Carbon dioxide is a pure substance. A pure substance can be an element or a compound. Carbon dioxide is a pure compound, consisting of two kinds of atoms that are bonded together. An element consists of merely one kind of atom. For example, oxygen is an element.
Two or more pure substances can be mixed together to form a mixture. An example of a mixture of two compounds is alcohol and water. An example of a mixture of elements and compounds is air. The major components of air are hydrogen, oxygen, nitrogen and carbon dioxide. The substances in a mixture can be separated physically via such methods as filtration or chromatography.