1. Elements, Compounds, and Chemical Reactions
Sec. 2.3 p
Sec. 2.4 p

2. Elements vs. Compounds
We can classify substances as either elements or compound Element- the simplest form of matter that has a unique set of properties. Ex. ________________ Chemists use chemical symbols to represent elements **Elements can NOT be broken down into simpler substances**

3. Some characteristics of compounds:
Elements vs. Compounds
Compound- a substance that contains 2 or more elements that are chemically combined
Some characteristics of compounds:
The elements in a compound are combined in fixed proportions in a chemical formula. Ex. H2O There are two atoms of hydrogen and one of oxygen.
The properties of a compound are different from the elements which make it up.
Sodium Chlorine gas = Table Salt
(Na+) (Cl-) (NaCl)
Metallic Solid Gas Solid
Shiny Yellow White and Dull
Reactive Reactive Non-reactive

4. Chemical Changes
Physical methods can not break compounds down into simpler substances but chemical changes can.
Chemical change- change that results in a substance with a different composition (compared to the original substance)
The ability of a substance to undergo a chemical reaction is called a chemical property.
Chemical properties can only be observed when the chemical change occurs
ex. A chemical property of iron is its ability to rust but we can only see this property when iron comes in contact with oxygen and the chemical change begins.

5. How do you recognize a chemical change?
Clue #1 Energy Transfer: Every chemical change involves a transfer of energy. Ex. When you cook food energy in the fuel combines with oxygen in the air when the burner is lit. This reaction gives off heat and light. The food absorbs some of the heat and causes a chemical change in the food.
Clue #2 Color Change: A change in color indicates a chemical change has occurred. Ex. Food may change color and brown as it cooks.
Clue #3 Formation of Precipitate: precipitate -a solid that forms when 2 liquids react Ex. Ring of soap scum in a tub, milk separating into solid curds and liquid whey
Clue#4 Production of a Gas: Ex. When you
spray cleaner on soap scum bubbles form because a
gas is being released. Dropping an antacid tablet in
water produces CO2 bubbles.

6. A chemical change is also called a chemical reaction
Chemical reaction- when one or more substances change into one or more new substances
Reactant- substance(s) present at the start of a chemical reaction
Product- substances(s) produced during a chemical reaction
Identify the reactants and the products in the following chemical reaction:
6CO2 + 6H2O + light C6H12O6 + O2
Reactants Products

7. BUT Remember: The composition of matter NEVER changes
during a physical change
BUT
The composition of matter ALWAYS changes
during a chemical change
Breaking Charcoal=Physical change
Burning Charcoal=Chemical change

8. Is it a chemical or physical change?
Identify the following as either a physical or chemical change
Sodium hydroxide dissolves in water
Ice melts
Iron rusts
Milk sours
Wood rots
A pellet of sodium is sliced in two

9. Matter can not be created nor destroyed, only conserved.
The Law of Conservation of Mass states that matter is conserved in any physical or chemical change
During a chemical reaction:
total mass of the products = total mass of the reactants
Ex. Conservation of mass says that the mass
of the wood before burning is equal to the
mass of the ashes and gasses
(if you could collect them) after burning.
The wood changes form, but its mass
is conserved.