Chemical substance


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In chemistry, a chemical substance is a form of matter that has constant chemical compositionand characteristic properties. It can not be separated into components by physical separation methods, i.e. without breaking chemical bonds. Chemical substances are often called pure to set them apart from mixtures. A common example of a chemical substance is pure water; it has the same properties […]

In chemistry, a chemical substance is a form of matter that has constant chemical compositionand characteristic properties.

It can not be separated into components by physical separation methods, i.e. without breaking chemical bonds. Chemical substances are often called pure to set them apart from mixtures. A common example of a chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory.

Other chemical substances commonly encountered in pure form are diamond, gold, salt (sodium chloride) and sugar (sucrose). Most chemical substances occur as mixtures with other chemical substances. For example, drinking water is a mixture of water, sodium chloride and many other chemical substances. Generally, chemical substances exist as a solid, liquid, gas, or plasma and may change between these phases of matter with changes in temperature or pressure. Chemical reactions convert one chemical substance into another.

Forms of energy, such as light and heat, are not considered to be matter, and thus they are not “substances” in this regard.

Chemical substances (also sometimes referred to as a pure substance) are often defined as “any material with a definite chemical composition” in most introductory general chemistry textbooks.

According to this definition a chemical substance can either be a pure chemical element or a pure chemical compound. But, there are exceptions to this definition; a pure substance can also be defined as a form of matter that has both definite composition and distinct properties.

The chemical substance index published by CAS also includes several alloys of uncertain composition. Non-stoichiometric compounds are a special case (in inorganic chemistry) that violates the law of constant composition, and for them, it is sometimes difficult to draw the line between a mixture and a compound, as in the case of palladium hydride.

Broader definitions of chemicals or chemical substances can be found, for example: “the term ‘chemical substance’ means any organic or inorganic substance of a particular molecular identity, including any combination of such substances occurring in whole or in part as a result of a chemical reaction or occurring in nature”.