today’s agenda…
Post on 30-Dec-2015
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Today’s Agenda….
• Bellringer: Explain why melting silver is a physical change. Give as much evidence as possible.
• Discuss tests• Discuss assignments• Notes on Chemical Reactions
State Changes
• Physical change• The substance remains the same • It can be reversed• Moo Glue? A state change?– It is not a state change because you are putting
two liquids together and they react to form a solid– The solid is a new substance– This cannot be easily reversed
Review
• What is a chemical change?– Creates a new substance– Not easily reversed
• What are some clues that a chemical change has taken place?– Change in color, odor, sound– Release or absorb heat or light– Produce a gas or solid
Chemical Reactions
• When you observe a chemical change, a chemical reaction has taken place
• Let’s look at some examples!• Create an input/output chart!
Iron Rusting
• Look at the before (right) and after (left).
• Why does the balloon look the way it does?
• What has happened?
Baking Soda and Vinegar
• Think about the before and after.• Why did the balloon look the way it did?• What has happened?
Roasting a Marshmallow
An Apple Rotting
Baking Bread
Why is this not just a change of state?
What do these all have in common?
• Law of conservation of mass• During a chemical OR physical change matter
cannot be created or destroyed.
Law of Conservation of Mass
• Chemical reactions are represented by symbols and formulas• The symbols come from the periodic table• The numbers tell you how many particles are in the
substances• Examples:
– O2
– H20– 3CaCl– 2CO2
– If there is a large number in front you must multiply the small number by that one!
Law of Conservation of Mass
• In a chemical reaction you have products (what you start with) and reactants (what you end up with).
• Reactants are on the right, products are on the left
• The mass must be the same on both sides• The number and type of particles must be the
same on both sides
Law of Conservation of Mass
• If my reactants have a mass of 45 grams, what will the mass of my products be?
• If I have 10 particles that make up my reactants, how many particles will make up my products?
Example:
• Iron Rusting• 4Fe + 3O2 2Fe2O3
• None of the particles are lost, they are just rearranged to form new substances!
Energy in Chemical Reactions
• Every chemical reaction involves a change of energy– Release energy or absorb energy– This energy is usually in the form of heat
• If it releases energy it is called an exothermic reaction– A candle burning
Energy in Chemical Reactions
• If it absorbs energy it is called an endothermic reaction– Baking a cake– An emergency ice pack
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