Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared and ultraviolet light. On Earth, sunlight is filtered through Earth's atmosphere, is obvious as daylight when the Sun is above the horizon; when the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by clouds or reflects off other objects, it is experienced as diffused light; the World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter. Other sources indicate an "Average over the entire earth" of "164 Watts per square meter over a 24 hour day"; the ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for vitamin D3 synthesis and a mutagen. Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun.
A photon starting at the center of the Sun and changing direction every time it encounters a charged particle would take between 10,000 and 170,000 years to get to the surface. Sunlight is a key factor in photosynthesis, the process used by plants and other autotrophic organisms to convert light energy from the Sun, into chemical energy that can be used to synthesize carbohydrates and to fuel the organisms' activities. Researchers can measure the intensity of sunlight using a sunshine recorder, pyranometer, or pyrheliometer. To calculate the amount of sunlight reaching the ground, both the eccentricity of Earth's elliptic orbit and the attenuation by Earth's atmosphere have to be taken into account; the extraterrestrial solar illuminance, corrected for the elliptic orbit by using the day number of the year, is given to a good approximation by E e x t = E s c ⋅, where dn=1 on January 1st. In this formula dn–3 is used, because in modern times Earth's perihelion, the closest approach to the Sun and, the maximum Eext occurs around January 3 each year.
The value of 0.033412 is determined knowing that the ratio between the perihelion squared and the aphelion squared should be 0.935338. The solar illuminance constant, is equal to 128×103 lux; the direct normal illuminance, corrected for the attenuating effects of the atmosphere is given by: E d n = E e x t e − c m, where c is the atmospheric extinction and m is the relative optical airmass. The atmospheric extinction brings the number of lux down to around 100 000 lux; the total amount of energy received at ground level from the Sun at the zenith depends on the distance to the Sun and thus on the time of year. It is 3.3 % lower in July. If the extraterrestrial solar radiation is 1367 watts per square meter the direct sunlight at Earth's surface when the Sun is at the zenith is about 1050 W/m2, but the total amount hitting the ground is around 1120 W/m2. In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared, 42 to 43 percent visible, 3 to 5 percent ultraviolet. At the top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet, with most of the extra UV consisting of biologically damaging short-wave ultraviolet.
Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux. Multiplying the figure of 1050 watts per square metre by 93 lumens per watt indicates that bright sunlight provides an illuminance of 98 000 lux on a perpendicular surface at sea level; the illumination of a horizontal surface will be less than this if the Sun is not high in the sky. Averaged over a day, the highest amount of sunlight on a horizontal surface occurs in January at the South Pole. Dividing the irradiance of 1050 W/m2 by the size of the Sun's disk in steradians gives an average radiance of 15.4 MW per square metre per steradian. Multiplying this by π gives an upper limit to the irradiance which can be focused on a surface using mirrors: 48.5 MW/m2. The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K; the Sun emits EM radiation across most of the electromagnetic spectrum. Although the Sun produces gamma rays as a result of the nuclear-fusion process, internal absorption and thermalization convert these super-high-energy photons to lower-energy photons before they reach the Sun's surface and are emitted out into space.
As a result, the Sun does not emit gamma rays from this process, but it does emit gamma rays from solar flares. The Sun emits X-rays, vis
An ore is an occurrence of rock or sediment that contains sufficient minerals with economically important elements metals, that can be economically extracted from the deposit. The ores are extracted from the earth through mining; the ore grade, or concentration of an ore mineral or metal, as well as its form of occurrence, will directly affect the costs associated with mining the ore. The cost of extraction must thus be weighed against the metal value contained in the rock to determine what ore can be processed and what ore is of too low a grade to be worth mining. Metal ores are oxides, silicates, or native metals that are not concentrated in the Earth's crust, or noble metals such as gold; the ores must be processed to extract the elements of interest from the waste rock and from the ore minerals. Ore bodies are formed by a variety of geological processes; the process of ore formation is called ore genesis. An ore deposit is an accumulation of ore; this is distinct from a mineral resource. An ore deposit is one occurrence of a particular ore type.
