What is the difference between a mixture & a pure substance?

"A pure substance contains only one kind of molecule, and a mixture is a combination of two or more pure substances. Distilled water, aluminum foil and table sugar are each made from only one type of particle. However, salt water is a mixture because it contains salt as well as water."

frequency, z, and the collision density, z, in carbon dioxide (co2), r 180 pm at 30 °c and 120 kpa