Chemistry is the scientific discipline involved with elements and compounds composed of atoms and ions: their composition, properties and the changes they undergo during a reaction with other substances. In the scope of its subject, chemistry occupies an intermediate position between physics and biology, it is sometimes called the central science because it provides a foundation for understanding both basic and applied scientific disciplines at a fundamental level. For example, chemistry explains aspects of plant chemistry, the formation of igneous rocks, how atmospheric ozone is formed and how environmental pollutants are degraded, the properties of the soil on the moon, how medications work, how to collect DNA evidence at a crime scene. Chemistry addresses topics such as how atoms and molecules interact via chemical bonds to form new chemical compounds. There are four types of chemical bonds: covalent bonds, in which compounds share one or more electron; the word chemistry comes from alchemy, which referred to an earlier set of practices that encompassed elements of chemistry, philosophy, astronomy and medicine.
It is seen as linked to the quest to turn lead or another common starting material into gold, though in ancient times the study encompassed many of the questions of modern chemistry being defined as the study of the composition of waters, growth, disembodying, drawing the spirits from bodies and bonding the spirits within bodies by the early 4th century Greek-Egyptian alchemist Zosimos. An alchemist was called a'chemist' in popular speech, the suffix "-ry" was added to this to describe the art of the chemist as "chemistry"; the modern word alchemy in turn is derived from the Arabic word al-kīmīā. In origin, the term is borrowed from the Greek χημία or χημεία; this may have Egyptian origins since al-kīmīā is derived from the Greek χημία, in turn derived from the word Kemet, the ancient name of Egypt in the Egyptian language. Alternately, al-kīmīā may derive from χημεία, meaning "cast together"; the current model of atomic structure is the quantum mechanical model. Traditional chemistry starts with the study of elementary particles, molecules, metals and other aggregates of matter.
This matter can be studied in isolation or in combination. The interactions and transformations that are studied in chemistry are the result of interactions between atoms, leading to rearrangements of the chemical bonds which hold atoms together; such behaviors are studied in a chemistry laboratory. The chemistry laboratory stereotypically uses various forms of laboratory glassware; however glassware is not central to chemistry, a great deal of experimental chemistry is done without it. A chemical reaction is a transformation of some substances into one or more different substances; the basis of such a chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. It can be symbolically depicted through a chemical equation, which involves atoms as subjects; the number of atoms on the left and the right in the equation for a chemical transformation is equal. The type of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws.
Energy and entropy considerations are invariably important in all chemical studies. Chemical substances are classified in terms of their structure, phase, as well as their chemical compositions, they can be analyzed using the tools of e.g. spectroscopy and chromatography. Scientists engaged in chemical research are known as chemists. Most chemists specialize in one or more sub-disciplines. Several concepts are essential for the study of chemistry; the particles that make up matter have rest mass as well – not all particles have rest mass, such as the photon. Matter can be a mixture of substances; the atom is the basic unit of chemistry. It consists of a dense core called the atomic nucleus surrounded by a space occupied by an electron cloud; the nucleus is made up of positively charged protons and uncharged neutrons, while the electron cloud consists of negatively charged electrons which orbit the nucleus. In a neutral atom, the negatively charged electrons balance out the positive charge of the protons.
The nucleus is dense. The atom is the smallest entity that can be envisaged to retain the chemical properties of the element, such as electronegativity, ionization potential, preferred oxidation state, coordination number, preferred types of bonds to form. A chemical element is a pure substance, composed of a single type of atom, characterized by its particular number of protons in the nuclei of its atoms, known as the atomic number and represented by the symbol Z; the mass number is the sum of the number of neutrons in a nucleus. Although all the nuclei of all atoms belonging to one element will have the same
Supersaturation is a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. It can refer to a vapor of a compound that has a higher pressure than the vapor pressure of that compound. Special conditions need to be met in order to generate a supersaturated solution. One of the easiest ways to do this relies on the temperature dependence of solubility; as a general rule, the more heat is added to a system, the more soluble a substance becomes.. Therefore, at high temperatures, more solute can be dissolved than at lower temperatures. If this solution were to be cooled at a rate faster than the rate of precipitation, the solution will become supersaturated until the solute precipitates to the temperature-determined saturation point; the precipitation or crystallization of the solute takes longer than the actual cooling time because the molecules need to meet up and form the precipitate without being knocked apart by water. Thus, the larger the molecule, the longer the solute will take to crystallize due to the principles of Brownian motion.
The condition of supersaturation does not have to be reached through the manipulation of heat. The ideal gas law suggests that pressure and volume can be changed to force a system into a supersaturated state. If the volume of solvent is decreased, the concentration of the solute can be above the saturation point and thus create a supersaturated solution; the decrease in volume is most generated through evaporation. An increase in pressure can drive a solution to a supersaturated state. All three of these mechanisms rely on the fact that the conditions of the solution can be changed quicker than the solute can precipitate or crystallize out. Supersaturated solutions will undergo crystallization under specific conditions. In a normal solution, once the maximum amount of solute is dissolved, adding more solute would either cause the dissolved solute to precipitate out and/or for the solute to not dissolve at all. There are cases wherein solubility of a saturated solution is decreased by manipulating temperature, pressure, or volume but a supersaturated state does not occur.
In these cases, the solute will precipitate out. This is. A supersaturated solution of gases in a liquid may form bubbles. Supersaturation may be defined as a sum of all gas partial pressures in the liquid which exceeds the ambient pressure in the liquid. Crystallization will occur to allow the solution to reach a lower energy state.. The activation energy comes in the form of a nuclei crystal being added to the liquid solution; this nuclei can be either added from another source, known as seeding, or can spontaneously form within the solution due to ion and molecule interactions. This process is known as primary nucleation, it is necessary for the nuclei to be identical to the solute, crystallizing. This will allow for the dissolved ions to build up on the nuclei and each other in the process of crystal growth or secondary nucleation. There are a multitude of factors that will affect the rate and order of magnitude with which crystallization proceeds as well as the difference in formation of crystallites and single crystals.
A crystallization phase diagram shows where undersaturation and supersaturation occur at certain concentrations. Concentrations below the solubility curve result in an undersaturation solution. Saturation occurs. If the concentrations are above the solubility curve, the solution is considered supersaturated. There are three mechanisms with which supersaturation occurs: precipitation and metastable. In the precipitation zone, the molecules in a solution are in excess and will separate from the solution to form amorphous aggregates; the excess of molecules aggregate to form a crystalline structure. In the metastable zone, the solution takes time to nucleate. In order to grow crystals while in the metastable zone, the conditions would require the formation of one nucleus while in the nucleation zone, just past the metastable region; the supersaturated solution can return to the metastable region. Supersaturation in vapor phase is related to the surface tension of liquids through the Kelvin equation, the Gibbs–Thomson effect and the Poynting effect.
The International Association for the Properties of Water and Steam provides a special equation for the Gibbs free energy in the metastable-vapor region of water in its Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam. All thermodynamic properties for the metastable-vapor region of water can be derived from this equation by means of the appropriate relations of thermodynamic properties to the Gibbs free energy. Table 1. Supersaturation measurement methods. Supersaturation has been a frequent topic of research throughout history. Early studies of these solutions were conducted with sodium sulfate known as Glauber’s Salt, due to the stability of the crystal and the rising role it had in industry. Through the use of this salt, an important scientific discovery was made by Jean-Baptiste Ziz, a botanist from Mayence, in 1809, his experiments allowed him to conclude that the crystallization of a supersaturated solution does not come from its agitation, but from solid matter entering and acting as a “starting” site for crystals to form