GED Science Practice

A compound in chemistry is correctly defined as a substance formed by the chemical union of two or more elements or ingredients. This definition highlights the fundamental nature of compounds, which are created when different elements chemically bond together, resulting in a substance with unique properties distinct from those of its individual elements. For instance, water (H₂O) is a compound made from the elements hydrogen and oxygen, where the chemical combination creates a substance with specific characteristics that differ from the gases hydrogen and oxygen. The other options do not pertain to the definition of a compound in chemistry. For example, the first option refers to a device rather than a chemical substance, while the second describes a physical process (phase change) rather than the formation of a compound. The last option discusses a type of rock, which is unrelated to chemical compounds specifically.