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Laws and theories Law – a generalization that describes the behavior of nature. i.e. Newton’s Laws of Motion Law of Definite Proportions Theory – an explanation of observations i.e. Darwin’s Theory of Evolution Dalton’s Atomic Theory Classes of matter Matter Pure substance element compound mixture homogeneous heterogeneous Solutions Solutions are homogeneous mixtures. A solute (the dissolved substance) is put in a solvent (the dissolving substance). The solvent is the substance there is more of. Examples elements Carbon Iron Gold Mercury compounds water carbon dioxide table salt (NaCl) aspirin homo mix air salt water white gold brass hetero mix concrete milk wood sand General classes of elements Metals – have luster (shine) are malleable (not brittle) are ductile (can be drawn into wires) conduct heat and electricity Nonmetals- poor conductors of heat and electricity brittle Metalloids- have characteristics of metals and nonmetals Brown, LeMay, Burstein “Chemistry: The Central Science Chemical symbols First letter is always capitalized. Second letter, if there is one, is never capitalized. Co and CO are very different! Some elements use the Latin name 2O means 2 oxygen atoms O2 means two oxygen atoms are chemically bonded together. Law of Definite CompositionEvery compound has a definite composition by weight. - the % weight of each element in a compound is always the same. Physical properties Can be determined without changing the identity of the material Freezing point Boiling point Color Odor Hardness Density Solubility Extensive Properties Depend on how much (the extent) matter there is i.e. length, mass, volume Intensive Properties Do NOT depend on how much matter there is i.e. temperature, density (m/V) Properties change at nanoscale Types of properties Optical (e.g. color, transparency) Electrical (e.g. conductivity) Physical (e.g. hardness, melting point, diffusion rate) Chemical (e.g. reactivity, reaction rates) Properties are usually measured by looking at large (~1023) aggregations of atoms or molecules Chemical Properties Describe the behavior of a material in reactions that change its identity Combustibility Reactivity with other substances Which chemical and physical properties should a roofer consider when selecting material for a metal roof? Physical Changes - - are those in which the identifying properties of a substance are unchanged Ex. : change of phase breaking, cutting dissolving Chemical Changes - - are those in which different substances are formed Ex.: burning rusting decaying or spoiling acid reacting with metal Chemical Changes - - are those in which different substances are formed Ex.: burning rusting decaying or spoiling acid reacting with metal Ice changing to liquid water Physical change Water breaking down into hydrogen and oxygen gases Chemical change Signs of a chemical change Heat gained or lost Production of a gas Formation of a precipitate Color change Methods of separating a mixture Use the different properties of the substances to separate them. Sand and iron filings Remove iron with magnet Sand and salt Dissolve salt, evaporate water filter Sand in water Water from salt water Distill the water Settle and extract Oil in water (immiscible liquids) less dense liquid Paper chromatography Mixture of pigments chromatography distillation Substance – a homogeneous material consisting of one kind of matter Compound – a substance that can be decomposed into simpler substances only by chemical means Element – substances that cannot be further broken down. Organic substance contain carbon