation of chemical bonds. In 1916, Gilbert N. Lewis explained valence and chemical bonding in terms of a tendency of atoms to achieve a stable octet of 8 valence-shell electrons.
According to Lewis, covalent bonding leads to octets by the sharing of electrons, ionic bonding leads to octets by the transfer of electrons from one atom to the other. The term covalence is attributed to Irving Langmuir, who stated in 1919 that "the number of pairs of electrons which any given atom shares with the adjacent atoms is called the covalence of that atom"; the prefix co - means "together". Subsequent to that, it is now more common to speak of covalent bonds rather than valence, which has fallen out of use in higher-level work from the advances in the theory of chemical bonding, but it is still used in elementary studies, where it provides a heuristic introduction to the subject. In the 1930s, Linus Pauling proposed that there are polar covalent bonds, which are intermediate between covalent and ionic, that the degree of ionic character depends on the difference of electronegativity of the two bonded atoms. Pauling also
Copper is a chemical element with symbol Cu and atomic number 29. It is a soft and ductile metal with high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, constantan used in strain gauges and thermocouples for temperature measurement. Copper is one of the few metals; this led to early human use in several regions, from c. 8000 BC. Thousands of years it was the first metal to be smelted from sulfide ores, c. 5000 BC, the first metal to be cast into a shape in a mold, c. 4000 BC and the first metal to be purposefully alloyed with another metal, tin, to create bronze, c. 3500 BC. In the Roman era, copper was principally mined on Cyprus, the origin of the name of the metal, from aes сyprium corrupted to сuprum, from which the words derived and copper, first used around 1530.
The encountered compounds are copper salts, which impart blue or green colors to such minerals as azurite and turquoise, have been used and as pigments. Copper used in buildings for roofing, oxidizes to form a green verdigris. Copper is sometimes used in decorative art, both in its elemental metal form and in compounds as pigments. Copper compounds are used as bacteriostatic agents and wood preservatives. Copper is essential to all living organisms as a trace dietary mineral because it is a key constituent of the respiratory enzyme complex cytochrome c oxidase. In molluscs and crustaceans, copper is a constituent of the blood pigment hemocyanin, replaced by the iron-complexed hemoglobin in fish and other vertebrates. In humans, copper is found in the liver and bone; the adult body contains between 2.1 mg of copper per kilogram of body weight. Copper and gold are in group 11 of the periodic table; the filled d-shells in these elements contribute little to interatomic interactions, which are dominated by the s-electrons through metallic bonds.
Unlike metals with incomplete d-shells, metallic bonds in copper are lacking a covalent character and are weak. This observation explains the low high ductility of single crystals of copper. At the macroscopic scale, introduction of extended defects to the crystal lattice, such as grain boundaries, hinders flow of the material under applied stress, thereby increasing its hardness. For this reason, copper is supplied in a fine-grained polycrystalline form, which has greater strength than monocrystalline forms; the softness of copper explains its high electrical conductivity and high thermal conductivity, second highest among pure metals at room temperature. This is because the resistivity to electron transport in metals at room temperature originates from scattering of electrons on thermal vibrations of the lattice, which are weak in a soft metal; the maximum permissible current density of copper in open air is 3.1×106 A/m2 of cross-sectional area, above which it begins to heat excessively. Copper is one of a few metallic elements with a natural color other than silver.
Pure copper acquires a reddish tarnish when exposed to air. The characteristic color of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells – the energy difference between these shells corresponds to orange light; as with other metals, if copper is put in contact with another metal, galvanic corrosion will occur. Copper does not react with water, but it does react with atmospheric oxygen to form a layer of brown-black copper oxide which, unlike the rust that forms on iron in moist air, protects the underlying metal from further corrosion. A green layer of verdigris can be seen on old copper structures, such as the roofing of many older buildings and the Statue of Liberty. Copper tarnishes when exposed to some sulfur compounds, with which it reacts to form various copper sulfides. There are 29 isotopes of copper. 63Cu and 65Cu are stable, with 63Cu comprising 69% of occurring copper. The other isotopes are radioactive, with the most stable being 67Cu with a half-life of 61.83 hours.
