UNIT III Lesson 3 Phase Changes

III.5 PHASE CHANGES A chemical change is a change in which NEW SUBSTANCES are formed. A physical change is a change in the _______________ of a substance. no new substances are formed! ***Note: chemical changes are often accompanied by phase changes (ie: physical changes may occur at the same time as a chemical change).

III.5 PHASE CHANGES

L ET US CHECK OUR UNDERSTANDING Physical and chemical changes check! ChangeLab.swf

III.5 PHASE CHANGES

P HASE CHANGES GRAPH

III.6 THE ROLE OF KINETIC ENERGY IN PHYSICAL CHANGES All molecules are constantly __________________! The energy that molecules possess as a result of their motion is _______________________________ There are THREE types of Kinetic energy that a molecule can possess:

1. ROTATIONAL ENERGY causes a molecule to rotate around one of its axes (no change in bond lengths or bond angles)

2. VIBRATIONAL ENERGY changes the bond lengths and/ or bond angles between atoms in a molecule.

3. TRANSLATIONAL ENERGY causes the molecule to travel in a straight line from place to place (no change in bond length or angles).

III.6 THE ROLE OF KINETIC ENERGY IN PHYSICAL CHANGES As a sample is heated, the particles gain KINETIC ENERGY. In a solid state.... The increase _____________ and _____________ causes the molecules to oscillate back and forth and bump into their neighbours. Once the _________________ energy increases to a certain point (melting point), the molecules can overcome the bonds holding them together (bonds break and solid melts).

III.6 THE ROLE OF KINETIC ENERGY IN PHYSICAL CHANGES In the liquid state... All three types of kinetic energy increase but the GREATEST increase is in ____________________ energy. Again, translational energy increases to a certain point (boiling point) and the molecules escape their neighbours and fly off into the gas phase!

P RACTICE TIME QUESTIONS: p. 64 # 64 – 71 QUESTIONS: p. 61# 59 ONLY.