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Glossary of chemical terms.Chemical glossary.

A
Absolute Zero
The zero point on the absolute temperature scale, -273.15°C or 0 K, theoretically, the temperature at which molecular motion ceases. The concept of an absolute zero temperature was first deduced from experiments with gases. When a fixed volume of gas is cooled, its pressure decreases with its temperature.

Absolute zero can't be reached through experiments, although it can be closely approached. According to the third law of thermodynamics the entropy of a pure crystal is zero at absolute zero temperature.

Absorption
In chemistry absorption can mean two things: Firstly it can imply that powerful forces exist holding two substances together, and that seperation of the two is not easily accomplished. Secondly it can mean absorption of heat, light etc.. . The absorption of ultraviolet, visible and infrared radiation is the basis of some forms of spectrometry which can be used to identify different chemical compounds.

Acid
A substance that when dissolved in water dissociates and can donate a hydrogen (proton) to another molecule. Examples include sulphuric acid and nitric acid .Under these conditions nitric acid dissociates according to the following equation:

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In water the protons attach themselves to the water molecules, giving the following equation:

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Actinides
This is a group of elements within the periodic table and are also known as the transuranics. The name refers to those elements with increasing atomic no. from actinium, which all have similar chemical properties, like the lanthanides. The first few members of the group are the naturally occuring elements actinium, thorium, proactinium and uranium. Beyond this elements have been made artificially by radioactive bombardment. These artificial elements are unstable, some have very short lifeimes and undergo spontaneous radioactive decay.

Adhesive
Any compound that can stick two surfaces together is classified as an adhesive. Simple adhesives are of cellulose , starch and rubbers. Modern adhesives are based upon complex polymeric materials. The adhesive is spread in an unpolymerised form, and the adhesive properties increase as polymerisation occurs between the two surfaces.

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Adiabatic
A chemical or mechanical process which takes place without heat entering or leaving the system. The term is only applicable to enclosed and isolated systems - so in essence is idealistic and purely theoretical and is important in the study of thermodynamics.

Adrenaline
This was the first naturally produced hormone to be isolated it a pure state. It is known as epinephrine, but its chemical name is 1-[3,4-dihydroxyphenol]-2-methylaminoethanol. Its main action is to raise blood pressure, producing a faster pulse rate.

Adsorption
If any compound, solid, liquid or gas, is loosely held by weak attraction to the surface of a solid it is said to have undergone adsorption. This process is much weaker and less permanent than absorption.

Air
The air that we breathe is a mixture of gases. The composition of dry-air at sea level is shown below:

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Alchemy
Several of the great Greek philosophers considered all matter to be composed of four basic "elements", fire, air, earth and water, and that all materials were had these components mixed in different proportions. If this theory was true, then it was believed that all substances could be converted (transmuted) into each other by varying the proportions of the mixture. This tansmutation theory was the basis of Alchemy from approx. 300 B.C. to 1500 A.D., after which it slowly gave way to the more scientific concepts of chemistry from the 17th century onwards. The alchemists represented their "element" by a variety of symbols (see below), some of which were based on astrological signs They were mainly concerned with trying to turn base metals into gold and looking for the elixir of life although they also developed medicinal drugs, developed couterfeiting techniques and the debasement of precious metals.

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Alcohol
An organic compound which has the general formula CnH2n+1OH, they consist of hydrocarbon chains terminated by hydroxyl groups, O-H. Smaller members are water soluble, flammable and are useful as organic solvents and fuels. As with hydrocarbons, each member differs from the previous by an additional -CH2- group. Alcohols with branced chains are also possible. Some of the simpler alcohols are listed below:

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Aldehyde
An organic compound containing the -CHO group (see below). An example of these distinct aromatic compounds is formaldehyde .

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Alicyclic compounds
An alicyclic compound contains rings of -CH2- units joined by single bonds. Their general formula is CnH2n, the simplest member being cyclopropane. An example of these compounds is cyclohexane (see below):

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Alkali
A water soluble hydroxide on one of the alkali metals. The term is virually synonymous with the term base. An example is Sodium Hydroxide, NaOH.