Most ore deposits are named according to their location, or after a discoverer, or after some whimsy, a historical figure, a prominent person, something from mythology or the code name of the resource company which found it. Ore deposits are classified according to various criteria developed via the study of economic geology, or ore genesis; the classifications below are typical. Mesothermal lode gold deposits, typified by the Golden Mile, Kalgoorlie Archaean conglomerate hosted gold-uranium deposits, typified by Elliot Lake, Ontario and Witwatersrand, South Africa Carlin–type gold deposits, including. Volcanic hosted massive sulfide Cu-Pb-Zn including. Stratiform arkose-hosted and shale-hosted copper, typified by the Zambian copperbelt. Stratiform tungsten, typified by the Erzgebirge deposits, Czechoslovakia Exhalative spilite-chert hosted gold deposits Mississippi valley type zinc-lead deposits Hematite iron ore deposits of altered banded iron formation Sudbury Basin nickel and copper, Canada The basic extraction of ore deposits follows these steps: Prospecting or exploration to find and define the extent and value of ore where it is located Conduct resource estimation to mathematically estimate the size and grade of the deposit Conduct a pre-feasibility study to determine the theoretical economics of the ore deposit.
This identifies, early on, whether further investment in estimation and engineering studies is warranted and identifies key risks and areas for further work. Conduct a feasibility study to evaluate the financial viability and financial risks and robustness of the project and make a decision as whether to develop or walk away from a proposed mine project; this includes mine planning to evaluate the economically recoverable portion of the deposit, the metallurgy and ore recoverability and payability of the ore concentrates, engineering and infrastructure costs and equity requirements and a cradle to grave analysis of the possible mine, from the initial excavation all the way through to reclamation. Development to create access to an ore body and building of mine plant and equipment The operation of the mine in an active sense Reclamation to make land where a mine had been suitable for future use Ores are traded internationally and comprise a sizeable portion of international trade in raw materials both in value and volume.
This is because the worldwide distribution of ores is unequal and dislocated from locations of peak demand and from smelting infrastructure. Most base metals are traded internationally on the London Metal Exchange, with
Total dissolved solids
Total dissolved solids is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized or micro-granular suspended form. The operational definition is that the solids must be small enough to survive filtration through a filter with two-micrometer pores. Total dissolved solids are discussed only for freshwater systems, as salinity includes some of the ions constituting the definition of TDS; the principal application of TDS is in the study of water quality for streams and lakes, although TDS is not considered a primary pollutant it is used as an indication of aesthetic characteristics of drinking water and as an aggregate indicator of the presence of a broad array of chemical contaminants. Primary sources for TDS in receiving waters are agricultural and residential runoff, clay rich mountain waters, leaching of soil contamination and point source water pollution discharge from industrial or sewage treatment plants; the most common chemical constituents are calcium, nitrates, sodium and chloride, which are found in nutrient runoff, general stormwater runoff and runoff from snowy climates where road de-icing salts are applied.
The chemicals may be cations, molecules or agglomerations on the order of one thousand or fewer molecules, so long as a soluble micro-granule is formed. More exotic and harmful elements of TDS are pesticides arising from surface runoff. Certain occurring total dissolved solids arise from the weathering and dissolution of rocks and soils; the United States has established a secondary water quality standard of 500 mg/l to provide for palatability of drinking water. Total dissolved solids are differentiated from total suspended solids, in that the latter cannot pass through a sieve of two micrometers and yet are indefinitely suspended in solution; the term "settleable solids" refers to material of any size that will not remain suspended or dissolved in a holding tank not subject to motion, excludes both TDS and TSS. Settleable solids may include insoluble molecules; the two principal methods of measuring total dissolved solids are gravimetric analysis and conductivity. Gravimetric methods are the most accurate and involve evaporating the liquid solvent and measuring the mass of residues left.
