What exactly is matter? Matter is anything that has mass and takes up space. There are two main ways of classifying matter. Matter is classified first by its physical state as a solid, liquid, or gas. Secondly, we classify matter by it chemical constitution as an element, a compound, or a mixture. We are going to learn about the differences between atoms, ions, and molecules, the differences between compounds, elements, and mixtures, and the difference between physical and chemical changes.