The correct statement is: a. Oxygen, nitrogen, and carbon dioxide are all classified as pure substances.
Let's analyze each options:
a. Oxygen, nitrogen, and carbon dioxide are indeed classified as pure substances. In the context of air, each of these gases exists in its molecular form.
Oxygen (O2), nitrogen (N2), and carbon dioxide (CO2) are composed of molecules consisting of the same type of atoms and have consistent and definite chemical compositions.
Pure substances can be elements or compounds, and in this case, these gases are considered pure substances, each with its distinct molecular structure.
b. This statement is incorrect because these gases can react with other substances. For example, oxygen supports combustion, and carbon dioxide can react with water to form carbonic acid.
c. This statement is incorrect because these gases are not chemically bonded to one another in the air. They exist as separate molecules with their own distinct chemical structures.
d. This statement is incorrect because while oxygen and nitrogen are elements, carbon dioxide is a compound composed of carbon and oxygen atoms.
Elements are substances made up of only one type of atom, while compounds are composed of two or more different types of atoms chemically bonded together.