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Pure Substances Elements vs. Compounds Today’s Focus Recall Matter Physical substance Occupies space Properties Mass Volume Density ● 3 states of matter ○ Solid ○ Liquid ○ Gas Atoms & Molecules Atoms Extremely small Made of: electrons (-), protons (+), and neutrons Molecules Made up of atoms Two or more atoms Ex. H2O, O2, CO2 What makes a substance “Pure”? Homogenous Uniform composition throughout MANY identical particles (elements or compounds) ● Properties are constant throughout the whole mixture i.e When you divide water in half both halves are identical Constant chemical composition i.e When you divide water in half it will be H2O in both halves NOT H2 in one half and O in the other A Pure Substance can be: 1. Element ONE type of atom Can be two or more of the SAME atom Impossible to separate into other substances e.x O2, C, H2, K, Na 2. Compound Substance CAN’T be separated by mechanical or physical means Composed of two or more DIFFERENT atoms Identical molecules make up the compound e.x CO2, H2O, NaCl (salt) Element (Oxygen) Water = Compound ● Composed of H2O molecules ● The molecule is made up of elements ○ H= Hydrogen ○ O= Oxygen Compound Molecule …. For example WATER Element (Hydrogen) Compound vs. Molecule **All Compounds are molecules BUT not all molecules are compounds** That means… O2 IS a molecule but is NOT a compound, but why?????? ...because… it only contains one type of element, Oxygen Take Away! Pure Substances Made of many identical molecules Smallest particle of matter