What defines a compound in chemistry?

A compound in chemistry is defined as a substance formed by the chemical combination of two or more elements in definite proportions. This means that when elements combine to create a compound, they do so in specific ratios that are consistent and fixed, leading to the formation of a unique chemical substance with distinct properties. For example, water (H₂O) is a compound made up of hydrogen and oxygen in a 2:1 ratio.

The concept of definite proportions is critical because it differentiates a compound from a mixture, where the components can vary in proportion and do not retain the specific characteristics of a chemical compound. This foundational understanding is essential for recognizing the differences between compounds and other forms of matter, such as elements or mixtures.

In the context of the other options, they describe different concepts that do not accurately reflect the definition of a compound. Understanding the precise definition of a compound is fundamental to studying broader chemical principles, such as chemical reactions, stoichiometry, and the behavior of substances in different states.