This method is the best, although it is time-consuming. If inorganic salts comprise the great majority of TDS, gravimetric methods are appropriate. Electrical conductivity of water is directly related to the concentration of dissolved ionized solids in the water. Ions from the dissolved solids in water create the ability for that water to conduct an electric current, which can be measured using a conventional conductivity meter or TDS meter; when correlated with laboratory TDS measurements, conductivity provides an approximate value for the TDS concentration to within ten-percent accuracy. The relationship of TDS and specific conductance of groundwater can be approximated by the following equation: TDS = keECwhere TDS is expressed in mg/L and EC is the electrical conductivity in microsiemens per centimeter at 25 °C; the correlation factor ke varies between 0.55 and 0.8. Hydrologic transport models are used to mathematically analyze movement of TDS within river systems; the most common models address surface runoff, allowing variation in land use type, soil type, vegetative cover and land management practice.
Runoff models have evolved to a good degree of accuracy and permit the evaluation of alternative land management practices upon impacts to stream water quality. Basin models are used to more comprehensively evaluate total dissolved solids within a catchment basin and dynamically along various stream reaches; the DSSAM model was developed by the U. S. Environmental Protection Agency; this hydrology transport model is based upon the pollutant-loading metric called "Total Maximum Daily Load", which addresses TDS and other specific chemical pollutants. The success of this model contributed to the Agency’s broadened commitment to the use of the underlying TMDL protocol in its national policy for management of many river systems in the United States; when measuring water treated with water softeners, high levels of total dissolved solids do not correlate to hard water, as water softeners do not reduce TDS. Hard water can cause scale buildup in pipes and filters, reducing performance and adding to system maintenance costs.
These effects can be seen in aquariums, swimming pools, reverse osmosis water treatment systems. In these applications, total dissolved solids are tested and filtration membranes are checked in order to prevent adverse effects. In the case of hydroponics and aquaculture, TDS is monitored in order to create a water quality environment favorable for organism productivity. For freshwater oysters and other high value seafood, highest productivity and economic returns are achieved by mimicking the TDS and pH levels of each species' native environment. For hydroponic uses, total dissolved solids is considered one of the best indices of nutrient availability for the aquatic plants being grown; because the threshold of acceptable aesthetic criteria for human drinking water is 500 mg/l, there is no general concern for odor and color at a level much lower than is required for harm. A number of studies have been conducted and indi
A fertilizer or fertiliser is any material of natural or synthetic origin, applied to soils or to plant tissues to supply one or more plant nutrients essential to the growth of plants. Many sources of fertilizer exist, both natural and industrially produced. Fertilizers enhance the growth of plants; this goal is met in the traditional one being additives that provide nutrients. The second mode by which some fertilizers act is to enhance the effectiveness of the soil by modifying its water retention and aeration; this article, like many on fertilizers, emphasises the nutritional aspect. Fertilizers provide, in varying proportions: three main macronutrients: Nitrogen: leaf growth Phosphorus: Development of roots, seeds, fruit. Of occasional significance are silicon and vanadium; the nutrients required for healthy plant life are classified according to the elements, but the elements are not used as fertilizers. Instead compounds containing these elements are the basis of fertilizers; the macro-nutrients are consumed in larger quantities and are present in plant tissue in quantities from 0.15% to 6.0% on a dry matter basis.
Plants are made up of four main elements: hydrogen, oxygen and nitrogen. Carbon and oxygen are available as water and carbon dioxide. Although nitrogen makes up most of the atmosphere, it is in a form, unavailable to plants. Nitrogen is the most important fertilizer since nitrogen is present in proteins, DNA and other components. To be nutritious to plants, nitrogen must be made available in a "fixed" form. Only some bacteria and their host plants can fix atmospheric nitrogen by converting it to ammonia. Phosphate is required for the production of DNA and ATP, the main energy carrier in cells, as well as certain lipids. Micronutrients are consumed in smaller quantities and are present in plant tissue on the order of parts-per-million, ranging from 0.15 to 400 ppm DM, or less than 0.04% DM. These elements are present at the active sites of enzymes that carry out the plant's metabolism; because these elements enable catalysts their impact far exceeds their weight percentage. Fertilizers are classified in several ways.
