Chemistry Notes 09/23 Physical and Chemical Changes

Types of substances Elements can NOT be chemically broken down to a simpler substance (examples: iron, hydrogen, krypton) Compounds are combinations of elements that can be broken down. (Ex. 2H 2 O  2H 2 + O 2 )

Elements and compounds are pure substances, meaning they always have the same composition (same proportions) Most substances are MIXTURES of elements and/or compounds

Physical Properties Characteristics of a substance: –Color, volume, odor, density, melting point, boiling point

Chemical properties The ability of a substance to form new substances

What is a chemical change? A chemical change occurs any time we have a rearrangement of the ways in which atoms are grouped. Since we cannot directly observe this rearrangement (atoms and compounds are too small), we need to look for evidence of the chemical change…

Evidence of a chemical reaction

Evidence for a chemical reaction Color change (except from dilution or color mixing) Temperature change (except when caused by external heating or cooling) Formation/release of a gas (except due to physical changes such as boiling) Formation of a solid precipitate when mixing two liquid solutions