In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent; the mixing process of a solution happens at a scale where the effects of chemical polarity are involved, resulting in interactions that are specific to solvation. The solution assumes the phase of the solvent when the solvent is the larger fraction of the mixture, as is the case; the concentration of a solute in a solution is the mass of that solute expressed as a percentage of the mass of the whole solution. The term aqueous solution is. A solution is a homogeneous mixture of two or more substances; the particles of solute in a solution cannot be seen by the naked eye. A solution does not allow beams of light to scatter. A solution is stable; the solute from a solution cannot be separated by filtration. It is composed of only one phase. Homogeneous means. Heterogeneous means; the properties of the mixture can be uniformly distributed through the volume but only in absence of diffusion phenomena or after their completion.
The substance present in the greatest amount is considered the solvent. Solvents can be liquids or solids. One or more components present in the solution other; the solution has the same physical state as the solvent. If the solvent is a gas, only gases are dissolved under a given set of conditions. An example of a gaseous solution is air. Since interactions between molecules play no role, dilute gases form rather trivial solutions. In part of the literature, they are not classified as solutions, but addressed as mixtures. If the solvent is a liquid almost all gases and solids can be dissolved. Here are some examples: Gas in liquid: Oxygen in water Carbon dioxide in water – a less simple example, because the solution is accompanied by a chemical reaction. Note that the visible bubbles in carbonated water are not the dissolved gas, but only an effervescence of carbon dioxide that has come out of solution. Liquid in liquid: The mixing of two or more substances of the same chemistry but different concentrations to form a constant.
Alcoholic beverages are solutions of ethanol in water. Solid in liquid: Sucrose in water Sodium chloride or any other salt in water, which forms an electrolyte: When dissolving, salt dissociates into ions. Solutions in water are common, are called aqueous solutions. Non-aqueous solutions are. Counter examples are provided by liquid mixtures that are not homogeneous: colloids, emulsions are not considered solutions. Body fluids are examples for complex liquid solutions. Many of these are electrolytes. Furthermore, they contain solute molecules like urea. Oxygen and carbon dioxide are essential components of blood chemistry, where significant changes in their concentrations may be a sign of severe illness or injury. If the solvent is a solid gases and solids can be dissolved. Gas in solids: Hydrogen dissolves rather well in metals in palladium. Liquid in solid: Mercury in gold, forming an amalgam Water in solid salt or sugar, forming moist solids Hexane in paraffin wax Solid in solid: Steel a solution of carbon atoms in a crystalline matrix of iron atoms Alloys like bronze and many others Polymers containing plasticizers The ability of one compound to dissolve in another compound is called solubility.
When a liquid can dissolve in another liquid the two liquids are miscible. Two substances that can never mix to form a solution are said to be immiscible. All solutions have a positive entropy of mixing; the interactions between different molecules or ions may be energetically favored or not. If interactions are unfavorable the free energy decreases with increasing solute concentration. At some point the energy loss outweighs the entropy gain, no more solute particles can be dissolved. However, the point at which a solution can become saturated can change with different environmental factors, such as temperature and contamination. For some solute-solvent combinations a supersaturated solution can be prepared by raising the solubility to dissolve more solute, lowering it; the greater the temperature of the solvent, the more of a given solid solute it can dissolve. However, most gases and some compounds exhibit solubilities that decrease with increased temperature; such behavior is a result of an exothermic enthalpy of solution.
Some surfactants exhibit this behaviour. The solubility of liquids in liquids is less temperature-sensitive than that of solids or gases; the physical properties of compounds such as melting point and boiling point change when other compounds are added. Together they are called colligative properties. There are several ways to quantify the amount of one compound dissolved in the other compounds collectively called concentration. Examples include molarity, volume fraction, mole fraction; the properties of ideal solutions can be calculated by the linear combination of the properties of
Fluid statics or hydrostatics is the branch of fluid mechanics that studies "fluids at rest and the pressure in a fluid or exerted by a fluid on an immersed body". It encompasses the study of the conditions under which fluids are at rest in stable equilibrium as opposed to fluid dynamics, the study of fluids in motion. Hydrostatics are categorized as a part of the fluid statics, the study of all fluids, incompressible or not, at rest. Hydrostatics is fundamental to hydraulics, the engineering of equipment for storing and using fluids, it is relevant to geophysics and astrophysics, to meteorology, to medicine, many other fields. Hydrostatics offers physical explanations for many phenomena of everyday life, such as why atmospheric pressure changes with altitude, why wood and oil float on water, why the surface of still water is always level; some principles of hydrostatics have been known in an empirical and intuitive sense since antiquity, by the builders of boats, cisterns and fountains. Archimedes is credited with the discovery of Archimedes' Principle, which relates the buoyancy force on an object, submerged in a fluid to the weight of fluid displaced by the object.