They are classified according to whether they provide a single nutrient, in which case they are classified as "straight fertilizers." "Multinutrient fertilizers" provide two or more nutrients, for example N and P. Fertilizers are sometimes classified as inorganic versus organic. Inorganic fertilizers exclude carbon-containing materials except ureas. Organic fertilizers are plant- or animal-derived matter. Inorganic are sometimes called synthetic fertilizers since various chemical treatments are required for their manufacture; the main nitrogen-based straight fertilizer is its solutions. Ammonium nitrate is widely used. Urea is another popular source of nitrogen, having the advantage that it is solid and non-explosive, unlike ammonia and ammonium nitrate, respectively. A few percent of the nitrogen fertilizer market has been met by calcium ammonium nitrate; the main straight phosphate fertilizers are the superphosphates. "Single superphosphate" consists of 14–18% P2O5, again in the form of Ca2, but phosphogypsum.
Triple superphosphate consists of 44-48% of P2O5 and no gypsum. A mixture of single superphosphate and triple superphosphate is called double superphosphate. More than 90% of a typical superphosphate fertilizer is water-soluble; the main potassium-based straight fertilizer is Muriate of Potash. Muriate of Potash consists of 95-99% KCl, is available as 0-0-60 or 0-0-62 fertilizer; these fertilizers are common. They consist of two or more nutrient components. Major two-component fertilizers provide both phosphorus to the plants; these are called NP fertilizers. The main NP fertilizers are diammonium phosphate; the active ingredient in MAP is NH4H2PO4. The active ingredient in DAP is 2HPO4. About 85% of MAP and DAP fertilizers are soluble in water. NPK fertilizers are three-component fertilizers providing nitrogen and potassium. NPK rating is a rating system describing the amount of nitrogen and potassium in a fertilizer. NPK ratings consist of three numbers separated by dashes describing the chemical content of fertilizers.
The first number represents the percentage of nitrogen in the product. Fertilizers do not contain P2O5 or K2O, but the system is a conventional shorthand for the amount of the phosphorus or potassium in a fertilizer. A 50-pound bag of fertilizer labeled 16-4-8 contains 8 lb of nitrogen, an amount of phosphorus equivalent to that in 2 pounds of P2O5, 4 pounds of K2O. Most fertilizers are labeled according to this N-P-K convention, although Australian convention, following an N-P-K-S system, adds a fourth number for sulfur, uses elemental values for all values including P and K; the main micronutrients are molybdenum, zinc and copper. These elements are provided as water-soluble salts
A combined sewer is a sewage collection system of pipes and tunnels designed to collect surface runoff and sewage water in a shared system. This type of gravity sewer design is no longer used in every instance worldwide when constructing new sewer systems. Modern-day sewer designs exclude surface runoff from sanitary sewers, but many older cities and towns continue to operate constructed combined sewer systems. Combined sewers can cause serious water pollution problems during combined sewer overflow events when combined sewage and surface runoff flows exceed the capacity of the sewage treatment plant, or of the maximum flow rate of the system which transmits the combined sources. In instances where exceptionally high surface runoff occurs, the load on individual tributary branches of the sewer system may cause a back-up to a point where raw sewage flows out of input sources such a toilets, causing inhabited buildings to be flooded with a toxic sewage-runoff mixture, incurring massive financial burdens for cleanup and repair.