Alkali metals
Group I of the periodic table consists of the alkali metals. They are the most electropositive elements known, are monovalent, have low melting points and react violently with water.

Alkaline earth metals
Group II of the periodic table consists of the alkaline earth metals. They are bivalent and react with water to produce water soluble hydroxides.

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Alkaloid
A naturally occuring organic compound containing nitrogen that acts as a base. Many alkaloids are physiologically active and can be used in small quantities as medicines, but if taken in larger doses they can be extremely poisonous. An example is caffeine .

Alkanes
This is the correct chemical term for compounds known as paraffins. They are considered the simplest organic compounds and are a family of chain hydrocarbons having the general formula C2H2n+2. All of the bonds are single bonds (-C-H-, and -C-C-). The chains can be straight or branched. The smaller members (less than 4 carbons) are gases, while larger ones (five to seventeen carbons) are liquids. Beyond seventeen carbons the alkanes are waxy solids. Some simple alkanes are listed below:

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Alkenes
These compounds are similar to alkanes, in that they can be straight or branched aliphatic hydrocarbons. The only difference is that the alkenes (also known as olefins) contain a carbon-carbon double bond, and have the general formula C2H2n. The simplest group member is ethene which is a gas. The double bond can be anywhere within the length of the carbon chain, resulting in a large number of possible isomers. The double bond means that these compounds are "unsaturated" and react readily with compounds capable of adding across the double bond. Some simple alkenes are listed below:

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Alkynes
These compounds are also similar to alkanes. They have the general formula C2H2n-2 corresponding to carbon chains with a triple carbon-carbon bond included. The simplest member of this family is ethyne (also known as acetylene). The triple bond makes these compounds very reactive. Some simple alkynes are listed below:

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Allotropy
Certain chemical elements have the ability to exist in two or more different structural forms known as allotropes. These allotropes may possess different physical properties such as density and melting points. Allotropic elements include carbon , tin, phosphorus and sulphur. Each allotrope is stable within a certain range of temperature and pressure only, and under certain conditions an allotrope can be converted into another.

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Alloy
An alloy consists of an intimate mixture of two elements, usually metals to give a metal compound or solid solution. Bronze is an alloy of copper and tin and brass on of copper and zinc. By varying the composition it is possible to alter the chemical and physical properties of the alloy such as conductivity, ductility and corrosion resistance.

Alpha particles
These are particles possessing a positive charge, and are emited from radioactive elements. The particles are actually helium nuclei, ie. helium atoms that had lost their outer electrons. The study of the alpha particle played a major part in the development of modern atomic theory. Rutherford bombarded other elements with alpha particles to produce nuclear disintegration.

Alum
An alum is a double salt comprising a sulphate salt of a monovalent element, with the sulphate salt of a trivalent element. The most well known example is potash alum (aluminium potassium sulphate) KAl(SO4)2.12H2O which is used to clear murky water.

Amalgam
Liquid mercury has the ability to dissolve other metals to produce alloys and these alloys are called amalgams. Amalgam examples are gold/mercury, silver/mercury and copper/mercury all of which are used in dentistry.

Amide
An organic compound containing the -CONH2- group (see below).

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Amine
An amine is a distinctive smelling organic compound with one or more of the hydrogen atoms in ammonia replaced by organic groups. There are three classes of amines, dependant on the number of hydrogens replaced (see below):

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The organic groups can be aliphatic, alicyclic or aromatic, and some common amines are listed below:

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Amino group
An organic compound containing the -NH2- group. Substances which contain this group are called amines. Examples include urea, and trimethylamine .