The Roman engineer Vitruvius warned readers about lead pipes bursting under hydrostatic pressure. The concept of pressure and the way it is transmitted by fluids was formulated by the French mathematician and philosopher Blaise Pascal in 1647; the "fair cup" or Pythagorean cup, which dates from about the 6th century BC, is a hydraulic technology whose invention is credited to the Greek mathematician and geometer Pythagoras. It was used as a learning tool; the cup consists of a line carved into the interior of the cup, a small vertical pipe in the center of the cup that leads to the bottom. The height of this pipe is the same as the line carved into the interior of the cup; the cup may be filled to the line without any fluid passing into the pipe in the center of the cup. However, when the amount of fluid exceeds this fill line, fluid will overflow into the pipe in the center of the cup. Due to the drag that molecules exert on one another, the cup will be emptied. Heron's fountain is a device invented by Heron of Alexandria that consists of a jet of fluid being fed by a reservoir of fluid.
The fountain is constructed in such a way that the height of the jet exceeds the height of the fluid in the reservoir in violation of principles of hydrostatic pressure. The device consisted of an opening and two containers arranged one above the other; the intermediate pot, sealed, was filled with fluid, several cannula connecting the various vessels. Trapped air inside the vessels induces a jet of water out of a nozzle, emptying all water from the intermediate reservoir. Pascal made contributions to developments in both hydrodynamics. Pascal's Law is a fundamental principle of fluid mechanics that states that any pressure applied to the surface of a fluid is transmitted uniformly throughout the fluid in all directions, in such a way that initial variations in pressure are not changed. Due to the fundamental nature of fluids, a fluid cannot remain at rest under the presence of a shear stress. However, fluids can exert pressure normal to any contacting surface. If a point in the fluid is thought of as an infinitesimally small cube it follows from the principles of equilibrium that the pressure on every side of this unit of fluid must be equal.
If this were not the case, the fluid would move in the direction of the resulting force. Thus, the pressure on a fluid at rest is isotropic; this characteristic allows fluids to transmit force through the length of tubes. This principle was first formulated, in a extended form, by Blaise Pascal, is now called Pascal's law. In a fluid at rest, all frictional and inertial stresses vanish and the state of stress of the system is called hydrostatic; when this condition of V = 0 is applied to the Navier-Stokes equation, the gradient of pressure becomes a function of body forces only. For a barotropic fluid in a conservative force field like a gravitational force field, pressure exerted by a fluid at equilibrium becomes a function of force exerted by gravity; the hydrostatic pressure can be determined from a control volume analysis of an infinitesimally small cube of fluid. Since pressure is defined as the force exerted on a test area, the only force acting on any such small cube of fluid is the weight of the fluid column above it, hydrostatic pressure can be calculated according to the following formula: p − p = 1 A ∫ z 0 z d z ′ ∬ A d x ′ d y ′ ρ g = ∫ z 0 z d z ′ ρ g, where: p is the hydrostatic pressure, ρ is the fluid density
Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, transporting molecules from one location to another. Proteins differ from one another in their sequence of amino acids, dictated by the nucleotide sequence of their genes, which results in protein folding into a specific three-dimensional structure that determines its activity. A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. Short polypeptides, containing less than 20–30 residues, are considered to be proteins and are called peptides, or sometimes oligopeptides; the individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues. The sequence of amino acid residues in a protein is defined by the sequence of a gene, encoded in the genetic code.
In general, the genetic code specifies 20 standard amino acids. Shortly after or during synthesis, the residues in a protein are chemically modified by post-translational modification, which alters the physical and chemical properties, stability and the function of the proteins. Sometimes proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors. Proteins can work together to achieve a particular function, they associate to form stable protein complexes. Once formed, proteins only exist for a certain period and are degraded and recycled by the cell's machinery through the process of protein turnover. A protein's lifespan covers a wide range, they can exist for years with an average lifespan of 1 -- 2 days in mammalian cells. Abnormal or misfolded proteins are degraded more either due to being targeted for destruction or due to being unstable. Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in every process within cells.
