1. Describing Reactions (Chp 7.1)

2. Chemical Reaction
A chemical change in which the atoms of one or more substances are rearranged to form new substances
Most reactions provide evidence
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (

3. Evidence of a chemical reaction
Temperature changes
Color changes
Odor
The formation of a solid

4. Chemical equations
Chemical equation - is a representation of a chemical reaction in which the reactants and products are expressed as formulas
Shows what was present before and after the reaction
Reactants are substances that undergo change during a chemical reaction
Products are the new substances that are formed as a result of the reaction

5. Representing Chemical Reactions
Use equations to show the reactants and products
Shows the identities and relative amounts
Shows the direction of the reaction
Example:
reactant1 + reactant product
Carbon + Oxygen Carbon Dioxide
C + O CO2

6. Word Equations Indicates the reactants and products using names
Example:
hydrogen + bromine --> hydrogen bromide

7. Skeleton Equations Uses chemical formulas instead of words
Word equation:
hydrogen + bromine --> hydrogen bromide
Skeleton equation:
H + Br --> HBr

8. The Law of Conservation of Mass
States that mass is neither created nor destroyed in a chemical reaction
The mass of the reactants must equal the mass of the products