Pure Substances: (Can be Elements or Compounds) Atoms consist only of one type of element. Molecules can consist of more than one type of element. – Molecules can have only one type of atom (an element). – Molecules can have more than one type of atom (a compound). If a pure substance can be decomposed into something else, then the substance is a compound. Mixtures: • If more than one atom, element, or compound are found together, then the substance is a mixture. • If matter is not uniform throughout, then it is a heterogeneous mixture. • If matter is uniform throughout, it is homogeneous (can be mixture or pure ) Tests for Substances: • If homogeneous matter can be separated by physical means, then the matter is a mixture. • If homogeneous matter cannot be separated by physical means, then the matter is a pure substance.