Amino acid
A large class of substances with molecules including the amino and carboxyl groups.In the human body, amino acids are joined together to form long chains as part of the structures of proteins. Because they possess two functional groups, amino acids exhibit both acidic and basic characteristics. Each amino acid has an abbreviation when in a protein chain, so that the protein GlyAlaVal would indicate that a molecule of Glycine is joined to one of Alanine which is in turn joined to one of Valine. A list of the more common amino acids are given below:

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Anhydride
These are compounds formed by the removal of water or Hydrogen and Oxygen together from another substance. In inorganic chemistry most anhydrides are formed by the loss of the water of crystallisation. eg. Copper(II) sulphate is a blue hydrated salt with the formula CuSO4.5H2O, but if heated, the water is driven off leaving the colourless anhydrous salt CuSO4. In organic chemistry an anhydride is usually the result of the loss of a water molecule from a dicarboxylic acid, eg. from phthalic acid to give phthalic anhydride (see below):

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Anion
A negatively charged ion or group of atoms.

Antioxidant
A substance which decrease the rate at which another substance is oxidised. An example includes BHA.

Aromatic compound
A substance containing one or more benzene rings. These compounds although unsaturated do not undergo normal reactions for unsaturated compounds as shown by their saturated open-chain analogues. This ability is related to the way in which their bonding electrons are spread evenly over the planar ring. Substances classified as being aromatic include those with one benzene ring such as:

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- or several rings such as:

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Certain other compounds can posses aromatic character - ie. they react in an aromatic manner even though they do not have the six-membered carbon ring. This group of compounds includes:

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Atactic
A polymer is atactic if the groups attatched to the backbone are not arranged in any regular geometric pattern.

Atom
The smallest part of an element that retains its properties. It is possible to "split" the atom up further into components; protons - which have a positive charge, electrons, which have a negative charge and neutrons which have no charge. The simplest atom is hydrogen, which consists of a single proton nucleus and has a single electron forming an orbit or shell around the proton. The next simplest is helium, with two protons and two neutrons in the nucleus with two electrons orbiting within the shell - see selow for diagram: In all atoms the number of protons and electrons are the same but the number of neutrons can vary. Most of the mass of an atom is composed of the protons and neutrons, while the different isotopes of the same element vary in their number of protons, neutrons and electrons in the atomic structure. In some elements the arrangement of protons and neutrons in the nucleus is unstable, and the atom may disintegrate forming radioactivity.

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Azo dye
An azo dye is one of an extensive range of synthetic organic dye made from aniline, by first converting it with sodium nitrite to a diazonium chloride salt which is then reacted with other aromatic amines, phenols and sulphonic acids. The well known indicator methyl-orange is formed by making the diazo salt of sulphanilic acid, and reacting the product with dimethylaniline (see below): All azo compounds have the general formula Ar-N=N-Ar' with the characteristic -N=N- group in the middle.

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B
Base
A substance that reacts with acid to produce a salt and water only. It does this by accepting a hydrogen ion from the acid. An example is ammonia which accepts a proton to become the ammonium ion - NH4+

Benzene
Carcinogenic. Harmful amounts may be absorbed through skin. Irritating to mucous membranes. Poisonous when ingested. Inhalation of fumes may be toxic. Cited by the EPA and OSHA as a threat to public health. Found in oven cleaners, detergents, furniture polishes, spot removers, and nail-polish removers.

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Beta particles
Radioactive elements can emit several types of radiation. Beta particles are very fast moving electrons. The particles have a velocity of between 30-99% speed of light, and it is this velocity which gives the beta particles greater penetrating power than alpha particles.

Bond
A chemical link between two atoms. In an ionic bond the attraction is between opposite charges on two neighbouring ions. In a covalent bond the two atoms share a pair of electrons.

C
Carbanion
This is an abbreviation of "carbon anion", and is applied to negatively charged ions that are believed to occur during organic reactions. The carbanion then reacts with positive species to produce new products. To acquire a negative carbon ion it is necessary for the atom to retain two electrons forming a bond between itself and another group - see below:

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Carbonyl group
An organic compound containing the >CO group (see below). When a hydrogen atom is attached to the carbon, the resulting compound is known as an aldehyde. When only carbon atoms are attached, the resulting compound is known as a ketone.

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Carboxyl group
An organic compound containing the -COOH group (see below),

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Where a carbonyl group is attached to a hydroxyl group.

