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Steam and liquid water are two different forms of the same chemical substance, water. A chemical substance is a form of matter having constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i. Chemical substances are often called ‘pure’ to set them apart from mixtures. Chemical substances exist as solids, liquids, gases, or plasma, and may change between these phases of matter with changes in temperature or pressure. Forms of energy, such as light and heat, are not matter, and are thus not «substances» in this regard.
UV light, showing how the chemical interacts dynamically with its solvent environment. A chemical substance may well be defined as «any material with a definite chemical composition» in an introductory general chemistry textbook. According to this definition a chemical substance can either be a pure chemical element or a pure chemical compound. In law, «chemical substances» may include both pure substances and mixtures with a defined composition or manufacturing process. Polymers almost always appear as mixtures of molecules of multiple molar masses, each of which could be considered a separate chemical substance.
The concept of a «chemical substance» became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate. Sulfur occurs naturally as elemental sulfur, in sulfide and sulfate minerals and in hydrogen sulfide. An element is a chemical substance made up of a particular kind of atom and hence cannot be broken down chemical abstract transformed by a chemical reaction into a different element, though it can be transmuted into another element through a nuclear reaction. A pure chemical compound is a chemical substance that is composed of a particular set of molecules or ions. Two or more elements combined into one substance through a chemical reaction form a chemical compound. All compounds are substances, but not all substances are compounds. A chemical compound can be either atoms bonded together in molecules or crystals in which atoms, molecules or ions form a crystalline lattice.
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Compounds in which components share electrons are known as covalent compounds. Compounds consisting of oppositely charged ions are known as ionic compounds, or salts. In organic chemistry, there can be more than one chemical compound with the same composition and molecular weight. Isomers usually have substantially different chemical properties, and often may be isolated without spontaneously interconverting.
A common example is glucose vs. However, tautomers are an exception: the isomerization occurs spontaneously in ordinary conditions, such that a pure substance cannot be isolated into its tautomers, even if these can be identified spectroscopically or even isolated in special conditions. A common example is glucose, which has open-chain and ring forms. Cranberry glass, while it looks homogeneous, is a mixture consisting of glass and gold colloidal particles of ca. 40 nm diameter, which give it a red color.
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