Air is considered a mixture, while water is considered a compound. Here are three reasons why this classification is made:
Composition: A mixture is formed by the physical combination of two or more substances, where each substance retains its individual properties and can be present in varying amounts. Air is composed of a combination of gases, primarily nitrogen (about 78%), oxygen (about 21%), and smaller amounts of other gases like carbon dioxide, argon, and trace gases. These gases exist in the mixture independently and can be separated by physical means like filtration or distillation.
Variable Composition: The composition of a mixture can vary, meaning the proportions of its components are not fixed. In the case of air, the percentages of nitrogen, oxygen, and other gases may vary slightly depending on factors such as location, altitude, and environmental conditions. This variability is characteristic of mixtures but not compounds.
Homogeneity: Mixtures can be homogeneous or heterogeneous. Air is considered a homogeneous mixture because its components are evenly distributed throughout the atmosphere. You can take a sample of air from different locations, and it will still consist of the same gases in similar proportions. In contrast, compounds like water are chemically bonded substances with a fixed composition and exhibit homogeneity at the molecular level.
Water, on the other hand, is a compound because it is formed by the chemical bonding of two hydrogen atoms with one oxygen atom, resulting in the molecule H2O. The chemical bonds in water are strong and involve the sharing of electrons between the hydrogen and oxygen atoms. This chemical composition is consistent throughout all water molecules, regardless of its source or location.
In summary, air is classified as a mixture because it is composed of various gases that can be separated physically and have variable compositions, while water is considered a compound because it is formed by the chemical combination of hydrogen and oxygen, and its composition is fixed and consistent.