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Carboxylic acid
A substance containing the carboxyl group. The carboxylic hydrogen can be lost as a hydrogen ion, so these substances are also acidic. An example includes acetic acid.

Carcinogen
A substance capable of causing cancer in living organisms. It includes substances such as blue asbestos (crocidolite), which is a mineral, and benzene. Benzene has been used extensively for decades in chemistry and strict saftey guidlines must now be followed befrore it is used. Any new pharmaceutical, cosmetic or food chemical has to be checked to ensure that they are not carcinogenic.

Catalyst
A substance that changes the rate at which a reaction equilibrium is attained, without itself being consumed. Catalysts can increase the rate of reaction (positive catalysts) or decrease them (negative catalysts or inhibitors).

Cation
A positively charged ion or group of atoms.

Chain reaction
A reaction where the product of one step is a reactant in a later step, which produces a reactant for a later step, and so on.

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Chiral
Means "handedness" - A chiral or asymmetric molecule is one which can be distinguished from its mirror image. An example includes lactic acid.

Cis-trans isomerism
Compounds with double bonds, or alicyclic rings can exhibit isomerism, due to the attached groups lying above or below the plane of the oduble bond or ring. The "cis" compound is the one with the groups on the same side of the bond, and the "trans" has the groups on the opposite sides. The different isomers have different physical and chemical properties. Examples are fumaric acid/maleic acid and 1,3-dimethylcyclohexane - see below:

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Combustion
Combustion is the combination of a substance with oxygen in the presence of a flame accompanied by the production of heat and light. Combustion requires a supply of both fuel and oxygen (air) and can take place in the open atmosphere such as an open fire, or in a closed system, such as a car engine.

Condensation reactions
This is mainly an organic reaction where a simple molecule such as water or ammonia is eliminated when two molecules combine to produce another compound. A typical reaction is the esterification of an acid by an alcohol - in this case water is eliminated - see below:

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Condensation reactions reaction play a major part in the polymer industry for the synthesis or artificial polymers.

Cross-links
To increase the rigidity and density of synthetic polymers groups are introduced into the monomer molecules which enable bonds to form between polymer chains - this is known as cross-linking. The degree of cross-likning can have a marked effect on the physical properties of the final product. An example are the silicones - they can vary from oils (low degree of cross-linking) to waxes (high degree of cross-linking).

Crystal
A discrete solid where the atoms,ions or molecules are arranged in an ordered 3-dimensional structure. The regular structures have the ability to "bend" or diffract beams of X-rays, and this aspect of crystallography is an extremely helpful tool in structure determination.

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Cyclic compounds
A cyclic compound is one where the atoms are joined together so as to form a closed ring.

D
Decomposition
Where chemical compounds are broken up into simple molecules and even as far as their original elements. These processes are normally irreversible. An example of decomposition is when ammonium nitrate is heated. This produces nitrous oxide and water which are unable to recombine - see below:

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Detergents
These are organic chemicals designed to clean surfaces or objects. They do this by reducing the surface tension and suspend dirt in suspension from the object to be cleaned. Early detergents were long chain alcohol derivatives of sodium sulphate salts, such as sodium lauryl sulphate CH3(CH2)11O.SO2.ONa they were superseded by alkyl benzene sulphonates but these could not be broken down easily by bacteria at sewage works so these were eventually replaced by more eco-friendly detergents. Detergent molecules come in two parts - the sulphate or phosphate end (water soluble hydrophilic portion) and an organic hydrocarbon chain (water insoluble hydrophobic portion).

Diazo compounds
This is a series of organic compounds possessing the general formula Ar-N=N-Y, where Ar is an aromatic group and Y is any group linked to the nitrogen except via a carbon atom. They are different from the azo compounds which have the general formula Ar-N=N-Ar'. Diazo compounds are formed from amines by a two stage diazotisation reaction. Stage one is the conversion of an aromatic amine into a diazonium chloride salt using cold sodium nitrite and hydrochloric acid - see below:

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The second stage is the reaction of the diazonium chloride salt with an appropriate compound - such as another aromatic amine, eg. the reaction with p-toluidine - see below:

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Distillation
The separation and purification of a mixture of components by vapourisation followed by condensation, based on the different volatilities of each component. A typical experimental set up for distillation is shown below:

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Drug
Any chemical compound used for medicianal purposes can be considered a drug. There are naturally occuring substances such as ethanol and caffeine as well as synthetic drugs such as aspirin and amphetamines. In general the term "drug" is used for compounds that can cause addiction with emphasis on narcotics.

