Chemical Properties and Changes By Mr. Pers

2 Review of Physical Changes Physical properties can change, but the identity of the substance does not change. - Broken glass is still glass. - An ice cube is still water (different state). - Ripped paper is still paper. In this lesson you will learn how chemical changes are different because they actually change the substance.

3 Common Signs of Chemical Changes  Change in color  What color does a marshmallow become when it is toasted over a campfire?  What happens when a pink T-shirt is placed in a sink full of bleach?  Bubbles  When bread bakes, gas bubbles form. What do you see when you look closely at a piece of bread that will prove this?  Odor, such as smoke  Once lit, a campfire creates smoke. Why?  Heat  Why does a car engine get warm when it is started? Hint: What liquid creates the change?

4 Chemical Change Why is baking a cake an example of a chemical change?

5 Are these examples of a chemical or physical change? Why?

6 Are these examples of a chemical or a physical change?  Hot gas forms when hydrogen and oxygen join to make water help blast the Space Shuttle into orbit. A log is made into a baseball bat. 14

7 Can Chemical Changes Be Undone?  Because new substances are formed, you cannot reverse most chemical changes.  Examples: You cannot change ashes into a log or sour milk into fresh milk or a pickle back to a cucumber.  What else cannot be undone?  However, some chemical changes can be reversed by other chemical changes.  The water formed in the Space Shuttle’s rockets could be split back into hydrogen and oxygen using an electric current.

8 The Law of Conservation of Mass Example  When the flammable material burns in a campfire, it combines with oxygen.  Ash, smoke and gases are produced.  The smoke and gas escape into the air.  If you measured the mass of the oxygen and the original wood, it would be equal to the mass of the ashes, smoke and gas.  Therefore, you end up with the same mass you started with. The mass of what you end with is always the same is the mass of what you start with. Definition: Mass is the amount of matter measured in kilograms.

9 What Have You Learned?  What are five examples of chemical properties?  What is a chemical change?  What are some examples?  Can chemical changes be reversed? Explain.  What is the Law of Conservation of Mass?