When combined sewer systems experience these higher than normal throughputs, relief systems cause discharges containing human and industrial waste to flow into rivers, streams, or other bodies of water. Such events cause both negative environmental and lifestyle consequences, including beach closures, contaminated shellfish unsafe for consumption, contamination of drinking water sources, rendering them temporarily unsafe for drinking and requiring boiling before uses such as bathing or washing dishes. Recent archaeological discoveries have shown that some of the earliest sewer systems were developed 2500 BC in the ancient city of Harappa; the primitive sewers were carved in the ground alongside buildings. This discovery reveals the conceptual understanding of waste disposal by the early civilizations; the earliest sewers were designed to carry street runoff away from inhabited areas and into surface waterways without treatment. Before the 19th century it was commonplace to empty human waste receptacles, e.g. chamberpots, into town and city streets, while the use of draft animals such as horses and herding of livestock through city streets meant that most contained large amounts of excrement.
Open sewers, consisting of gutters and urban streambeds, were common worldwide before the 20th century. In the majority of developed countries, large efforts were made during the late 19th and early 20th centuries to cover the open sewers, converting them to closed systems with cast iron, steel, or concrete pipes and concrete arches. Most sewage collection systems of the 19th and early to mid 20th century used single-pipe systems that collect both sewage and urban runoff from streets and roofs This type of collection system is referred to as a combined sewer system; the rationale for combining the two was. Most cities at that time did not have sewage treatment plants, so there was no perceived public health advantage in constructing a separate "surface water sewerage" or "storm sewer" system. Moreover, runoff was, pre-automobile to be highly contaminated with animal waste; the widespread replacement of horses with automotive propulsion, paving of city streets and surfaces, provision of mains water in the 20th century changed the nature and volume of urban runoff to be cleaner, include water that soaked away and to include saved rooftop rainwater after combined sewers were widely adopted.
When constructed, combined sewer systems were sized to carry three to 160 times the average dry weather sewage flows. It is infeasible to treat the volume of mixed sewage and surface runoff flowing in a combined sewer during peak runoff events caused by snowmelt or convective precipitation; as cities built sewage treatment plants, those plants were built to treat only the volume of sewage flowing during dry weather. Relief structures were installed in the collection system to bypass untreated sewage mixed with surface runoff during wet weather, protecting sewage treatment plants from damage caused if peak flows reached the headworks; these relief structures, called storm-water regulators are constructed in combined sewer systems to divert flows in excess of the peak design flow of the sewage treatment plant. Combined sewers are built with control sections establishing stage-discharge or pressure differential-discharge relationships which may be either predicted or calibrated to divert flows in excess of sewage treatment plant capacity.
A leaping weir may be used as a regulating device allowing typical dry-weather sewage flow rates to fall into an interceptor sewer to the sewage treatment plant, but causing a major portion of higher flow rates to leap over the interceptor into the diversion outfall. Alternatively, an orifice may be sized to accept the sewage treatment plant design capacity and cause excess flow to accumulate above the orifice until it overtops a side-overflow weir to the diversion outfall. CSO statistics may be confusing because the term may describe either the number of events or the number of relief structure locations at which such events may occur. A CSO event, as the term is used in American English, occurs when mixed sewage and stormwater are bypassed from a combined sewer system control section into a river, lake, or ocean through a designed diversion outfall, but without treatment. Overflow frequency and duration varies both from system to system, from outfall to outfall, within a single combined sewer s
The Dead Sea is a salt lake bordered by Jordan to the east and Israel and the West Bank to the west. It lies in the Jordan Rift Valley, its main tributary is the Jordan River, its surface and shores are 430.5 metres below Earth's lowest elevation on land. It is 304 m deep, the deepest hypersaline lake in the world. With a salinity of 342 g/kg, or 34.2%, it is one of the world's saltiest bodies of water – 9.6 times as salty as the ocean – and has a density of 1.24 kg/litre, which makes swimming similar to floating. This salinity makes for a harsh environment in which plants and animals cannot flourish, hence its name; the Dead Sea's main, northern basin is 50 kilometres long and 15 kilometres wide at its widest point. The Dead Sea has attracted visitors from around the Mediterranean Basin for thousands of years, it was one of the world's first health resorts, it has been the supplier of a wide variety of products, from asphalt for Egyptian mummification to potash for fertilisers. The Dead Sea is receding at an alarming rate.