Dyes
Dyes are chemical compounds that can attach themselves to fabrics or surfaces to give them colour. Most dyes are complex organic molecules and need to be resistant to many things such as the weather and the action of detergents. Indigo, otained from plants was being used by the Egyptians 5000 years ago, and natural dyes obtained from plants and animal sources are stil used today. In 1856, Perkin developed the first synthetic dye based upon aniline.

E
Ebullioscopic constant
Ebullioscopic constant (Eb) is the constant that expresses the amount by which the boiling point Tb of a solvent is raised by a nondissociating solute, through the relation
ΔTb = Eb b
where b is the molality of the solute.

Electrical resistivity
Electrical resistivity (ρ) is electric field strength divided by current density when there is no electromotive force in the conductor. Resistivity is an intrinsic property of a material. For a conductor of uniform cross section with area A and length L, and whose resistance is R, the resistivity is given by
ρ = R*A/ι
The SI unit is Ω m.

Electrochemical series
Electrochemical series is a series of chemical elements arranged in order of their standard electrode potentials. The hydrogen electrode
H+(aq) + e- 1/2H2(g)
is taken as having zero electrode potential. An electrode potential is, by definition, a reduction potential.
Elements that have a greater tendency than hydrogen to lose electrons to their solution are taken as electropositive; those that gain electrons from their solution are below hydrogen in the series and are called electronegative.
The series shows the order in which metals replace one another from their salts; electropositive metals will replace hydrogen from acids.

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Electrode potential
Electrode potential is defined as the potential of a cell consisting of the electrode in question acting as a cathode and the standard hydrogen electrode acting as an anode. Reduction always takes place at the cathode, and oxidation at the anode. According to the IUPAC convention, the term electrode potential is reserved exclusively to describe half-reactions written as reductions. The sign of the half-cell in question determines the sign of an electrode potential when it is coupled to a standard hydrogen electrode.
Electrode potential is defined by measuring the potential relative to a standard hydrogen half cell
H2(g) 2H+(aq) + 2e-
The convention is to designate the cell so that the oxidized form is written first. For example
Pt(s)|H2(g)|H+(aq)||Zn2+(aq)|Zn(s)
The e.m.f. of this cell is
e.m.f. = Eright - Eleft
By convention, at p(H2) = 101325 Pa and a(H+) = 1.00, the potential of the standard hydrogen electrode is 0.000 V at all temperatures. As a consequence of this definition, any potential developed in a galvanic cell consisting of a standard hydrogen electrode and some other electrode is attributed entirely to the other electrode
e.m.f. = E(Zn2+/Zn)

Electrolysis
Electrolysis is the decomposition of a substance as a result of passing an electric current between two electrodes immersed in the sample.

Electromagnetic waves
All matter absorbs and emmits radiation covering a broad band of frequencies and wavelengths. This electromagnetic radiation has the velocity of the speed of light (2.998 x 108 ms-1) and arises from the vibrating electric charges in atoms and bonds. The range of wavelengths, also known as the electromagnetic spectrum is shown below:

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Electromotive force
Electromotive force (e.m.f. or EMF) is the difference in electric potential that exists between two dissimilar electrodes immersed in the same electrolyte or otherwise connected by ionic conductors.

