Dictionary C (Part 3 of 3)

Coagulation

Coagulation is the process in which colloidal particles come together to form lager masses. Coagulation can be brought about by adding ions to neutralize the charges stabilizing the colloid. Ions with a high charge are particularly effective. Another example of ionic coagulation is in the formation of river deltas, which occurs when colloidal silt particles in rivers are coagulated by ions in sea water. Heating is another way of coagulating certain colloids.

Coal

Coal is a brown or black carbonaceous deposit derived from the accumulation and alteration of ancient vegetation, which originated largely in swamps or other moist environments. As the vegetation decomposed it formed layers of peat, which were subsequently buried. Under the increased pressure and the resulting higher temperatures the peat was transformed into coal.

Coal Gas

Coal gas is a gas produced by the destructive distillation of coal, and contains approximately 50% hydrogen, 35% methane and 8% carbon monoxide.

The by products of the production of coal gas are coal tar and Coke.

Coal Tar

Coal tar is a material obtained from the destructive distillation of coal in the production of coal gas. The crude tar contains a large number of organic compounds (e.g. benzene, naphthalene, methylbenzene, etc.), which can be separated by fractional distillation.

Coke

Coke is an impure form of carbon, which is obtained as a byproduct from the destructive distillation of coal.

Colorimetric Analysis

Colorimetric analysis is a quantitative analysis of solutions by estimating the colour produced by the reaction of the sample under analysis with a reagent and comparing it with the colours produced by known standard solutions.

Colour of Halogens

The halogen molecules are all coloured due to the absorption of visible light. This absorption of light results in the excitation of outer electrons to higher energy levels.

The energy required to bring about these electronic transitions follows the same pattern as the ionisation potential values for the elements. Thus, the energy required to bring about such a transition in the smaller fluorine atom is much larger than the energy required for the same transition for the larger iodine atom. Fluorine absorbs the higher energy violet light and transmits yellow light whilst the iodine absorbs the lower energy yellow light and transmits violet light.

Combined Gas Law

The combined gas law is the mathematical relationship between the pressure, volume and temperature of a fixed quantity of an ideal gas, which states that the product of the pressure, P, and the volume, V, of one mole of gas is equal to the product of the gas constant, R, and the absolute temperature, T.


			P V  =  R T

The combined gas law is derived from Boyle's Law and Charles's Law.

Common Group

Common groups which are derived from hydrocarbons include :

Alkyl groups
			CH3		methyl	
			C2H5		ethyl	
			C3H7		propyl	
			C4H9		butyl	
			C9H29		nonyl

Aromatic groups
			C6H5		phenyl	
			C10H9		naphthyl

Alkenyl groups
			C2H3		vinyl	
			C3H5		allyl

Comparison of Physical and Chemical Changes

A substance is said to undergo a physical change when it changes from one state to another. The changes of water when it is boiled or frozen are examples of physical changes. When water is heated until it boils, bubbles of gas rise through the hot liquid. We know this gas as steam. If the steam that is produced when water is boiled is cooled down to room temperature, a liquid (i.e. water) is formed. If water is cooled much below room temperature, a solid known as ice is formed.

Whenever a process takes place with the production of one or more new materials, the process itself is called a chemical change or a chemical reaction. This process takes place within a chemical system. In a chemical reaction, the initial materials called reactants, and these are replaced as a result of the chemical change by a new set of materials called products. In order to distinguish between a physical and chemical change, it is necessary to find out if a new substance has been formed. If a new substance has been formed in the change, a chemical change has taken place.

As matter may undergo changes which may be either physical changes or chemical changes, the following table indicates the differences between physical and chemical changes.

Characteristics of a physical change

This may be a change from solid to liquid to gas, or solid to gas no new kind of matter being formed.
This change of state is reversible, e.g. gas to liquid to solid.
No change in weight.

Characteristics of a chemical change

A new type of matter is formed.
This change is not easily reversible.
The weight of each reacting substance changes, although the total weight remains unchanged.

