solid liquid gas tightly packed, in a regular pattern vibrate, but do not move from place to place...

Phase Changes

Overcoming Intermolecular Forces

STATES OF MATTER

SOLID LIQUID GAS

Tightly packed, in a regular

patternVibrate, but do not move from place to place

Close together with no regular arrangement.Vibrate, move

about, and slide past each other

Well separated with no regular arrangement.Vibrate and

move freely at high speeds

What causes a substance to be in solid or liquid state? Attractive forces between particles.

STATES OF MATTER

SOLID LIQUID GAS

Tightly packed, in a regular

patternVibrate, but do not move from place to place

Close together with no regular arrangement.Vibrate, move

about, and slide past each other

Well separated with no regular arrangement.Vibrate and

move freely at high speeds

How do we overcome these attractive forces?

By adding energy (heat) to the particles.

Kinetic Energy & Temperature

Adding energy (heat) makes the particles vibrate or move faster.This kinetic energy (energy of motion) is the basis of temperature.More Kinetic Energy = Higher temperature

Kinetic Energy & Temperature

So why are substances gases at high temperature?The particles are moving faster… There is less opportunity for attractive forces (IMFs) to pull molecules together as solids or liquids.

Kinetic Energy & Temperature

If the substance is in a gaseous state, energy can be removed (it can be cooled down)

As soon particles are moving slowly enough for their kinetic energy to be overcome by attractive forces between them (IMFs), the substance will liquefy or solidify.

Overcoming IMFs with kinetic energy

Heating Curve (water)

Tem

pera

ture

Energy Added

solid

liqui

d

gas

100º C -

0º C -

melting point(s) (l)-

(s) (l)-freezing point

boiling point(l) (g)-

(l) (g)-condensation point

Heating Curve (water)

Tem

pera

ture

Energy Added

Heat of fusion

Heat of vaporization

“Fusion” means melting (fusing bits of solid into a liquid) when it is used to talk about phase changes.

∆Hfusion ∆Hvaporization

Where did the energy go?

Tem

pera

ture

Energy Added

Kinetic energy (T) does not increase during phase change.

The energy that overcomes attractive forces between particles is stored as potential energy.

0º C -

-25º C -

Heating solid(kinetic energy)

Latent heat (heat

of fusion)(potenti

al energy)

Heating liquid(kinetic energy)

Latent heat (heat of

vaporization)

(potential energy)

Phase change energy is often called a “latent” heat. (Latent = hidden)

Heating gas

(kinetic energy)

100º C -

165º C -

MeltingChange from Solid to Liquid

The melting point is the temperature at which a material changes phase from solid to liquid.

Melting occurs when the kinetic energy of individual atoms equals the attractive force between atoms.

Kinetic Energy =

temperature

BoilingChange from Liquid to Gas

The boiling point is the temperature at which a material changes phase from liquid to gas.

Boiling occurs when the kinetic energy of individual atoms causes a vapor pressure equal to atmospheric pressure.

BoilingChange from Liquid to Gas

VAPOR PRESSURE:The pressure exerted by gas molecules of a (specific) substance.

This is temperature dependent. (Vapor pressure increases with temperature.)

So boiling point = the temperature where vapor pressure = atmospheric pressure

Vapor Pressure:

Vapor Pressure:

At the boiling point, bubbles of gas form throughout the liquid.

The bubbles only form if their pressure matches surrounding pressure.

BoilingChange from Liquid to Gas

What’s the boiling temp of chloroform?

Ethanol?

Water?

Ethanoic acid?A substance cannot boil until its vapor pressure = atmospheric pressure.

Standard Pressure: 1 atm = 101.3 kPa = 760 mmHg

60 ºC

80 ºC

100 ºC

120 ºC

“Standard Pressure” = At sea level

We’re not at sea level.Higher altitude =

Thinner air = Lower air pressure.

Our air pressure is about 84.2 kPa.

BoilingChange from Liquid to Gas

What’s the boiling temp of water in Alpine?Air pressure ≈ 84.2 kPa

Boiling point ≈ 95 ºC

Does spaghetti take more time or less time to cook in Alpine, compared to San Diego?

