48. : Colloidal Chemistry. Types of disperse systems. Materials may be mixed together to form a true solution, a coloidal dispersion, or a coarse dispersion.

A true solution - mixture of two or more components that form a homogenous molecular dispersion, i.e. a one-phase system, the composition of which can vary over a wide range.

A colloidal dispersion - represents a system havin a particle size intermediate between that of a true solution and a coarse dispersion, roughly 10Å to 5000Å (0.1mm = 1000Å)

A coarse dispersion the diameter of the particles in emulsions and suspensions for the most part being larger than 0.1mm (1000Å).

A colloid is any substance which is dispersed throughout another substance very evenly, to the point of even distribution on the microscopic level. In order to be distributed in this way, the colloidal mixture has to be broken down into very small particles, called colloidal particles, which are too small to be directly seen by a conventional microscope. There are both biological and man-made examples of colloids which are evenly distributed throughout another substance in the world, such as within milk, fog, smoke and pears, or aerosol sprays, marshmallows, styrofoam, and shaving lather.

A colloid is prepared by reducing large particles to colloidal size (generally between 1 nanometer and 1 micrometer), or increasing small particles (usually single molecules) to the size of colloidal particles. There are a wide variety of techniques in practice, and the type of method used depends on what form state of colloid is desired. Aerosols are produced by using a jet of pressured gas to tear away liquid from the mixture, sometimes facilitated by the ionization of the liquid, then using the repulsion between the similarly ionized liquid to separate into easily removed droplets. Emulsions are prepared by vigorously shaking the two constituent liquids together, sometimes with the use of surfactants like soap in order to help emulsify and stabilize the product formed here.

Semi-solid colloids are what are known as gels, sometimes formed by taking lycophilic (attracting solvents) sols (a stable dispersed mixture of a solid and a liquid) and cooling them, such that their large linear molecules and their natural viscosity cause the solution to disperse. Colloids can be purified through dialysis, which is the process of removing any ionic material that may have accompanied the colloid during its formation. In order to carry out this dialysis, a membrane that allows ions and solvents to pass through, but which stops colloids is used, exhibiting the concepts of diffusion, osmosis, and ultrafiltration.

Colloids have a number of unique properties. Colloids have strong absorption qualities, caused by the large exposed surface area when colloidal particles are finely divided. Colloidal particles can carry an electrical charge, resulting in attraction or repulsion among itself, known as electrostatic interaction.

Stable colloidal systems are simply colloids with the capability to remain in the colloidal state. Unstable colloidal dispersion results in aggregation, where the dispersed substance builds up, and becomes unevenly distributed. Aggregation can be avoided through the use of electrostatic stabilization, by which the colloidal substance is induced with a like electrical charge throughout, resulting in repulsion between all dispersed particles.

A solution is a homogeneous mixture of one or more solutes dissolved in a solvent.

solvent: the substance in which a solute dissolves to produce a homogeneous mixture

solute: the substance that dissolves in a solvent to produce a homogeneous mixture

Note that the solvent is the substance that is present in the greatest amount.

Many different kinds of solutions exist. For example, a solute can be a gas, a liquid or a solid. Solvents can also be gases, liquids or solids.

Colloids are on the dividing line between solutions and heterogeneous mixtures. Like solutions, colloids can be gases, liquids, or solids.

Disperse Phase	Dispersion Medium	Solution	Collodial Dispersion	Coarse Dispersion
Gas	Gas	Air	n/a	n/a
Liquid	Gas	Water Vapour	Fog	Spray
Solid	Gas	Sublimed Idoine	Smoke	Dust
Gas	Liquid	Carbonated Water	Foam	Foam
Liquid	Liquid	Alcohol in Water	Liquid surfactant micelles	Emulsions
Solid	Liquid	NaCl in Water	Polymer or protein solutions	Suspensions
Gas	Solid	Hydrogen in palladium	Solid Foam	Solid Foam
Liquid	Solid	Mineral oil in paraffin		Solid Emulsion
Solid	Solid	"Solid dispersion"	Colloidal Gold in glass	"Solid suspensions"

A suspension is a heterogeneous mixture in which do not dissolve fully, but remain suspended throughout the solution. The particles are larger than 10,000 Angstroms which allows them to be filtered. If a suspension is allowed to stand the particles will separate out.

A colloid is somewhere between a solution and a suspension. A suspension separates out but a colloid does not. When light passes through a colloidal dispersion, such as smoky air, it gets reflected by the larger particles and the light beam becomes visible.

66. Calcium is a chemical element with symbol Ca and atomic number 20. Calcium is a soft gray alkaline earth metal, fifth-most-abundant element by mass in the Earth's crust. The ion Ca²⁺ is also the fifth-most-abundant dissolved ion in seawater by both molarity and mass, after sodium, chloride, magnesium, and sulfate.^[3] Free calcium metal is too reactive to occur in nature. Calcium is produced in the explosions at the end of the life of massive stars

Calcium is essential for living organisms, in particular in cell physiology, where movement of the calcium ion into and out of the cytoplasm functions as a signal for many cellular processes. As a major material used in mineralization of bone, teeth andshells, calcium is the most abundant metal by mass in many animals. In chemical terms, calcium is reactive and soft for a metal; though harder than lead, it can be cut with a knife with difficulty. It is a silvery metallic element that must be extracted byelectrolysis from a fused salt like calcium chloride.^[4] Once produced, it rapidly forms a gray-white oxide and nitride coating when exposed to air. In bulk form (typically as chips or "turnings"), the metal is somewhat difficult to ignite, more so even than magnesium chips; but, when lit, the metal burns in air with a brilliant high-intensity orange-red light. Calcium metal reacts with water, generating hydrogen gas at a rate rapid enough to be noticeable, but not fast enough at room temperature to generate much heat, making it useful for generating hydrogen. With a density of 1.54 g/cm³,^[6] calcium is the lightest of the alkaline earth metals; magnesium (specific gravity 1.74) and beryllium (1.84) are denser though lighter in atomic mass. From strontium onward, the alkali earth metals become denser with increasing atomic mass.Calcium has two allotropes.^[7]Calcium has a higher electrical resistivity than copper or aluminium, yet weight-for-weight, due to its much lower density, it is a better conductor than either. Its use as such in terrestrial applications is usually limited by its high reactivity with air; however, it has potential for use as wiring in off-world applications. Calcium, combined with phosphate, forming hydroxylapatite, is the mineral portion of human and animal bones and teeth. The mineral portion of some corals can also be transformed into hydroxylapatite.

