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What do all of the following examples share in common? Crushing a can Melting an ice cube Boiling water Mixing sand & water Breaking a glass Dissolving sugar in water Shredding paper Chopping wood Mixing green & red marbles Sublimating dry ice All of these examples represent a physical change -although the matter may change in appearance, no new substance is actually formed -the changes are reversible (a crushed can is able to be put back to its original shape or shredded paper can be put back together to make its original whole paper, etc.)
In this chapter…. You will explore what happens during a chemical change -In a chemical change, the atoms rearrange themselves and bond together differently to form new products -The result is a whole new substance with different properties than before!
Candle Demo In most chemical reactions, 2 or more substances, called reactants, interact to create different substances called products -burning a candle is an example of a chemical reaction What do you think are the reactants in this chemical reaction? 1.) wax 2.) oxygen from the air -molecules that make up the wax combine with oxygen from the air to make the products carbon dioxide (CO 2 ) and water vapor (H 2 O) In a chemical reaction, atoms in the reactants combine in new and different ways to form the molecules of the products
Candle Demo Continued…. Will the candle still burn if one of the reactants (wax or oxygen) is no longer available? -jar is used to “cut off” the oxygen from the candle -now the candle has limited oxygen to deal with Both reactants are required for the chemical reaction to continue When a candle burns for a while, it eventually gets smaller & smaller. Where does the candle wax go? -the wax reacts chemically, and the new products go into the air!
Chemical Equations Wax is made of long molecules called paraffin and paraffin is made up of only carbon atoms & hydrogen atoms -molecules made of only C & H are called hydrocarbons We will use the simplest hydrocarbon, methane, as a model to show how the wax, or any other hydrocarbon, burns ReactantsProducts
Chemical Equations Where do the atoms come from that make the carbon dioxide and the water on the right side of the equation? -from the atoms in the reactants -in a chemical reaction, bonds between atoms in the reactants are broken and the atoms rearrange and form new bonds to make the products
Chemical Equations www.middleschoolchemistry.com/multimedia/chapter6/lesson1#combustion_ of_methane How many carbon, hydrogen, and oxygen atoms are in the reactants compared to the number of carbon, hydrogen, and oxygen atoms in the products? CH 4 + 2O 2 CO 2 + 2H 2 O Subscript tells you how many atoms of that element are in each molecule Coefficient tells you how many of a particular molecule there is
Conservation of Mass Are atoms created/destroyed in a chemical reaction? How do you know? No. Same # of each type of atom on both the reactant and product side of the chemical equation In a physical change, the substance itself doesn’t really change. How is a chemical change different from a physical change? In a chemical change, the molecules in the reactants interact to form new substances. Physical changes do not produce new substances
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