The recession of the Dead Sea has begun causing problems, multiple canals and pipelines proposals exist to reduce its recession. One of these proposals is the Red Sea–Dead Sea Water Conveyance project, carried out by Jordan, which will provide water to neighbouring countries, while the brine will be carried to the Dead Sea to help stabilise its water level; the first phase of the project is scheduled to begin in 2018 and be completed in 2021. In Hebrew, the Dead Sea is Yām ha-Melaḥ, meaning "sea of salt"; the Bible uses this term alongside two others: the Sea of the Arabah, the Eastern Sea. The designation "Dead Sea" never appears in the Bible. In prose sometimes the term Yām ha-Māvet is due to the scarcity of aquatic life there. In Arabic the Dead Sea is called al-Bahr al-Mayyit, or less baḥrᵘ lūṭᵃ. Another historic name in Arabic was the "Sea of Zoʼar", after a nearby town in biblical times; the Greeks called it Lake Asphaltites. The Dead Sea is an endorheic lake located in the Jordan Rift Valley, a geographic feature formed by the Dead Sea Transform.
This left lateral-moving transform fault lies along the tectonic plate boundary between the African Plate and the Arabian Plate. It runs between the East Anatolian Fault zone in Turkey and the northern end of the Red Sea Rift offshore of the southern tip of Sinai, it is here that the Upper Jordan River/Sea of Galilee/Lower Jordan River water system comes to an end. The Jordan River is the only major water source flowing into the Dead Sea, although there are small perennial springs under and around the Dead Sea, forming pools and quicksand pits along the edges. There are no outlet; the Mujib River, biblical Arnon, is one of the larger water sources of the Dead Sea other than the Jordan. The Wadi Mujib valley, 420 m below the sea level in the southern part of the Jordan valley, is a biosphere reserve, with an area of 212 km2. Other more substantial sources are Wadi Darajeh /Nahal Dragot, Nahal Arugot. Wadi Hasa is another wadi flowing into the Dead Sea. Rainfall is scarcely 100 mm per year in the northern part of the Dead Sea and 50 mm in the southern part.
The Dead Sea zone's aridity is due to the rainshadow effect of the Judaean Mountains. The highlands east of the Dead Sea receive more rainfall than the Dead Sea itself. To the west of the Dead Sea, the Judaean mountains rise less steeply and are much lower than the mountains to the east. Along the southwestern side of the lake is a 210 m tall halite formation called "Mount Sodom". There are two contending hypotheses about the origin of the low elevation of the Dead Sea; the older hypothesis is that the Dead Sea lies in a true rift zone, an extension of the Red Sea Rift, or of the Great Rift Valley of eastern Africa. A more recent hypothesis is that the Dead Sea basin is a consequence of a "step-over" discontinuity along the Dead Sea Transform, creating an extension of the crust with consequent subsidence. Around 3.7 million years ago, what is now the valley of the Jordan River, Dead Sea, the northern Wadi Arabah was inundated by waters from the Mediterranean Sea. The waters formed in a narrow, crooked bay, called by geologists the Sedom Lagoon, connected to the sea through what is now the Jezreel Valley.
The floods of the valley went depending on long-scale climate change. The Sedom Lagoon deposited beds of salt that became 2.5 km thick. Two million years ago, the land between the Rift Valley and the Mediterranean Sea rose to such an extent that the ocean could no longer flood the area. Thus, the long lagoon became a landlocked lake; the Sedom Lagoon extended at its maximum from the Sea of Galilee in the north to somewhere around 50 km south of the current southern end of the Dead Sea, the subsequent lakes never surpassed this expanse. The Hula Depression was never part of any of these water bodies due to its higher elevation and the high threshold of the Korazim block separating it from the Sea of Galilee basin; the first prehistoric lake to follow the Sedom Lagoon is named Lake Amora, followed by Lake Lisan and by the Dead Sea. The water levels and salinity of these lakes have either risen or fallen as an effect of the tectonic dropping of the valley bo
Heavy metals are defined as metals with high densities, atomic weights, or atomic numbers. The criteria used, whether metalloids are included, vary depending on the author and context. In metallurgy, for example, a heavy metal may be defined on the basis of density, whereas in physics the distinguishing criterion might be atomic number, while a chemist would be more concerned with chemical behaviour. More specific definitions have been published, but none of these have been accepted; the definitions surveyed in this article encompass up to 96 out of the 118 known chemical elements. Despite this lack of agreement, the term is used in science. A density of more than 5 g/cm3 is sometimes quoted as a used criterion and is used in the body of this article; the earliest known metals—common metals such as iron and tin, precious metals such as silver and platinum—are heavy metals. From 1809 onwards, light metals, such as magnesium and titanium, were discovered, as well as less well-known heavy metals including gallium and hafnium.
