Phase Changes
“It’s just a phase”
States of Matter
Solid, liquid and gas (plasma and BEC).
 Changes between states are called
“phase changes”.
 Caused by a change of heat or
pressure. More often heat.
 HEAT and TEMPERATURE are not the
same thing.

Temperature
Measures the average kinetic energy of
the particles in a substance.
 Is measured in Celsius or Kelvin.
 Kinetic energy is directly related to the
speed of the molecules.
 The faster the particles/molecules are
moving the higher the temperature.

Heat
Heat is a measure of energy (Joules).
 HEAT and TEMPERATURE are not the
same thing.
 Ex. A cold Lake Superior has more heat
energy than a boiling pot of water.
 For our class, higher temperature
means more heat.

Solids
Molecules are tightly packed together.
 High potential energy – more bonds.
 Molecules greatly affected by
intermolecular forces.
 Particles vibrate in place.
 Very Dense.
 Not easily compressed.

Liquids
Particles are not so tightly packed
(liquids flow and can be poured).
 Medium potential energy.
 Particles are affected by intermolecular
forces.

Tension-ability of a liquid’s
molecules to “stick” together.
 Adhesion-stick to other compounds
 Cohesion- stick to self
 Surface
Cool Videos
Number One
 Number Two
 Number Three

Gases
Particles spread out as the container
will allow.
 High kinetic energy, particles are
moving very quickly (1000 km/sec).
 Little effect of intermolecular forces.
 Low density, can be compressed, very
fluid.

Cool Videos

Number Two
Phase Changes
When a substance changes states.
 Requires the input or the removal of
energy or change in pressure.
 During a phase change the temperature
does not change, but the amount of
energy does.
 Phase Change Diagram.

Intermolecular Forces
Interactions between particles that
cause them to “stick” together.
 Strongest in solids
 Weakest in gases.
 Strongest when particles move slowly.
 During a phase change IMF are either
weakened or strengthened.

Sublimation
Transformation of a substance to a gas
from a solid state with no liquid
transition.
 Dry Ice does this.

Deposition
When a gas transforms into a solid
without transitioning through a liquid
state.
 Ex. Frost forming on windows.

Where does all the energy go?
During a phase change energy is
added, but the temperature does not
increase.
 The energy goes toward breaking up
weak intermolecular forces between the
particles.

Some Vocab
Triple Point: Pressure and temperature
where a substance exists as a solid,
liquid, and gas.
 Critical Point: Indicates critical temp and
pressure.

Critical Temperature
The temperature at and above which no
additional pressure can be added to
maintain a liquid.
 The critical pressure is the highest
pressure that will keep a compound a
liquid at the critical temperature.
 The two correspond to a point on a
graph.

Helpful Info
Melting point and freezing point are the
same thing. It just depends if the
substance is getting hotter or colder.
 During a phase change temperature
does not change, but the amount of
heat does.

Intermolecular Forces

Intermolecular Forces are the forces
of attraction that are between
molecules.

This is why lakes form lakes, gold can be
melting together, and why any matter can
be held together.
Phases


Phases is the term scientist use to more
properly define solid, liquids and gases.
It means the same as the term “state”
Phase is defined as “a part of matter
that has uniform properties through
out the entire substance”
Phases change



As scientists we can change phases.
We can change a solid into a liquid and
a liquid into a gas.
When two phases exist at the same
time it is called equilibrium.
Equilibrium is a dynamic condition in
which two opposing changes occur in
equal rates in a closed system
Equilibrium



To have equilibrium we need to have
both a temperature and pressure.
When we have both a measured
temperature and pressure two or three
states will exist at the same time.
Ice melting into water, water freezing
into ice.
Phase Diagrams


Scientist have phase diagrams to show
exactly the temperature and pressure
must be achieved to have a solid, liquid,
gas, two phases or all three phases
A phase diagram by definition is “a
relationship between physical states
that deals with temperature and
pressure.”
Phase diagram of
water.
Pressure is in
atmopheres (normal
pressure is 1 atm)
Temperature is in
celcius (water boils at
100)
Each of the lines represent equilibrium (two
states exist at the same time)
Point C is called the critical point
Critical point: is the point on which liquids and
gases are indistinguishable (can’t tell the
difference
Point A is called the Triple Point
The Triple point is the temperature and
pressure in which gas, liquid and solid all exist
in equilibrium
Video
Sublimation
 Freezing
 Freezing Waves

Vapor Pressure
The molecules in a liquid are constantly
moving and running into each other.
 Sometimes they will “bounce” out of the
container they are in.
 As a result, a faint vapor will exist over
most liquids.

Vapor Pressure
Vapor Pressure is the pressure of vapor
over a liquid at equilibrium with the
atmosphere.
 When water boils the vapor pressure
exceeds the atmospheric pressure and
the molecules leave the liquid with little
impediment.

Vapor Pressure
So then how do puddles evaporate and
why can clothes dry outside when the
temperature is below freezing???
 Water molecules are always moving
and are always escaping from their
container. Heat makes them move
faster or slower.
