reactantsproducts Depict the kind of reactants and products including their relative amounts in a reaction. 4 Al(s) + 3 O 2 (g) 2 Al 2 O 3 (s) The numbers in the front are called coefficients coefficients. The letters (s), (g), and (l) are the physical states of compounds. aluminum oxide

This equation means: 4 Al(s) + 3 O 2 (g) 2 Al 2 O 3 (s) 4 Al atoms + 3 O 2 molecules yield 2 molecules of Al 2 O 3 aluminum oxide sandpaper

Because the same atoms are present in a reaction at the beginning (reactants) and at the end (products), the amount of mass in a system does not change. Law of Conservation of Mass The Law of Conservation of Mass Kotz web

conservation of mass. Because of the principle of the conservation of mass. equation must be balanced An equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788

Reactants – the substances that exist before a chemical change (or reaction) takes place. Products – the new substance(s) that are formed during the chemical changes. CHEMICAL EQUATION indicates the reactants and products of a reaction. REACTANTS  PRODUCTS

Cl H H H HH H H H 2 + Cl 2  HCl H 2 + Cl 2  2 HCl reactants products H Cl reactants products H Cl (unbalanced) (balanced) Unbalanced and Balanced Equations

Na + Cl 2 NaCl2 2

Chemical Symbol Meaning Composition H 2 O One molecule of water: Two H atoms and one O atom 2 H 2 O Two molecules of water: Four H atoms and two O atoms H 2 O 2 One molecule of hydrogen peroxide: Two H atoms and two O atoms