Seven metastable isotopes have been characterized. Isotopes with a mass number above 64 decay by β−, whereas those with a mass number below 64 decay by β+. 64Cu, which has a half-life of 12.7 hours, decays both ways.62Cu and 64Cu have significant applications. 62Cu is used in 62Cu-PTSM as a radioactive tracer for positron emission tomography. Copper is produced in massive stars and is present in the Earth's crust in a proportion of about 50 parts per million. In nature, copper occurs in a variety of minerals, including native copper, copper sulfides such as chalcopyrite, digenite and chalcocite, copper sulfosalts such as tetrahedite-tennantite, enargite, copper carbonates such as azurite and malachite, as copper or copper oxides such as cuprite and tenorite, respectively; the largest mass of elemental copper discovered weighed 420 tonnes and was found in 1857 on the Keweenaw Peninsula in Michigan, US. Native copper is a polycrystal
Food additives are substances added to food to preserve flavor or enhance its taste, appearance, or other qualities. Some additives have been used for centuries. With the advent of processed foods in the second half of the twentieth century, many more additives have been introduced, of both natural and artificial origin. Food additives include substances that may be introduced to food indirectly in the manufacturing process, through packaging, or during storage or transport. To regulate these additives, inform consumers, each additive is assigned a unique number, termed as "E numbers", used in Europe for all approved additives; this numbering scheme has now been adopted and extended by the Codex Alimentarius Commission to internationally identify all additives, regardless of whether they are approved for use. E numbers are all prefixed by "E", but countries outside Europe use only the number, whether the additive is approved in Europe or not. For example, acetic acid is written as E260 on products sold in Europe, but is known as additive 260 in some countries.
Additive 103, alkannin, is not approved for use in Europe so does not have an E number, although it is approved for use in Australia and New Zealand. Since 1987, Australia has had an approved system of labelling for additives in packaged foods; each food additive has to be numbered. The numbers are the same as in Europe, but without the prefix "E"; the United States Food and Drug Administration lists these items as "generally recognized as safe". See list of food additives for a complete list of all the names. Food additives can be divided into several groups, although there is some overlap because some additives exert more than one effect. For example, salt is both a preservative as well as a flavor. Acidulents Acidulents confer sour or acid taste. Common acidulents include vinegar, citric acid, tartaric acid, malic acid, fumaric acid, lactic acid. Acidity regulators Acidity regulators are used for controlling the pH of foods for stability or to affect activity of enzymes. Anticaking agents Anticaking agents keep powders such as milk powder from sticking.
Antifoaming and foaming agents Antifoaming agents prevent foaming in foods. Foaming agents do the reverse. Antioxidants Antioxidants such as vitamin C are preservatives by inhibiting the degradation of food by oxygen. Bulking agents Bulking agents such as starch are additives that increase the bulk of a food without affecting its taste. Food coloring Colorings are added to food to replace colors lost during preparation or to make food look more attractive. Fortifying agents Vitamins and dietary supplements to increase the nutritional value Color retention agents In contrast to colorings, color retention agents are used to preserve a food's existing color. Emulsifiers Emulsifiers allow water and oils to remain mixed together in an emulsion, as in mayonnaise, ice cream, homogenized milk. Flavors Flavors are additives that give food a particular taste or smell, may be derived from natural ingredients or created artificially. Flavor enhancers Flavor enhancers enhance a food's existing flavors. A popular example is monosodium glutamate.
Some flavor enhancers have their own flavors. Flour treatment agents Flour treatment agents are added to flour to improve its color or its use in baking. Glazing agents Glazing agents provide a shiny appearance or protective coating to foods. Humectants Humectants prevent foods from drying out. Tracer gas Tracer gas allow for package integrity testing to prevent foods from being exposed to atmosphere, thus guaranteeing shelf life. Preservatives Preservatives prevent or inhibit spoilage of food due to fungi and other microorganisms. Stabilizers Stabilizers and gelling agents, like agar or pectin give foods a firmer texture. While they are not true emulsifiers, they help to stabilize emulsions. Sweeteners Sweeteners are added to foods for flavoring. Sweeteners other than sugar are added to keep the food energy low, or because they have beneficial effects regarding diabetes mellitus, tooth decay, or diarrhea. Thickeners Thickening agents are substances which, when added to the mixture, increase its viscosity without modifying its other properties.