Comparison of Ionic and Covalent Compounds

There are a number of differences in the physical properties of chemical compounds which depend on the type of bonds which are present in the molecules of the compound. These differences are summarised in the table below.

Ionic Compounds

Formed by transferring electrons
High melting and boiling points
Form lattices
Dissolve in water
Do not dissolve in organic solvents
Conduct electricity when melted or dissolved in water
Strong forces holding whole ionic structure together

Covalent Compounds

Formed by sharing electrons
Low melting and boiling points
No lattices, except diamond and graphite
Do not dissolve in water
Dissolve in organic solvents
Do not conduct electricity (graphite is the only exception)
Atoms in molecule held by strong forces, but weak forces between the molecules.

Comparison of Mixtures and Compounds

Mixtures

Properties related to those of the constituents.
Composition is variable.
Constituents can be mechanically separated.

Compounds

Properties need not be related to constituents.
Composition is fixed and can be represented by a formula.
Constituents cannot be mechanically separated.

Component

A component is a distinct chemical species in a mixture. If there are no chemical reactions between the components in a mixture, the number of components is the number of distinct chemical species.

Compound

A compound is a substance composed of two or more elements that are bound together by chemical bonds. A compound cannot be broken down into its individual elements of which it is composed by physical means. However, a compound can be broken down to the elements of which it is composed by chemical means.


Compounds				Elements
Calcium chloride			Calcium, Chlorine
Calcium oxide				Calcium, Oxygen
Sodium chloride				Sodium, Chlorine
Hydrogen oxide				Hydrogen, Oxygen
Iron oxide				Iron, Oxygen

Just as every element has its own symbol, every compound has its own formula. The formula identifies the elements present in the compound and the number of atoms of each kind that are in a molecule of the compound.

As all compounds are made from elements, so the elements are the building blocks of compounds. When a compound is formed by the combination of elements, this reaction is called a synthesis reaction.

Unlike the elements, compounds can be split up into simpler substances (i.e. the elements of which they are composed). The splitting up of compounds into simpler substances is called decomposition.

Compound Ions

Compound ions are groups of atoms which remain unchanged through a series of chemical reactions, but which are incapable of independent existence. They have an overall charge. Examples include the carbonate ion, CO3(-.-), nitrate ion, NO3(-), ammonium ion, NH4(++), etc.

Concentration

Concentration is the quantity of dissolved substance per unit quantity of solvent in a solution. The mass concentration is the mass of solute per unit volume of solvent. The molal concentration is the amount of substance per unit mass of solvent.

Condensation

Condensation is the change of a vapour or gas into a liquid. The change of phase is accompanied by the evolution of heat.

Condensation Reaction

Condensation reaction are those chemical reactions in which two molecules combine to form a larger molecule with elimination of a small molecule, typically water, during the course of the reaction.

Condenser

A condenser, (i.e. a Liebig Condenser) consists of a straight glass tube enclosed in a glass jacket through which water is passed. The condenser speeds the cooling and condensing process, when vapour passes through the condenser's center tube and is cooled by the water flowing around the condenser's outer jacket.

Conductimetric Titration

A coductimetric titration is one in which the electrical conductivity of the reaction mixture is continuously monitored as one reactant is added. The equivalence point (i.e. the end-point of the titration) is the point at which the conductivity undergoes a sudden change.

Conductivity

No text prepared for this topic yet

Configuration

Two definitions are associated with the term, configuration.

Configuration is the arrangement of atoms or groups of atoms in a molecule, or
Configuration is the arrangement of electrons about the nucleus of an atom.

Conical Flask

A conical flask is used for titration's to facilitate the return of liquid splashed onto the walls of the flask to the body of the liquid at the bottom of the flask.

Conjugated Bonds

Conjugated bonds describe the alternating pattern of double and single bonds, or triple bonds and single bonds, in a molecule. In such molecules, there is some delocalisation of electrons into the pi orbitals of the carbon atoms linked by the single bond.

Consolute Temperature

Consolute temperature is a temperature at which two partially miscible liquids become fully miscible as the temperature is increased.

