Chemistry is the study of matter (anything that takes up space and has mass desk, book, salt, sugar, etc.)
Substance: Any variety of matter that has the same properties and composition throughout. A substance is homogeneous, made up of only one thing. (A bag of sugar is homogeneous made entirely of sugar.) There are two types of substances: elements and compounds: An element cannot be decomposed (broken down) into anything simpler by chemical change. Examples are: hydrogen, oxygen, nitrogen. All elements are written down on the Periodic Table on page Reference Tables-15 in Appendix I. A compound can be decomposed (broken down) by a chemical change. It is made of two or more different elements chemically united in a definite ratio. Examples of compounds are sodium chloride, NaCl, magnesium oxide, MgO, and sulfuric acid, H₂SO₄. A binary compound consists of only two elements, e.g., NaCl.

In a mixture , two or more substances are mixed together (not united). Example: salt and sugar mixed together. A mixture could have different proportions different amounts i.e., cup salt mixed with cup sugar or cup salt mixed with cup sugar. Mixtures can be heterogeneous (example: salt mixed with sugar, two different things) or homogeneous (example: solutions, which are considered like one thing; for example, salt solution, salt dissolved evenly throughout the water).