Explain why elements and compounds are pure substances

N2 - Element; Pure substance O2 - Element; Pure substance N2O - Compound; Pure Substance Air - Homogeneous; Solution

Elements and compounds are substances because we know exactly what is in them. Elements are only one kind of atom and have a symbol. Compounds have a formula telling us exactly how many of what kind of atom make up that compound

Solutions, homogeneous mixtures, not only contain 2 compounds, but we do not know how much of each. We can make salt water with one grain of salt in a cup of water or a drop of water in a cup of salt. Mixtures do not have a formula because they can be any concentration.