Physical vs. Chemical Changes

Goal: How do you tell the difference between chemical and physical

changes?

Physical Change*• A change in any object’s physical

properties

• any change in an object’s:

- mass - color

- weight - shape

- volume - hardness

- density - texture

Physical Changes*

Changes in Density, Mass or Volume D = M/V

Physical Changes*

Creating mixtures that can be separated out - mixtures can be reversed.

Physical Changes*

Changes in Phase of Matter

Example of a Physical Change:

A Melting Icicle

• 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!

Review: Other Physical Properties*

ColorSize

ShapeTexture

Magnetism

Chemical Changes*

• Chemical Changes occur when atoms react with other atoms to form chemical bonds.

• When atoms are bonded together they make a molecule.

Chemical Changes*

• When a chemical reaction happens the new molecules are a new substance or compound.

• A + B AB

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

Evidence of Chemical Changes, Reactions or Bonding

• Energy being taken in or given off (heat, coolness, light)

Evidence of Chemical Changes, Reactions or Bonding

• Gas or Bubbles

Evidence of Chemical Changes, Reactions or Bonding

• Color or Texture Change

Evidence of Chemical Changes, Reactions or Bonding

• Color and Texture Change that is not easily reversed

Evidence of Chemical Changes, Reactions or Bonding

• Forming of a Precipitate (adding to solutions and a solid is formed)

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

Examples of Chemical Change*

–The rusting of iron

–The baking of bread

–Burning of a match

–Fireworks

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 )