physical vs. chemical changes. goal: how do you tell the difference between chemical and physical...
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Physical vs. Chemical Changes
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Goal: How do you tell the difference between chemical and physical
changes?
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Physical Change*• A change in any object’s physical
properties
• any change in an object’s:
- mass - color
- weight - shape
- volume - hardness
- density - texture
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Physical Changes*
Changes in Density, Mass or Volume D = M/V
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Physical Changes*
Creating mixtures that can be separated out - mixtures can be reversed.
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Physical Changes*
Changes in Phase of Matter
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Example of a Physical Change:
A Melting Icicle
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• physical properties = characteristics of a substance that can be observed without changing the identity of the substance.
ex.) wood is still wood whether it is shaped into a baseball bat or used to build the walls of your house – or if it’s still in the shape of a tree!
ex.) water is still water whether it is in the form of ice, liquid water, or water vapor. It’s still H2O!
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Review: Other Physical Properties*
ColorSize
ShapeTexture
Magnetism
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Chemical Changes*
• Chemical Changes occur when atoms react with other atoms to form chemical bonds.
• When atoms are bonded together they make a molecule.
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Chemical Changes*
• When a chemical reaction happens the new molecules are a new substance or compound.
• A + B AB
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Chemical Change
> changes that substances undergo when they turn into other substances
> Also called a “Chemical Reaction”
ex.) as coal burns, O2 (in the air) combines chemically with C (the substance that makes up most of the coal) and forms a completely new substance = CO2
- C and O2 have changed chemically! They
no longer exist in their original forms!
- Written out: C + O2 = CO2
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Evidence of Chemical Changes, Reactions or Bonding
• Energy being taken in or given off (heat, coolness, light)
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Evidence of Chemical Changes, Reactions or Bonding
• Gas or Bubbles
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Evidence of Chemical Changes, Reactions or Bonding
• Color or Texture Change
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Evidence of Chemical Changes, Reactions or Bonding
• Color and Texture Change that is not easily reversed
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Evidence of Chemical Changes, Reactions or Bonding
• Forming of a Precipitate (adding to solutions and a solid is formed)
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How to decide type of change
• Ask yourself: Can I easily turn substance back to the way it was?
If yes: physical changeIf no: chemical change
• Was the change awesome to watch? If yes: Probably chemical changeIf no: Probably physical change
• Did the name of the new substance change?If yes: Probably chemical If no: Probably physical
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Examples of Chemical Change*
–The rusting of iron
–The baking of bread
–Burning of a match
–Fireworks
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Example of a Chemical Change: The Electrolysis of Water
(H2O)
The Chemical Identity of Water ( H2O ) is changed into the elements Hydrogen ( H2 ) and Oxygen ( O2 )