Physical and Chemical Changes Objective To recognize chemical

Physical and Chemical Changes Objective: To recognize chemical changes and distinguish them from physical changes

Changes in Matter • During a chemical change, the composition of matter always changes – Something new is made • It is also called a chemical reaction • Example: Burning paper produces CO 2, H 2 O and ash • During a physical change, the composition of matter never changes – The substances present before the change, are the same substances present after the change • Example: Ripping paper

How can you tell a chemical reaction has taken place? • Possible clues that a chemical change has occurred: – A change in color (fall leaves turning orange) – A change in odor (food rotting) – A change in energy (usually in the form of heat, may be in the form of light) – Production of gas or precipitate – Not easily reversed (can’t get original substance back)

Another hint… • Ask yourself this question… – Can you reverse the change easily? • If you can, the change is likely not a chemical change, but a physical one. • If you cannot, the change is likely a chemical change. – This does not always work, but it can help

• Energy is always absorbed or given off in a chemical reaction • This also may happen during a physical change as well. • Many times this is in the form of heat.

The change in energy is often in the form of heat Exothermic Process • A process that releases heat to the surroundings • These reactions make the surroundings get warmer • Example: hand warmers Endothermic Process • A process that absorbs heat from the surroundings • These reactions make the surroundings get cooler • Example: chemical ice packs