MORE time– because we cook it at a slightly lower temperature.

Phase Diagram

One Graph to Rule them ALL!The phase diagram combines:Temperature/Energy information from a heating curve

Temperature/Pressure

information from a vapor pressure curve.

Temperature

Pre

ssure

Phase Diagram

All phase changes are shown on this graphWhich phase is likely

at low temp?At high temp?At high pressure?

High pressure favors the most dense phase.

Low pressure favors the least dense.

Temperature

Pre

ssure

SOLID

GAS

LIQ

UID

Phase Diagram

All the phase changes:

Temperature

Pre

ssure

SOLID

GAS

LIQ

UIDMelting = fusion

Freezing = crystallization = solidifying

Boiling = evaporation = vaporization

Condensing

Every point on this line is a MP & FP !

Every point on this line is a BP & CP !

Phase Diagram

All the phase changes:

Temperature

Pre

ssure

SOLID

GAS

LIQ

UIDSublimation

(solid to gas)Deposition

(gas to solid) Every point on this line is a SP & DP !

Sublimation From Solid Straight to Gas

Requires the input of energy

Sublimation of snow occurs more readily under at high altitudes with less air pressure, with dry winds.

Examples: dry ice, iodine and snow

Sublimation occurs (on a small scale) in your freezer– when things get freezer burn.

Deposition: from gas to solid (without liquefying)

(The opposite of sublimation)

Deposition of iodine

Requires the release of energy.

Deposition commonly occurs when water vapor freezes to form snow or frost.

(On your windshield, for example.)

This also occurs in your freezer with the freezer burn!

Phase Diagram

“Normal” phase change:

Temperature

Pre

ssure

SOLID

GAS

LIQ

UIDMeans:

Standard Pressure1 atm, 101.3 kPa, 760 mmHg

This graph (for CO2) doesn’t have mp or bp, because CO2 sublimes at 1 atm.

Phase Diagram

Can we force it to boil?

Temperature

Pre

ssure

SOLID

GAS

LIQ

UIDYes! If we raise

the pressure above 5.2 atm, will melt and boil.

If we raise the pressure to exactly 5.2 atm?

Triple point: all three phases coexist. (It boils and freezes!)

Phase Diagram

Critical point

Temperature

Pre

ssure

SOLID

GAS

LIQ

UIDAbove this

temperature, the particles have so much kinetic energy that they ONLY exist in the gas phase.

Even if the graph stops here, assume the line goes straight up.

Phase diagram for water:

The 1 atm line crosses all three phases, so we experience all phases of water.

To reach the triple point, you would need a vacuum chamber. (Or extreme high altitude.)

Boiling Point and Pressure As altitude increases, air

pressure decreases. As air pressure decreases

boiling point decreases.

At what altitude would your food cook most quickly? Hint: at what temperature would

it cook most quickly?

Phase diagram for water:

This is why boiling ramen doesn’t work well for (high altitude) High Adventure camps

The boiling water is not as hot as you hoped… so cooking takes forever.

Heating Curve: At High Altitude

Tem

pera

ture

Energy Added

solid

liqui

d

gas

85º C -

2º C -

melting point(s) (l)-

(s) (l)-freezing point

boiling point(l) (g)-

(l) (g)-freezing point

Even in a raging bonfire, your water will refuse to get any hotter once it’s boiling.

Phase diagram for water:

If only there was a way to increase the pressure…

They can boil water at a temp above 100 ºC, so food cooks faster.

Pressure cookers do that!

Evaporation

Molecules leaving the liquid take energy away from a liquid in order to convert to gas.

The average energy of the molecules left behind is lowered:the temp. of the fluid left behind is lower.

Evaporation can occur when molecules go from liquid to gas at temperatures below the boiling point.

Evaporation cools the surface of a liquid because the fastest molecules escape and carry energy away.

Condensation Condensation can occur when molecules

go from gas to liquid at temperatures below the boiling point. Molecules slow down and begin

to stick together. (Due to IMFs!) As they change from gas to

liquid, the molecules release latent heat to the surroundings Condensation is a heating

process!The release of energy from

condensation drives weather including wind, thunderstorms, hurricanes etc…

Energy and Phase Change