Which of the following is NOT a classification of matter?

Matter is commonly classified into three primary categories: elements, compounds, and mixtures. Elements are pure substances that consist of only one type of atom, while compounds are substances formed when two or more elements chemically bond together. Mixtures, on the other hand, are combinations of two or more substances that retain their individual properties and can be separated by physical means.

Gases, however, are a state of matter rather than a classification of matter. The states of matter include solids, liquids, and gases, which describe how the particles of a substance are arranged and how they behave. Since gases do not represent a fundamental classification like elements, compounds, or mixtures, they do not belong to the same category of matter classification. This distinction clarifies why gases are not considered a classification of matter but rather a physical state that matter can exist in.