Electron
An elementary negatively charged particle orbiting within an atom, the electrons are arranged in shells - and it is the outermost shell electrons which take part in bond formation. Electron is discovered by J.J. Thompson (1856-1940) in 1896. The electron is an elementary particle with a negative electric charge of (1.602 189 2 ± 0.000 004 6) × 10-19 C and a mass of 1/1837 that of a proton, equivalent to (9.109 534 ± 0.000 047) × 10-31 kg. Electrons are arranged in from one to seven shells around the nucleus; the maximum number of electrons in each shell is strictly limited by the laws of physics (2n2). The outer shells are not always filled: sodium has two electrons in the first shell (2×12 = 2), eight in the second (2×22 = 8), and only one in the third (2×32 = 18). A single electron in the outer shell may be attracted into an incomplete shell of another element, leaving the original atom with a net positive charge. Valence electrons are those that can be captured by or shared with another atom.
Electrons can be removed from the atoms by heat, light, electric energy, or bombardment with high-energy particles. Decaying radioactive nuclei spontaneously emits free electrons, called β particles.

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Electron affinity
Electron affinity (EA) is the energy change occurring when an atom or molecule gains an electron to form a negative ion. For an atom or molecule X, it is the energy released for the electron-attachment reaction
X(g) + e- X- (g)
Often this is measured in electronvolts. Alternatively, the molar enthalpy change, ΔH, can be used.

Electron pair
Two electrons within one orbital with opposite spins responsible for a chemical bond.

Electron spin
Electron spin (s) is the quantum number, equal to 1/2, that specifies the intrinsic angular momentum of the electron.

Electron volt
Electron volt (eV) is a non-SI unit of energy used in atomic and nuclear physics, equal to approximately 1.602177 × 10-19 J. The electron volt is defined as the kinetic energy acquired by an electron upon acceleration through a potential difference of 1 V.

Electronegativity
Electronegativity is a parameter originally introduced by L. Pauling which describes, on a relative basis, the power of an atom to attract electrons. For example, in hydrogen chloride, the chlorine atom is more electronegative than the hydrogen and the molecule is polar, with negative charge on the chlorine atom.
There are various ways of assigning values for the electronegativity of an element. Pauling electronegativities are based on bond dissociation energies using a scale in which fluorine, the most electronegative element, has value 4 and francium, the lowest electronegative element, has value 0.7.

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Electrophoresis
Electrophoresis is a technique for the analysis and separation of colloids, based on the movement of charged colloidal particles in an electric field. The migration is toward electrodes of charge opposite to that of the particles. The rate of migration of the particles depends on the field, the charge on the particles, and on other factors, such as the size and shape of the particles.
Electrophoresis is important in the study of proteins. The acidity of the solution can be used to control the direction in which a protein moves upon electrophoresis.

Emulsion
A dispersal, with a variety of industrial uses of one liquid as small particles in another liquid. An example would be milk. Emulsion is colloidal system in which the dispersed and continuous phases are both liquids (e.g. oil in water or water in oil). Such systems require an emulsifying agent to stabilize the dispersed particles.

Enantiomers
Enantiomers are a chiral molecule and its non-superposable mirror image. The two forms rotate the plane of polarized light by equal amounts in opposite directions. Also called optical isomers.

Energy
Energy (E, U) is the characteristic of a system that enables it to do work. Like work itself, it is measured in joules (J).
The internal energy of a body is the sum of the potential energy and the kinetic energy of its component atoms and molecules.
Potential energy is the energy stored in a body or system as a consequence of its position, shape, or state (this includes gravitational energy, electrical energy, nuclear energy, and chemical energy).
Kinetic energy is energy of motion and is usually defined as the work that will be done by body possessing the energy when it is brought to rest. For a body of mass m having a speed v, the kinetic energy is mv2/2. Kinetic energy is most clearly exhibited in gases, in which molecules have much greater freedom of motion than in liquids and solids.

Enthalpy
Enthalpy (H) is a thermodynamic property of a system defined by
H = U + pV
where U is the internal energy of the system, p its pressure, and V its volume. J.W. Gibbs put the concept of an ensemble forward in 1902. In a chemical reaction carried out in the atmosphere the pressure remains constant and the enthalpy of reaction (ΔH), is to equal
ΔH = ΔU + pΔV
For an exothermic reaction ΔH is taken to be negative.