W H AT H A P P E N S W H E N T H I N G S G E T H O T

PHASE CHANGES

• Pick up a copy of the notes• Draw a solid, liquid, and gas

THE PHASES

• Solid LiquidGas

• We are drawing a lot today!

PHASE CHANGES

• The first couple you should know• Melting• Solid to liquid

• Freezing• Liquid to solid

• Vaporizing• Liquid to gas

• Condensing• Gas to liquid

• Now for the tricky ones• Sublimation• Solid to Gas

• Deposition• Gas to Solid

• All Blue ones are Endothermic• All Red ones are

Exothermic

ENDO/EXO-THERMIC

• Endothermic• Takes in energy• Energy transfers as

heat

• The molecules gain energy• The molecules are

now moving faster

• Exothermic• Lose energy• NOT LOOSE

• Heat leaves the system• Molecules are now

moving slower

LOOK AT THIS PICTURE

• Would ya look at that

• Before hot plate was 25 C• More heat = faster speed of molecules

• When speed changes enough, there is a phase change

DEMO

PHASE CHANGES

• Phase changes are determined by Intermolecular Forces• There’s that word again!

• Phase changes happen when intermolecular forces either dominate speed or the intermolecular forces are broken• Molecules with stronger intermolecular forces

have higher boiling/melting points

WHAT IS A PHASE CHANGE

• The kinetic energy (temperature) of a molecule becomes large enough to break intermolecular forces

• OR• The kinetic energy (temperature) becomes small

enough such that intermolecular forces take over• The distance between molecules changes• Further if IMFs are broken• Closer when IMFs dominate

• Temperature DOES NOT CHANGE during phase change• All energy goes into breaking/forming IMFs

• The temperature has reached the freezing or boiling point

Freezing Point Boiling Point

(or Temperature)

MELTING

• Draw• You’ve seen it! Ice!• Solid to Liquid

• Molecules gain energy and move faster• Thus energy goes in• Endothermic

• Occurs at Melting Point

FREEZING

• Draw• You’ve seen it! Ice! (Again!)• Liquid to Solid

• Molecules lose energy and move slower• Thus energy goes out• Exothermic

• Occurs at Melting Point• The freezing point is the same as the melting point! 0 Celsius

SUBLIMATION

• Draw it• Solid to Gas (no liquid!)

• Molecules gain energy and move faster• Thus energy goes in• Endothermic

• Dry Ice! Very cold, -78 Celsius

CONDENSATION

• Draw• You’ve seen it every morning!• Tomorrow go outside and say ‘Look at all that phase change’

• Gas to Liquid

• Molecules lose energy and move slower• Thus energy goes out• Exothermic

• Occurs at Boiling point• The boiling point is the same as the condensing point!

VAPORIZATION

• Draw• You did this in lab. Why did the acetone boil before water?• Liquid to Gas

• Molecules gain energy and move faster• Thus energy goes in• Endothermic

• Occurs at Boiling point• The boiling point is the same as the condensing point!

QUICK ASIDE

• Anything can undergo a phase change• EVERY element/compound can vaporize, freeze,

or anything else• All at their own Temperature/Pressure• Copper can vaporize• Oxygen can freeze• Helium can condense

DEPOSITION

• Draw• You probably saw this this morning! Frost on the grass

• Frost on the car, frost on the window, frost everywhere

• Gas to Solid (No liquid phase!)

• Molecules lose energy and move slower• Thus energy goes out• Exothermic

DIFFERENT SPEEDS

• Not all molecules are going the same speed• Just like everyone in this room cannot run the same

speed• This means some molecules have more kinetic energy

VAPOR PRESSURE

• Some molecules have enough kinetic energy to enter the vapor phase even though most of the other molecules are still a liquid• This is vapor pressure

• The higher the temp the higher the Vapor Pressure• You already know this!• Green bottle

VOLATILITY

• Strong intermolecular forces = Low Vapor Pressure• Less volatile

• Weak Intermolecular Forces = High Vapor Pressure• More volatile

• Acetone is more volatile than water• It evaporated faster

SURFACE TENSION

• Surface Tension holds liquids together• It clings together!

• Strong Intermolecular Forces = Strong Surface Tension• Liquids want to minimize surface area• A surfactant can change all this