Some heavy metals are either essential nutrients, or harmless, but can be toxic in larger amounts or certain forms. Other heavy metals, such as cadmium and lead, are poisonous. Potential sources of heavy metal poisoning include mining, industrial wastes, agricultural runoff, occupational exposure and treated timber. Physical and chemical characterisations of heavy metals need to be treated with caution, as the metals involved are not always defined; as well as being dense, heavy metals tend to be less reactive than lighter metals and have much less soluble sulfides and hydroxides. While it is easy to distinguish a heavy metal such as tungsten from a lighter metal such as sodium, a few heavy metals, such as zinc and lead, have some of the characteristics of lighter metals, lighter metals such as beryllium and titanium, have some of the characteristics of heavier metals. Heavy metals are scarce in the Earth's crust but are present in many aspects of modern life, they are used in, for example, golf clubs, antiseptics, self-cleaning ovens, solar panels, mobile phones, particle accelerators.
There is no agreed criterion-based definition of a heavy metal. Different meanings may be attached depending on the context. In metallurgy, for example, a heavy metal may be defined on the basis of density, whereas in physics the distinguishing criterion might be atomic number, a chemist would be more concerned with chemical behaviour. Density criteria range from above 3.5 g/cm3 to above 7 g/cm3. Atomic weight definitions can range from greater than sodium. Atomic numbers of heavy metals are given as greater than 20. Definitions based on atomic number have been criticised for including metals with low densities. For example, rubidium in group 1 of the periodic table has an atomic number of 37 but a density of only 1.532 g/cm3, below the threshold figure used by other authors. The same problem may occur with atomic weight based definitions; the United States Pharmacopeia includes a test for heavy metals that involves precipitating metallic impurities as their coloured sulfides." In 1997, Stephen Hawkes, a chemistry professor writing in the context of fifty years' experience with the term, said it applied to "metals with insoluble sulfides and hydroxides, whose salts produce colored solutions in water and whose complexes are colored".
On the basis of the metals he had seen referred to as heavy metals, he suggested it would useful to define them as all the metals in periodic table columns 3 to 16 that are in row 4 or greater, in other words, the transition metals and post-transition metals. The lanthanides satisfy Hawkes' three-part description. In biochemistry, heavy metals are sometimes defined—on the basis of the Lewis acid behaviour of their ions in aqueous solution—as class B and borderline metals. In this scheme, class A metal ions prefer oxygen donors. Class A metals, which tend to have low electronegativity and form bonds with large ionic character, are the alkali and alkaline earths, the group 3 metals, the lanthanides and actinides. Class B metals, which tend to have higher electronegativity and form bonds with considerable covalent character, are the heavier transition and post-transition metals. Borderline metals comprise the lighter transition and post-transition metals; the distinction between the class A metals and the other two categories is sharp.
A cited proposal to use these classification categories instead of the more evocative name heavy metal has not been adopted. A density of more than 5 g/cm3 is sometimes mentioned as a common heavy metal defining factor and, in the absence of a unanimous definition, is used to populate this list and guide the remainder of the article. Metalloids meeting the applicable criteria–arsenic and antimony for example—are sometimes counted as heavy metals in environmental chemistry, as is the case here. Selenium is include