Packaging Bisphenols and perfluoroalkyl chemicals are indirect additives used in manufacturing or packaging. In July 2018 the American Academy of Pediatrics called for more careful study of those three substances, along with nitrates and food coloring, as they might harm children during development. With the increasing use of processed foods since the 19th century, food additives are more used. Many countries regulate their use. For example, boric acid was used as a food preservative from the 1870s to the 1920s, but was banned after World War I due to its toxicity, as demonstrated in animal and human studies. During World War II, the urgent need for cheap, available food preservatives led to it being used again, but it was banned in the 1950s; such cases led to a general mistrust of food additives, an application of the precautionary principle led to the conclusion that only additives that are known to be safe should be used in foods. In the United States, this led to the adoption of the Delaney clause, an amendment to the Federal Food and Cosmetic Act of 1938, stating that no carcinogenic substances may be used as food additives.
Iron is a chemical element with symbol Fe and atomic number 26. It is a metal, that belongs to group 8 of the periodic table, it is by mass the most common element on Earth, forming much of Earth's inner core. It is the fourth most common element in the Earth's crust. Pure iron is rare on the Earth's crust being limited to meteorites. Iron ores are quite abundant, but extracting usable metal from them requires kilns or furnaces capable of reaching 1500 °C or higher, about 500 °C higher than what is enough to smelt copper. Humans started to dominate that process in Eurasia only about 2000 BCE, iron began to displace copper alloys for tools and weapons, in some regions, only around 1200 BCE; that event is considered the transition from the Bronze Age to the Iron Age. Iron alloys, such as steel and special steels are now by far the most common industrial metals, because of their mechanical properties and their low cost. Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts with oxygen and water to give brown to black hydrated iron oxides known as rust.
Unlike the oxides of some other metals, that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing fresh surfaces for corrosion. The body of an adult human contains about 3 to 5 grams of elemental iron in hemoglobin and myoglobin; these two proteins play essential roles in vertebrate metabolism oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals. Chemically, the most common oxidation states of iron are +2 and +3. Iron shares many properties of other transition metals, including the other group 8 elements and osmium. Iron forms compounds in a wide range of oxidation states, −2 to +7. Iron forms many coordination compounds. At least four allotropes of iron are known, conventionally denoted α, γ, δ, ε; the first three forms are observed at ordinary pressures.
As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a body-centered cubic crystal structure. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a face-centered cubic crystal structure, or austenite. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope; the physical properties of iron at high pressures and temperatures have been studied extensively, because of their relevance to theories about the cores of the Earth and other planets. Above 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into another hexagonal close-packed structure, known as ε-iron; the higher-temperature γ-phase changes into ε-iron, but does so at higher pressure. Some controversial experimental evidence exists for a stable β phase at pressures above 50 GPa and temperatures of at least 1500 K, it is supposed to have a double hcp structure. The inner core of the Earth is presumed to consist of an iron-nickel alloy with ε structure.
The melting and boiling points of iron, along with its enthalpy of atomization, are lower than those of the earlier 3d elements from scandium to chromium, showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus. This same trend appears for ruthenium but not osmium; the melting point of iron is experimentally well defined for pressures less than 50 GPa. For greater pressures, published data still varies by tens of gigapascals and over a thousand kelvin. Below its Curie point of 770 °C, α-iron changes from paramagnetic to ferromagnetic: the spins of the two unpaired electrons in each atom align with the spins of its neighbors, creating an overall magnetic field; this happens because the orbitals of those two electrons do not point toward neighboring atoms in the lattice, therefore are not involved in metallic bonding. In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into magnetic domains, about 10 micrometres across, such that the atoms in each domain have parallel spins, but different domains have other orientations.
Thus a macroscopic piece of iron will have a nearly zero overall magnetic field. Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field; this effect is exploited in devices that needs to channel magnetic fields, such as electrical transformers, magnetic recording heads, electric motors. Impurities, lattice defects, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists after the external field is removed -- thus turning the iron object into a magnet. Similar behavior is exhibited by some iron compounds, such as the fer