What are compounds that people think are elements

I am making a practice science final for my self and some of my friends, and i need to know some compounds people think are elements, and also and elements people think are compounds.

Steel may be a good example of a compound which is often thought of as an element. Steel is an iron alloy. Iron is an element, but an alloy is a mixture of two or more metals. Steel contains Nickel, Chromium and Vanadium, plus other minor metals. Steel also contains a non-metal, Carbon.

There are 4 astrological elements which are not chemical elements at all. Fire is a state of matter known as a plasma. It is a collection of atoms and electrons, and the electrons can move freely between the atoms. Plasma also exists inside a fluorescent tube. Water is a compound composed of the gaseous elements Hydrogen and Oxygen. Earth is a mixture of silicate minerals and decomposing organic material. It is mostly composed of the elements Silicon, Oxygen, Carbon and Hydrogen. Air is a mixture of gasses, mostly of which are elements. There is Oxygen at 21%, Nitrogen at 78% and the rest is either the compound Carbon Dioxide or the element Argon.

Water (H2O)
Fire (combustion)
Soil (heterogeneous mixture)
table salt (NaCl)
Air (composed of Nitrogen, Oxygen, Hydrogen, Carbon Dioxide etc...)