1. Chapter 2.3 Elements and Compounds -Pure Substances: Elements- simplest form of matter; unique properties; are on the Periodic Table; Au, Ag, Hg, K, N, O, C, etc… Names and Symbols: first letter always Capital Letter + Second letter (if the case) always lower case; Compound- two or more elements combined chemically; CaCl2; H20; H2SO4; CaCO3; HCl, etc… Properties of Compounds: different from the properties of the elements they are coming from.

2. 2 Physical and Chemical Change Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

3. 3 Some Examples of Chemical and Physical Changes Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

4. 4 Chapter 2.4 Chemical Reactions Reactants → Products - new substances - different physical properties - different compositions N° of atoms reactants = N° of atoms products Mass reactants = Mass products Old bonds are broken and new bonds are formed. Atoms in the reactants are rearranged to form one or more different substances. Evidences of chemical reactions: - 1. Change in the color - 2. Formation of a solid (precipitate) - 3. Formation of a gas (bubbles) - 4. Heat (or a flame) is produced or heat is absorbed Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

5. 5 Chapter 2.4 Chemical equations Symbols used to represent a chemical equation: “→” – means the reactants form products; shows which way the reaction goes “+” - separates two or more formulas on the same side “ ∆” - means reactants are heated to start the reaction “ s” - solid substance “ l “ - liquid “ g” - gas “ aq” - aqueous solution – substance dissolved in water Example: C (s) + O 2 (g) → CO 2 (g) Diatomic molecules: - only exist in nature as pairs of atoms, never alone H 2 – hydrogen ; O 2 - oxygen; N 2 - nitrogen; F 2 - fluorine; Cl 2 - Chlorine ; I 2 - iodine and Br 2 - bromine