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What happens to the weight of an iron bar as it rusts? (Assume the rust doesn’t fall off the bar.) Chemical Reactions Substances change into other substances A. The weight increases B. The weight decreases C. The weight stays the same Why? Chemistry and the “atom” Chemistry and the “atom” The Law of Definite Proportions Examples of Definite Proportions In chemical reactions, materials combine in definite proportions by weight, weight which do not change between instances of a reaction. { 29 pounds of cinnabar yields: z z { 4 pounds of sulfur 25 pounds of mercury To neutralize 4 pounds of lime, z z 5 pounds of sulfuric acid or 7 pounds of nitric acid 1 Chemistry and the “atom” Chemistry and the “atom” Oxygen and atomic weights Atomic weights and water { Oxygen forms compounds with all the elements discovered before 1894! z z { Most of these compounds are solids and easy to work with Therefore atomic weights were determined relative to oxygen { Water can be made from z z { 1 pound of hydrogen 8 pounds of oxygen What’s the formula? z Dalton: “HO” { But… oxygen is not the lightest element! z Oxygen weighs 8 times as much as hydrogen Others: “H2O” { Oxygen weighs 16 times as much as hydrogen Chemistry and the “atom” Chemistry and the “atom” Definite Proportions of gases Atomic weights and water hydrogen oxygen { { Decomposition of water z z 2 volumes of hydrogen 1 volume of oxygen Water can be made from z z { 1 pound of hydrogen = 2 volumes 8 pounds of oxygen = 1 volume What’s the formula? z z Avogadro’s hypothesis: equal volumes of gas contain equal numbers of atoms Water is therefore H2O { Oxygen weighs 16 times as much as hydrogen 2 Chemistry and the “atom” Atoms in electrolysis of water Stoichiometry { “Atoms on the left must be equal to atoms on the right” __Na + __Cl2 → __NaCl Stoichiometry { In the open crystal structure of ice, what is in the spaces? “Atoms on the left must be equal to atoms on the right” __CH4 + __O2 → __CO2 + __H2O A. Air B. Water vapor C. Nothing How do we know from the diagram? 3