Chemical Reactions Chapter 2 Section 1 & 2

Physical Background Physical Property: Characteristics of matter that can be seen through direct observations Density Melting point Boiling Point Physical Change: Change in which the identity of the substance does not change

Chemical Background Chemical Property: characteristics of matter that can only be observed when one substance changes into a different substance Iron to Rust Chemical Change: New Substance is formed

A Change of Properties The properties of these substances are very different from the starting substances In a chemical reaction, bonds are broken. Atoms are then rearranged to form new substances

Signs of a Chemical Reaction Gas Solid Energy Endothermic Exothermic Color change

Chemical or Physical Glass Breaking Hammering wood together Burning Leaves Melting Ice Frying an egg Melting butter Bleaching your hair Fireworks Mixing salt with water

Chemical Formula A shorthand way to use chemical symbols and numbers to represent a substance Shows how many of each atom is present in a molecule 3H2O Subscript 2 – (applies to symbol before it) Coefficient 3 – (applies to entire molecule)

Be Accurate Symbols can be mistaken!!

Chemical Equations Use chemical symbols and formulas as a shortcut to describe a chemical reaction Reactants = starting material Products = substances formed from a reaction C + O2  CO2 reactants before arrow products after arrow

Law of conservation of mass During a reaction, mass is neither created nor destroyed Equations must be balanced because atoms are neither created nor destroyed Mass of Reactants = Mass of Products

How to balance an equation Step 1 Count the number of atoms on each side Step 2 You can only add or change coefficients to balance the number of atoms on each side.

Chemical Equation Shows the reaction between reactants and products Reactants-starting material Products-what is formed C + O2 CO2 C=1 C=1 O=2 O=2 http://www.explorelearning.com/index.cfm?method=cResource.dspView&ResourceID=408