Are elements always pure substances?

Yes, there are two types of pure substances, compounds and elements. Elements consist of a single atom whereas compounds are made of molecules. If a substance consists of only one element or one compound, it is a pure substance. If it consists of two compounds or elements that have been mixed together in solution, than it is a mixture. Salt water is a solution containing dissolved salt (NaCl) and water. Since it consists of two compounds, it is a mixture and not a pure substance. Air is also a mixture since it is a mixture of nitrogen, oxygen, and other trace gases in Earth’s atmosphere. An atmosphere consisting of 100% oxygen would represent a pure substance.

Pure substances have to be separated using complex physical methods. One method is through evaporation. If a solid at room temperature is mixed with water, for example, the water can be evaporated leaving behind the solid. This is how salt rock deposits, such as gypsum, are formed.