– In order to get many chemicals to function the way we want them to, we need them to change.  Example: no one would eat a raw hamburger.  By applying heat to the ground beef, it turns into its finished product

PHYSICAL CHANGES

– Physical changes involve a change in state.  Do you remember what the three states of matter are?  Solid, liquid, and gas.

-In a physical change, the chemical substances DOES NOT become a new substance.

-Example: water can change state from being a solid (ice) to a liquid (water) to a gas (water vapour).

– Physical changes can always be reversed.

– There are six changes of state (which are physical changes)

• Melting – This turns a solid into a liquid.  Water melts at ^oC.
• Freezing – This is the opposite process of melting.  This turns a liquid into a solid.  Water freezes at ^oC.
• Condensation – This turns a gas into a liquid.  Water condenses at 100^oC.
• Evaporation – This is the opposite process of condensation.  This turns a liquid into a gas.  Water evaporates at 100^oC.
• Sublimation – This is a much rarer physical change that results from a substance going through a massive temperature change.  This process turns a solid into a gas.  One example of this is when dry ice (frozen carbon dioxide (therefore a solid)) is exposed to room temperature.  The temperature change is so large that the solid skips the liquid phase and moves right into being a gas.
• Deposition – This is another rare physical change.  It is the opposite of sublimation, a gas will change into a solid.

CHEMICAL CHANGES

– A chemical change results in at least one new substance being created.  The old substance becomes a completely different substance.

– Chemical changes are very difficult to reverse.

-The chemical change you are most familiar with is called “combustion.”  Combustion occurs when the heat is applied to a chemical.  For example, lighting a match, fireworks, baking a cake.  As you can imagine, it is impossible to reverse these reactions.  Once you cook a hamburger, you cannot uncook it and start off with fresh ground beef again.

– Another chemical change you are familiar with is rusting.  Rusting occurs when metals such as iron react with the oxygen gas in the air.  The iron and the oxygen combine to form a new substance called iron oxide, or rust.

– Chemical changes are very hard to observe.  Physical changes are easy to observe; you can watch an ice cube melt and see how the solid is turning into a liquid.  When rust forms (a chemical change), you cannot see the individual molecules changing into a new substance, you can only see the final product, which is rust.