Many proteins are enzymes that are vital to metabolism. Proteins have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, the cell cycle. In animals, proteins are needed in the diet to provide the essential amino acids that cannot be synthesized. Digestion breaks the proteins down for use in the metabolism. Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation, precipitation and chromatography. Methods used to study protein structure and function include immunohistochemistry, site-directed mutagenesis, X-ray crystallography, nuclear magnetic resonance and mass spectrometry. Most proteins consist of linear polymers built from series of up to 20 different L-α- amino acids. All proteinogenic amino acids possess common structural features, including an α-carbon to which an amino group, a carboxyl group, a variable side chain are bonded.
Only proline differs from this basic structure as it contains an unusual ring to the N-end amine group, which forces the CO–NH amide moiety into a fixed conformation. The side chains of the standard amino acids, detailed in the list of standard amino acids, have a great variety of chemical structures and properties; the amino acids in a polypeptide chain are linked by peptide bonds. Once linked in the protein chain, an individual amino acid is called a residue, the linked series of carbon and oxygen atoms are known as the main chain or protein backbone; the peptide bond has two resonance forms that contribute some double-bond character and inhibit rotation around its axis, so that the alpha carbons are coplanar. The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone; the end with a free amino group is known as the N-terminus or amino terminus, whereas the end of the protein with a free carboxyl group is known as the C-terminus or carboxy terminus.
The words protein and peptide are a little ambiguous and can overlap in meaning. Protein is used to refer to the complete biological molecule in a stable conformation, whereas peptide is reserved for a short amino acid oligomers lacking a stable three-dimensional structure. However, the boundary between the two is not well defined and lies near 20–30 residues. Polypeptide can refer to any single linear chain of amino acids regardless of length, but implies an absence of a defined conformation. Proteins can interact with many types of molecules, including with other proteins, with lipids, with carboyhydrates, with DNA, it has been estimated. Smaller bacteria, such as Mycoplasma or spirochetes contain fewer molecules, on the order of 50,000 to 1 million. By contrast, eukaryotic cells are larger and thus contain much more pro
Gelatin or gelatine is a translucent, flavorless food ingredient, derived from collagen taken from animal body parts. Brittle when dry and gummy when moist, it is called hydrolyzed collagen, collagen hydrolysate, gelatine hydrolysate, hydrolyzed gelatine, collagen peptides, it is used as a gelling agent in food, medications and vitamin capsules, photographic films and papers, cosmetics. Substances containing gelatin or functioning in a similar way are "gelatinous". Gelatin is an irreversibly hydrolyzed form of collagen, wherein the hydrolysis reduces protein fibrils into smaller peptides. Gelatin is in gelatin desserts. Gelatin for cooking comes as powder and sheets. Instant types can be added to the food. Hydrolysis results in the reduction of collagen protein fibrils of about 300,000 Da into smaller peptides. Depending upon the process of hydrolysis, peptides will have broad molecular weight ranges associated with physical and chemical methods of denaturation; the amino acid content of hydrolyzed collagen is the same as collagen.
Hydrolyzed collagen contains 19 amino acids, predominantly glycine and hydroxyproline, which together represent around 50% of the total amino acid content. Hydrolyzed collagen contains 8 out of 9 essential amino acids, including glycine and arginine—two amino-acid precursors necessary for the biosynthesis of creatine, it contains no tryptophan and is deficient in isoleucine and methionine. The bioavailability of hydrolyzed collagen in mice was demonstrated in a 1999 study. A 2005 study in humans found hydrolyzed collagen absorbed as small peptides in the blood. Ingestion of hydrolyzed collagen may affect the skin by increasing the density of collagen fibrils and fibroblasts, thereby stimulating collagen production, it has been suggested, based on mouse and in vitro studies, that hydrolyzed collagen peptides have chemotactic properties on fibroblasts or an influence on growth of fibroblasts. Some clinical studies report that the oral ingestion of hydrolyzed collagen decreases joint pain, those with the most severe symptoms showing the most benefit.
