To be classified as a compound, a substance must consist of two or more elements in a fixed proportion held together by chemical bonds. It has a uniform composition, can only be separated by chemical reactions, and displays unique properties different from its constituent elements.
If a chemist determines a substance is a compound, there are certain characteristics that must be true for the substance:
The substance consists of two or more elements chemically combined in a fixed proportion.
Chemical bonds hold together the atoms within the compound.
The compound has a unique composition that is always the same, meaning it cannot vary like a mixture.
The substance can only be separated into its individual elements through chemical reactions that break the chemical bonds.
A compound has distinct physical and chemical properties that are different from those of its component elements.
Using water (H₂O) as an example, it always contains one atom of oxygen and two atoms of hydrogen. This specific ratio is constant for any sample of pure water, which demonstrates a key characteristic of a compound.