Contact Process

The contact process is used for manufacturing sulphuric acid and fuming sulphuric acid from sulphur dioxide, which is made by burning sulphur or by roasting sulphide ores and oxygen (in the form of air) which combine to form sulphur trioxide in the presence of a Catalyst. The reaction is exothermic and the conditions are controlled to keep the temperature at 450 degC. The catalyst used is valadium (V) oxide (V2O5). The sulphur trioxide is dissolved in sulphuric acid to form fuming sulphuric acid, this called oleum.


	S(s)	+	O2(g)		==>		SO2(g)	
							Sulphur	
							Dioxide	


	2SO2(g)	+	O2(g)		==>		2SO3(g)	
							Sulphur	
							Trioxide	


	SO3(g)	+	H2SO4(l) 	==>		H2S2O7(l)
                                                          Oleum

This Oleum is then diluted with water into concentrated sulphuric acid.



	H2S2O7(l) 	+	H2O (l)	==>	2 H2SO4(l) 	
	Oleum					Sulphuric	
						Acid

Continuous Spectra

A continuous spectrum consisting of a band of coloured light is produced when a narrow beam of sunlight is allowed to pass through a prism. In a continuous spectrum, each colour merges into the next, so that it is difficult to say where one colour ends and the next colour begins. Each colour corresponds to radiation of different energy.

Convection

Convection is the process by which heat is transferred from one part of a fluid to another by movement of the fluid itself. There are two methods in which this can be carried out. One is by natural convection, in which movement occurs as a result of gravity, the hot part of the fluid expands, become less dense and is displaced by the colder denser part of the fluid as this drops below it. Forced convection is where hot fluid is transferred from one region to another by a pump.

Coordination Complex

A coordination complex is a compound in which molecules or ions form coordinate bonds to a central metal atom or Ion. The complex may contain positive ions, negative ions or neutral molecules. The formation of such coordination complexes is typical behaviour of Transition Metals.

Corrosion

Corrosion is the chemical changes which occurs in a material over time due to its contact with air or moisture, and which result in the loss of physical strength and the mechanical properties of the material, rendering it unsuitable for its intended use.

For example, metals are oxidised in moist air to their oxides which have low mechanical strength.

Corrosion Cells

Corrosion cells are the arrangements of electrodes and solutions which result in electrochemical changes to the contents of the cells.

Coulomb

The coulomb is the charge carried by a current of one ampere flowing for a period of one second.

Coupling Reaction

A coupling reaction is a chemical reaction in which two molecules join together to form a single product.

Covalent Bond

A covalent bond is the principal type of bond between atoms whose Electronegativity's are of almost equal (i.e. it is formed by sharing electrons between any two constituent atoms in a molecule such as the elements in the middle of the periodic table, who do not lose or gain electrons easily). Thus, in the covalent bond, the Electron Pair in the covalent bond is shared equally between the atoms, so that stable outer shells are created in each atom by this sharing.

For example, in the chlorine molecule each chlorine atom contributes one electron from its outer shell to form a shared pair, thus giving both a stable outer shell.

The atoms in carbon dioxide are also covalently bonded. Each oxygen atom shares two pairs of electrons with the carbon atom. Thus, there is a double covalent bond between the carbon atom and each oxygen atom.

Covalent Chlorides

The chlorides of non-metals (e.g. phosphorus trichloride) are covalent compounds.

With the exception of carbon tetrachloride, these chlorides fume in air and are readily hydrolysed to yield either an acid or an acidic oxide.

For example, phosphorus trichloride fumes in air


	PCl3   +   3 H2O   ==>   H3PO3   +   3 HCl

Cracking

Cracking is the process of breaking down chemical compounds at high temperature. The term is applied particularly to the cracking of hydrocarbons in the kerosene fraction obtained from petroleum refining to give smaller hydrocarbons. This process involves the decomposition of petroleum fractions into hydrocarbons of lower molecular weight, by heating them under pressure in the presence of a suitable Catalyst.