Beneficial action is due to hydrolyzed collagen accumulation in the cartilage and stimulated production of collagen by the chondrocytes, the cells of cartilage. Several studies have shown that a daily intake of hydrolyzed collagen increases bone mass density in rats, it seems that hydrolyzed collagen peptides stimulated differentiation and osteoblasts activity - the cells that build bone - over that of osteoclasts. However, other clinical trials have yielded mixed results. In 2011, the European Food Safety Authority Panel on Dietetic Products and Allergies concluded that "a cause and effect relationship has not been established between the consumption of collagen hydrolysate and maintenance of joints". Four other studies reported benefit with no side effects. One study found that oral collagen only improved symptoms in a minority of patients and reported nausea as a side effect. Another study reported no improvement in disease activity in patients with rheumatoid arthritis. Another study found that collagen treatment may cause an exacerbation of rheumatoid arthritis symptoms.
Hydrolyzed collagen, like gelatin, is made from animal by-products from the meat industry, including skin and connective tissue. In 1997, the U. S. Food and Drug Administration, with support from the TSE Advisory Committee, began monitoring the potential risk of transmitting animal diseases bovine spongiform encephalopathy known as mad cow disease. An FDA study from that year stated: "...steps such as heat, alkaline treatment, filtration could be effective in reducing the level of contaminating TSE agents. On March 18, 2016 the FDA finalized three previously-issued interim final rules designed to further reduce the potential risk of BSE in human food; the final rule clarified that "gelatin is not considered a prohibited cattle material if it is manufactured using the customary industry processes specified."The Scientific Steering Committee of the European Union in 2003 stated that the risk associated with bovine bone gelatin is low or zero. In 2006, the European Food Safety Authority stated that the SSC opinion was confirmed, that the BSE risk of bone-derived gelatin was small, that it recommended removal of the 2003 request to exclude the skull and vertebrae of bovine origin older than 12 months from the material used in gelatin manufacturing.
In cosmetics, hydrolyzed collagen may be found in topical creams, acting as a product texture conditioner, moisturizer. Gelatin is a mixture of peptides and proteins produced by partial hydrolysis of collagen extracted from the skin and connective tissues of animals such as domesticated cattle, chicken and fish. During hydrolysis, the natural molecular bonds between individual collagen strands are broken down into a form that rearranges more easily, its chemical composition is, in many aspects similar to that of its parent collagen. P
Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is defined as the measure of the tendency of a solution to take in pure solvent by osmosis. Potential osmotic pressure is the maximum osmotic pressure that could develop in a solution if it were separated from its pure solvent by a semipermeable membrane. Osmosis occurs when two solutions, containing different concentration of solute, are separated by a selectively permeable membrane. Solvent molecules pass preferentially through the membrane from the low-concentration solution to the solution with higher solute concentration; the transfer of solvent molecules will continue. Jacobus van't Hoff found a quantitative relationship between osmotic pressure and solute concentration, expressed in the following equation. Π = i C R T where Π is osmotic pressure, i is the dimensionless van't Hoff index, C is the molar concentration of solute, R is the ideal gas constant, T is the temperature in kelvins.
This formula applies when the solute concentration is sufficiently low that the solution can be treated as an ideal solution. The proportionality to concentration means. Note the similarity of this formula to the ideal gas law in the form p = n V R T = c gas R T where n is the total number of moles of gas molecules in the volume V, n/V is the molar concentration of gas molecules. Harmon Northrop Morse and Frazer showed that the equation applied to more concentrated solutions if the unit of concentration was molal rather than molar. For more concentrated solutions the van't Hoff equation can be extended as a power series in solute concentration, C. To a first approximation, Π = Π 0 + A C 2 where Π 0 is the ideal pressure and A is an empirical parameter; the value of the parameter A can be used to calculate Pitzer parameters. Empirical parameters are used to quantify the behaviour of solutions of ionic and non-ionic solutes which are not ideal solutions in the thermodynamic sense; the Pfeffer cell was developed for the measurement of osmotic pressure.
Osmotic pressure measurement may be used for the determination of molecular weights. Osmotic pressure is an important factor affecting cells. Osmoregulation is the homeostasis mechanism of an organism to reach balance in osmotic pressure. Hypertonicity is the presence of a solution. Hypotonicity is the presence of a solution. Isotonicity is the presence of a solution; when a biological cell is in a hypotonic environment, the cell interior accumulates water, water flows across the cell membrane into the cell, causing it to expand. In plant cells, the cell wall restricts the expansion, resulting in pressure on the cell wall from within called turgor pressure. Turgor pressure allows herbaceous plants to stand upright, it is the determining factor for how plants regulate the aperture of their stomata. In animal cells excessive osmotic pressure can result in cytolysis. Osmotic pressure is the basis of filtering, a process used in water purification; the water to be purified is placed in a chamber and put under an amount of pressure greater than the osmotic pressure exerted by the water and the solutes dissolved in it.