Calcium hydroxide (Ca(OH)₂) (slaked lime) is used in many chemical refinery processes and is made by heating limestone at high temperature (above 825 °C) and then carefully adding water to it. When lime is mixed with sand, it hardens into a mortarand is turned into plaster by carbon dioxide uptake. Mixed with other compounds, lime forms an important part of Portland cement.

Calcium carbonate (CaCO₃) is one of the common compounds of calcium. It is heated to form quicklime (CaO), which is then added to water (H₂O). This forms another material known as slaked lime (Ca(OH)₂), which is an inexpensive base material used throughout the chemical industry. Chalk, marble, and limestone are all forms of calcium carbonate.

When water percolates through limestone or other soluble carbonate rocks, it partially dissolves the rock and causes cave formation with their characteristic stalactites andstalagmites, and also forms hard water. Other important calcium compounds are calcium nitrate, calcium sulfide, calcium chloride, calcium carbide, calcium cyanamide and calcium hypochlorite.

A few calcium compounds where calcium is in the oxidation state +1 have also been investigated recently.^[2] Charlotte Froese Fischer predicted that a Ca⁻ ion would be stable; this ion was discovered experimentally in 1987.^[11]

Isotopes

Main article: Isotopes of calcium

Calcium has five stable isotopes (⁴⁰Ca, ⁴²Ca, ⁴³Ca, ⁴⁴Ca and ⁴⁶Ca), plus one more isotope (⁴⁸Ca) that has such a long half-life that for all practical purposes it can also be considered stable. The 20% range in relative mass among naturally occurring calcium isotopes is greater than for any other element except hydrogen and helium. Calcium also has a cosmogenic isotope, radioactive ⁴¹Ca, which has a half-life of 103,000 years.

75. Mercury is a chemical element with symbol Hg and atomic number 80. It is commonly known as quicksilver and was formerly named hydrargyrum (/haɪˈdrɑrdʒərəm/).^[3] A heavy, silvery d-block element, mercury is the only metallic element that is liquid at standard conditions for temperature and pressure; the only other element that is liquid under these conditions is bromine, though metals such as caesium, gallium, and rubidium melt just above room temperature.

Mercury occurs in deposits throughout the world mostly as cinnabar (mercuric sulfide). The red pigment vermilion, a pure form of mercuric sulfide, is mostly obtained by reaction of mercury (produced by reduction from cinnabar) with sulfur.Mercury poisoning can result from exposure to water-soluble forms of mercury (such as mercuric chloride ormethylmercury), inhalation of mercury vapor, or eating seafood contaminated with mercury.

Mercury is used in thermometers, barometers, manometers, sphygmomanometers, float valves, mercury switches, mercury relays, fluorescent lamps and other devices, though concerns about the element's toxicity have led to mercury thermometers and sphygmomanometers being largely phased out in clinical environments in favour of alternatives such asalcohol- or galinstan-filled glass thermometers and thermistor- or infrared-based electronic instruments. Likewise, mechanical pressure gauges and electronic strain gauge sensors have replaced mercury sphygmomanometers. Mercury remains in use in scientific research applications and in amalgam material for dental restoration in some locales. It is used in lighting: electricity passed through mercury vapor in a fluorescent lamp produces short-wave ultraviolet light which then causes the phosphor in the tube to fluoresce, making visible light. Mercury is a heavy, silvery-white metal. As compared to other metals, it is a poor conductor of heat, but a fair conductor of electricity.^[4] Mercury has a freezing pointof −38.83 °C and a boiling point of 356.73 °C,^[5][6][7] both exceptionally low for a metal, and it is the only elemental metal known to melt at a generally cold temperature. In addition, mercury's boiling point of 629.88 K (674.11 °F) is the lowest for any metal.^[8] A complete explanation of this delves deep into the realm ofquantum physics, but it can be summarized as follows: mercury has a uniqueelectron configuration where electrons fill up all the available 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, and 6s subshells. Because this configuration strongly resists removal of an electron, mercury behaves similarly to noble gas elements, which form weak bonds and hence melt at relatively low temperatures. Mercury does not react with most acids, such as dilute sulfuric acid, although oxidizing acids such as concentrated sulfuric acid and nitric acid or aqua regia dissolve it to give sulfate, nitrate, and chloride salts. Like silver, mercury reacts with atmospheric hydrogen sulfide. Mercury even reacts with solid sulfur flakes, which are used in mercury spill kits to absorb mercury vapors (spill kits also use activated carbon and powdered zinc). There are seven stable isotopes of mercury with 202Hg being the most abundant (29.86%). The longest-lived radioisotopes are 194Hg with a half-life of 444 years, and 203 Hg with a half-life of 46.612 days. Most of the remaining radioisotopes have half-lives that are less than a day. 199Hg and 201Hg are the most often studied NMR-active nuclei, having spins of¹⁄₂ and ³⁄₂ respectively.^[4]