A chemical compound (or just compound if used in the context of chemistry) is an entity consisting of two or more atoms, at least two from different elements, which associate via chemical bonds. There are four types of compounds, depending on how the constituent atoms are held together: molecules held together by covalent bonds, ionic compounds held together by ionic bonds, intermetallic compounds held together by metallic bonds, and certain complexes held together by coordinate covalent bonds. Many chemical compounds have a unique numerical identifier assigned by the Chemical Abstracts Service (CAS): its CAS number. A chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using the standard abbreviations for the chemical elements, and subscripts to indicate the number of atoms involved. For example, water is composed of two hydrogen atoms bonded to one oxygen atom: the chemical formula is H2O.

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A compound can be converted to a different chemical composition by interaction with a second chemical compound via a chemical reaction. In this process, bonds between atoms are broken in both of the interacting compounds, and then bonds are reformed so that new associations are made between atoms. Schematically, this reaction could be described as AB + CD -->AC + BD, where A, B, C, and D are each unique atoms; and AB, CD, AC, and BD are each unique compounds. The physical and chemical properties of compounds differ from those of their constituent elements.

This is one of the main criteria that distinguish a compound from a mixture of elements or other substances—in general, a mixture's properties are closely related to, and depend on, the properties of its constituents. Another criterion that distinguishes a compound from a mixture is that constituents of a mixture can usually be separated by simple mechanical means, such as filtering, evaporation, or magnetic force, but components of a compound can be separated only by a chemical reaction. However, mixtures can be created by mechanical means alone, but a compound can be created (either from elements or from other compounds, or a combination of the two) only by a chemical reaction. Compounds may have several possible phases. All compounds can exist as solids, at least at low enough temperatures. Free Download Smscaster Keygen.

Molecular compounds may also exist as liquids, gases, and, in some cases, even plasmas. All compounds decompose upon applying heat. The temperature at which such fragmentation occurs is often called the decomposition temperature. Decomposition temperatures are not sharp and depend on pressure, temperature, and the concentration of each species in the compound.

Serial Communicator Mecanique Rationnelle. Electricians were originally people who demonstrated or studied the principles of electricity, often electrostatic generators of one form or another. In the United States, electricians are divided into two primary categories: linemen, who work on electric utility company distribution systems at higher voltages, and wiremen, who work with the lower voltages utilized inside buildings.

Wiremen are generally trained in one of five primary specialties: commercial, residential, light industrial, industrial, and low-voltage wiring, more commonly known as Voice-Data-Video, or VDV. Other sub-specialties such as control wiring and fire-alarm may be performed by specialists trained in the devices being installed, or by inside wiremen. Electricians are trained to one of three levels: Apprentice, Journeyman, and Master Electrician. In the US and Canada, apprentices work and receive a reduced compensation while learning their trade. They generally take several hundred hours of classroom instruction and are contracted to follow apprenticeship standards for a period of between three and six years, during which time they are paid as a percentage of the Journeyman's pay.

Journeymen are electricians who have completed their Apprenticeship and who have been found by the local, State, or National licensing body to be competent in the electrical trade. Master Electricians have performed well in the trade for a period of time, often seven to ten years, and have passed an exam to demonstrate superior knowledge of the National Electrical Code, or NEC. Service electricians are tasked to respond to requests for isolated repairs and upgrades. They have skills troubleshooting wiring problems, installing wiring in existing buildings, and making repairs. Construction electricians primarily focus on larger projects, such as installing all new electrical system for an entire building, or upgrading an entire floor of an office building as part of a remodeling process. Other specialty areas are marine electricians, research electricians and hospital electricians. 'Electrician' is also used as the name of a role in stagecraft, where electricians are tasked primarily with hanging, focusing, and operating stage lighting.

In this context, the Master Electrician is the show's chief electrician. Although theater electricians routinely perform electrical work on stage lighting instruments and equipment, they are not part of the electrical trade and have a different set of skills and qualifications from the electricians that work on building wiring. In the film industry and on a television crew the head electrician is referred to as a Gaffer.

Electrical contractors are businesses that employ electricians to design, install, and maintain electrical systems. Contractors are responsible for generating bids for new jobs, hiring tradespeople for the job, providing material to electricians in a timely manner, and communicating with architects, electrical and building engineers, and the customer to plan and complete the finished product.