Part of the chamber opens to a differentially permeable membrane that lets water molecules through, but not the solute particles. The osmotic pressure of ocean water is about 27 atm. Reverse osmosis desalinates fresh water from ocean salt water. Consider the system at the point when it has reached equilibrium; the condition for this is that the chemical potential of the solvent on both sides of the membrane is equal. The compartment containing the pure solvent has a chemical potential of μ 0, where p is the pressure. On the other side, in the compartment containing the solute, the chemical potential of the solvent depends on the mole fraction of the solvent, 0 < x v < 1. Besides, this compartment can assume a different pressure, p ′. We can therefore write the chemical potential of the solvent as μ v. If we write p ′ = p + Π, the balance of the chemical potential is therefore: μ v 0 = μ v. Here, the difference in pressure of the two compartments Π ≡ p ′ − p is defined as the osmotic pressure exerted by the solutes.
Holding the pressure, the addition of solute decreases the chemical potential
The albumins are a family of globular proteins, the most common of which are the serum albumins. All the proteins of the albumin family are water-soluble, moderately soluble in concentrated salt solutions, experience heat denaturation. Albumins are found in blood plasma and differ from other blood proteins in that they are not glycosylated. Substances containing albumins, such as egg white, are called albuminoids. A number of blood transport proteins are evolutionarily related, including serum albumin, alpha-fetoprotein, vitamin D-binding protein and afamin. Albumin binds to the cell surface receptor albondin. Serum albumin is the main protein of human blood plasma, it binds water, fatty acids, bilirubin and pharmaceuticals: its main function is to regulate the oncotic pressure of blood. Alpha-fetoprotein is a fetal plasma protein that binds fatty acids and bilirubin. Vitamin D-binding protein binds to its metabolites, as well as to fatty acids; the isoelectric point of albumin is 4.9. The 3D structure of human serum albumin has been determined by X-ray crystallography to a resolution of 2.5 ångströms.
Albumin is a 65–70 kDa protein. Albumin comprises three homologous domains; each domain is a product of two subdomains. The principal regions of ligand binding to human serum albumin are located in hydrophobic cavities in subdomains IIA and IIIA, which exhibit similar chemistry. Structurally, the serum albumins are similar, each domain containing five or six internal disulfide bonds, as shown schematically below: Serum albumin is the most abundant blood plasma protein and is produced in the liver and forms a large proportion of all plasma protein; the human version is human serum albumin, it constitutes about 50% of human plasma protein. Serum albumins are important in regulating blood volume by maintaining the oncotic pressure of the blood compartment, they serve as carriers for molecules of low water solubility this way isolating their hydrophobic nature, including lipid-soluble hormones, bile salts, unconjugated bilirubin, free fatty acids, calcium and some drugs like warfarin, clofibrate & phenytoin.
For this reason, it is sometimes referred as a molecular "taxi". Competition between drugs for albumin binding sites may cause drug interaction by increasing the free fraction of one of the drugs, thereby affecting potency. Specific types include: human serum albumin bovine serum albumin or BSA used in medical and molecular biology labs; the normal range of human serum albumin in adults is 3.5 to 5 g/dL. For children less than three years of age, the normal range is broader, 2.9–5.5 g/dL. Low albumin may be caused by liver disease, nephrotic syndrome, protein-losing enteropathy, malnutrition, late pregnancy, genetic variations and malignancy. High albumin is always caused by dehydration. In some cases of retinol deficiency, the albumin level can be elevated to high-normal values; this is. This swelling likely occurs during treatment with 13-cis retinoic acid, a pharmaceutical for treating severe acne, amongst other conditions. In lab experiments it has been shown that all-trans retinoic acid down regulates human albumin production.
Other albumin types include the storage protein ovalbumin in egg white, different storage albumins in the seeds of some plants, including hemp. Note that the protein "albumin" is spelled with an "i", while "albumen" with an "e", is the white of an egg, which contains several dozen types of albumin ovalbumin. For patients with low blood volume, there is no evidence that albumin reduces mortality when compared with cheaper alternatives such as normal saline, or that albumin reduces mortality in patients with burns and low albumin levels. Therefore, the Cochrane Collaboration recommends. In acoustic droplet vaporization, albumin is sometimes used as a surfactant. ADV has been proposed as a cancer treatment by means of occlusion therapy. Human serum albumin may be used to reverse drug/chemical toxicity by binding to free drug/agent. Worldwide, certain traditional Chinese medicines contain wild bear bile, banned under CITES legislation. Dip sticks, similar to common pregnancy tests, have been developed to detect the presence of bear albumin in traditional medicine products, indicating that bear bile had been used in their creation.
