Compound A pure substance consisting

Chemical bond—The force or "glue" that holds atoms together in chemical compounds. Compound—A pure substance that consists of two or more elements, in specific proportions, joined by chemical bonds. The properties of the compound may differ greatly from those of the elements it is made from. 

Furthermore, we may say that a compound is a pure substance consisting of two or more different elements in a fixed ratio. Water is 11.1% hydrogen and 88.9% oxygen by mass. Similarly, carbon dioxide is 27.3% carbon and 72.7% oxygen by mass, and calcium oxide (the white solid A in the previous discussion) is 71.5% calcium and 28.5% oxygen by mass. We could also combine the numbers in the previous paragraph to show that calcium carbonate is 40.1% calcium, 12.0% carbon, and 47.9% oxygen by mass. Observations such as these on innumerable pure compounds led to the statement of the Law of Definite Proportions (also known as the Law of Constant Composition) ... 

Furthermore, we may say that a compound is a pure substance consisting of two or more dijferent elements in a fixed ratio. Water is 11.1% hydrogen and 88.9% oxygen by mass. 

Many of us have a bottle in our medicine cabinet containing a mild disinfectant consisting of hydrogen peroxide and water. The liquid is about 3% hydrogen peroxide, H2O2, and about 97% water. Classify each of the following as a pure substance or a mixture. If it is a pure substance, is it an element or a compound ... 

Some elements do not behave as if their smallest particle is a single atom. For example, at common temperatures seven elements form diatomic molecules. A molecule is the tiniest independent particle of a pure substance—element or compound— in a sample of that substance. Di- is a prefix that means two. Thus, a diatomic molecule consists of two atoms that are chemically bonded to each other. This molecule is a formula unit of the element. Its chemical formula is the elemental symbol followed by a subscript 2. For example, the chemical formula of hydrogen (symbol H) is H2. 

Matter comes in many forms. The fundamental building blocks of matter are atoms and molecules. These particles make up elements and compounds. An atom is the smallest unit of an element that maintains the chemical identity of that element. An element is a pure substance that cannot be broken down into simpler, stable substances and is made of one type of atom. Carbon is an element and contains one kind of atom. The model of diamond in Figure 2.1a consists of carbon atoms. 

A compound is also a pure substance, but it consists of two or more elements chemically combined in the same proportion. 

With a theory of atoms, chemists began to have a more exact way to define elements and compounds elements are pure substances that contain only one kind of atom compounds are pure substances that contain more than one kind of atom and that are present in very definite proportions. Mixtures, on the other hand, were recognized as impure substances that consist of more than one element or compoimd, but which are present in variable proportions. 

Organic matter extracted from earth materials usually is fractionated on the basis of solubility characteristics. The fractions commonly obtained include humic acid (soluble in alkaline solution, insoluble in acidic solution), fulvic acid (soluble in aqueous media at any pH), hymatomelamic acid (alcohol-soluble part of humic acid), and humin (insoluble in alkaline solutions). This operational fractionation is based in part on the classical definition by Aiken et al. (1985). It should be noticed, however, that this fractionation of soil organic matter does not lead to a pure compound each named fraction consists of a very complicated, heterogeneous mixture of organic substances. Hayes and Malcom (2001) emphasize that biomolecules, which are not part of humic substances, also may precipitate at a pH of 1 or 2 with the humic acids. Furthermore, the more polar compounds may precipitate with fulvic acids. 

This method amounts to a complete resolution of the type described in Section 19-3D, but on an analytical scale. For example, assume that you have a partially resolved compound, A, consisting of unequal amounts of the enantiomers Aand A. By reaction with a second chiral enantiomerically pure substance, B+, A is converted to a mixture of diastereomers A+B+ and A B+. Because these diastereomers are chemically and physically different, the mixture usually can be analyzed by gas-liquid chromatography (Section 9-2A). If the reaction of B+ with A+ and A was quantitative, the relative areas of the two peaks eluting from the column correspond to the ratio of the diastereomers A,.B+/AJB+, and thus to the ratio of enantiomers A+/A, from which the enantiomeric purity of the partially resolved mixture can be calculated. 

The growth rate of the layer of any chemical compound in reaction couples consisting of one of elementary substances A or B and another compound of a multiphase binary system or of two other compounds, is higher than the rate of its growth between pure components, provided that this layer is the only one in all the reaction couples under comparison. 

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