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. A pure chemical substance cannot be separated into other substances by a process that does not involve any chemical reaction and is rarely found in nature. Some typical chemical substances can be diamond, gold, salt (sodium chloride) and sugar (sucrose). Generally, chemical substances exist as a solid, liquid, or gas, and may change between these phases of matter with changes in temperature or pressure.

Forms of energy, such as light and heat, are not considered to be matter, and thus they are not "substances" in the scientific regard.

Definition

Chemical substances (also sometimes referred to as a pure substances) 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. However, 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. and 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.

Elements

Main: Chemical element

An element is a chemical substance that is made up of a particular kind of atoms and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmutated into another element through a nuclear reaction. This is so, because all of the atoms in a sample of an element have the same number of protons, though they may be different isotopes, with differing numbers of neutrons.

There are about 120 known elements, about 80 of which are stable, that is, they do not change by radioactive decay into other elements, however the number of chemical substances that are elements can be more than 120, because some elements can occur as more than a single chemical substance (allotropes), for instance oxygen as oxygen and ozone. The majority of elements are classified as metals. These are elements with a characteristic lustre such as iron, copper, and gold. Metals typically conduct electricity and heat well, and they are malleable and ductile. Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S. General Chemistry, 4th ed., pp45-46, Pearson Prentice Hall, Upper Saddle River, New Jersey, 2005. Around a dozen elements, such as carbon, nitrogen, and oxygen, are classified as non-metals. Non-metals lack the metallic properties described above, they also have a high electronegativity and a tendency to form negative ions called anions. Certain elements such as silicon sometimes resemble metals and sometimes resemble non-metals, and are known as metalloids.