Cohn process Serum albumin Bovine serum albumin Human serum albumin Albumins at the US National Library of Medicine Medical Subject Headings The Albumin website Albumin binding prediction
Water is a transparent, tasteless and nearly colorless chemical substance, the main constituent of Earth's streams and oceans, the fluids of most living organisms. It is vital for all known forms of life though it provides no calories or organic nutrients, its chemical formula is H2O, meaning that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. Water is the name of the liquid state of H2O at standard ambient pressure, it forms precipitation in the form of rain and aerosols in the form of fog. Clouds are formed from suspended droplets of its solid state; when finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is water vapor. Water moves continually through the water cycle of evaporation, condensation and runoff reaching the sea. Water covers 71% of the Earth's surface in seas and oceans. Small portions of water occur as groundwater, in the glaciers and the ice caps of Antarctica and Greenland, in the air as vapor and precipitation.
Water plays an important role in the world economy. 70% of the freshwater used by humans goes to agriculture. Fishing in salt and fresh water bodies is a major source of food for many parts of the world. Much of long-distance trade of commodities and manufactured products is transported by boats through seas, rivers and canals. Large quantities of water and steam are used for cooling and heating, in industry and homes. Water is an excellent solvent for a wide variety of chemical substances. Water is central to many sports and other forms of entertainment, such as swimming, pleasure boating, boat racing, sport fishing, diving; the word water comes from Old English wæter, from Proto-Germanic *watar, from Proto-Indo-European *wod-or, suffixed form of root *wed-. Cognate, through the Indo-European root, with Greek ύδωρ, Russian вода́, Irish uisce, Albanian ujë; the identification of water as a substance Water is a polar inorganic compound, at room temperature a tasteless and odorless liquid, nearly colorless with a hint of blue.
This simplest hydrogen chalcogenide is by far the most studied chemical compound and is described as the "universal solvent" for its ability to dissolve many substances. This allows it to be the "solvent of life", it is the only common substance to exist as a solid and gas in normal terrestrial conditions. Water is a liquid at the pressures that are most adequate for life. At a standard pressure of 1 atm, water is a liquid between 0 and 100 °C. Increasing the pressure lowers the melting point, about −5 °C at 600 atm and −22 °C at 2100 atm; this effect is relevant, for example, to ice skating, to the buried lakes of Antarctica, to the movement of glaciers. Increasing the pressure has a more dramatic effect on the boiling point, about 374 °C at 220 atm; this effect is important in, among other things, deep-sea hydrothermal vents and geysers, pressure cooking, steam engine design. At the top of Mount Everest, where the atmospheric pressure is about 0.34 atm, water boils at 68 °C. At low pressures, water cannot exist in the liquid state and passes directly from solid to gas by sublimation—a phenomenon exploited in the freeze drying of food.
At high pressures, the liquid and gas states are no longer distinguishable, a state called supercritical steam. Water differs from most liquids in that it becomes less dense as it freezes; the maximum density of water in its liquid form is 1,000 kg/m3. The density of ice is 917 kg/m3. Thus, water expands 9% in volume as it freezes, which accounts for the fact that ice floats on liquid water; the details of the exact chemical nature of liquid water are not well understood. Pure water is described as tasteless and odorless, although humans have specific sensors that can feel the presence of water in their mouths, frogs are known to be able to smell it. However, water from ordinary sources has many dissolved substances, that may give it varying tastes and odors. Humans and other animals have developed senses that enable them to evaluate the potability of water by avoiding water, too salty or putrid; the apparent color of natural bodies of water is determined more by dissolved and suspended solids, or by reflection of the sky, than by water itself.
Light in the visible electromagnetic spectrum can traverse a couple meters of pure water without significant absorption, so that it looks transparent and colorless. Thus aquatic plants and other photosynthetic organisms can live in water up to hundreds of meters deep, because sunlight can reach them. Water vapour is invisible as a gas. Through a thickness of 10 meters or more, the intrinsic color of water is visibly turquoise, as its absorption spectrum has