Elements, Compounds, and Chemical Reactions Sec. 2.3 p. 36-37 Sec. 2.4 p. 43-47

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**

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

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.

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.

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

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

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

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.