The Particle View of Matter

Everything is made of atoms
• Only about 100 varieties (about 90 naturally occurring).
• Everything is made from a combination of these 100 elements.
• Think of them as the ingredients that go into a recipe
where each substance we know has it’s own recipe.
Image from wikipedia

What are the properties of a gas?
The atmosphere is a gas. Does it:
• Have weight? How do you know?
What do you notice when you move quickly? When the wind blows?
What’s going on when a sailboat moves?
• What can we say about the particles that
make up the atmosphere?
Is air solid?
Can air be compressed?

In a SCUBA tank, 100 cubic feet of air can
be compressed into ½ cubic foot (the air is
compressed by a factor of 200).
What do the particles in the air look like?
In the tank?
What occupies the space between the
particles?

Consider a balloon full of air:
what happens when it is heated?
• Why does this
happen?
• What is happening
with the particles that
make up the air?
• How does increasing
the temperature affect
the particles that
make up the air?
HOT PLATE

How do we know that the molecules in air
are moving?
• What happens when you open a perfume?
The smell spreads until eventually the
whole room picks up the scent.
How can we model this?

• Consider four adjacent boxes
• Place 10 air particles in each box
– Assume that on any given turn 50% of the particles in a box
will move one box to the left, and 50% will move one box to
the right.
– Particles that would hit a side wall stay in their box.
etc.

Consider the same 4 boxes with 32 ‘perfume’
particles in box 1 at the start.
• As time passes, perfume particles spread.
• Over time, they become more uniform
throughout the space.

Boyle’s Law
Stated mathematically: P1 * V1 = P2 * V2
Practically, what does this mean?
• If you have a sealed container of gas and
you decrease the volume the observed
pressure increases.
An example...balloon in a vacuum

Balloon Demo
• What will happen to the balloon when we
reduce the pressure around it?
• Why will this happen?

Demo: Marshmallow in a vacuum
• A marshmallow consists of many little
bubbles of air within a sugary, stretchy
matrix.
• At normal pressure the bubbles have their
normal volume. This P times this starting
V equals a constant number which, no
matter how P or V may be changed later,
must remain constant.

Liquids
• Are liquid compressible? Are there
spaces between the particles that make
up a liquid?
• Do liquids expand when heated?
• If you put a drop of dye into water, what
eventually happens?
– Hot water?
– Cold water?
– How do the results compare?

Dip your fingertip into a glass of water, then
lift it out.
• What do you notice?
• What is holding the bottom of the water drop to the rest
of the drop?
Look at a small drop of mercury on a glass
surface.
• What do you notice?
What does the behavior of the drop tell you
about liquids?

What happens when you heat a
liquid? (Demonstration)
• Marked spot on tube. What will happen as
we heat the water?
• What is our view in particle terms?
• Did what we expected to happen actually
happen?

Demo – surface tension
• Particles in a liquid
feel a ‘sticky’ force
which tends to hold
them together
• For substances like
water, where this
force is unusually
strong, it can be
exploited...

Solids
• Are they compressible?
• Do solids expand when heated? (DEMO--ball and ring)
– So are the particles moving?
• If you leave a book on a table will it evaporate?
• Can you poke your finger into a rock?
• What does this tell you is an appropriate model for
solids?

Structure of a Solid
• Molecules held to
well-defined positions.
• Molecules close
together.
• Significant energy
must be expended to
push a molecule
aside.

Demo: Thermal Expansion
• Materials tend to expand when heated.
• Ball will not pass through ring until the ring
has been heated.

Demo – bimetallic strips
• All materials undergo
thermal expansion.
• This expansion is
different for different
materials.
(Dental filling materials are
chosen based on how
close their expansion is to
human teeth – why?)
Image authored by Joh3.16
released under GNU Free Document license
The upper material expands more than
lower under heating. When materials
are fixed to one another this results in
bending.

Recap: phases and the particle
view of matter
• Gases are spread out,
little force between
particles
• Liquids have a
cohesive force that
holds them together,
more dense than
gases
• Solids are more
dense than gases but
have a rigid structure.
gas
liquid
solid

Evaporation
When you put a glass of water on the
counter the water will slowly evaporate.
• Which evaporates faster, a pint of water in a
wide pan or a pint of water in a tall glass? What
does this tell us?
• In terms of the particle model, what is happening
during evaporation?

Demonstration: Evaporation
• Feel the bottle of rubbing alcohol. Notice
that it doesn’t feel particularly warm or
cool.
• Now put a drop of alcohol on your finger.
What do you notice as the alcohol
evaporates?

• What does this graph tell you about?
• What is the most probable age of a Mt. SAC Student?
• What would happen to the average age of a Mt. SAC
student if the oldest students (those 50+ years old) left?
AGE
19 or less
20-24
25-29
30-34
35-39
40-44
45-49
50+
PERCENT
23
32.4
11.2
7.1
5.2
4.6
4
12.5
35
30
25
20
15
10
5
0
19 or less 20-24 25-29 30-34 35-39 40-44 45-49 50+
Age distribution of Mt. SAC Students, Spring 2003
Percent of
total student
population
Age

• The highest energy particles escape.
• What happens to the average energy of the
liquid?
Energy
required to
escape liquid
surface.

Examples of Evaporative Cooling
• Sweating
• A Dog’s panting
• Evaporative coolers

Condensation
• The lowest energy molecules in the gas get stuck to the liquid.
• Low energy gas >= highest energy liquid :: mean energy of liquid
increases.
Fractionof
moleculesinthe
sample
Energy of the
molecules
Lowest energy
gas molecules
may stick to
liquid surface.

Melting and Freezing
• What happens to our picture of a solid
during melting?
• What happens to our view of a liquid
during freezing?

Recap: Thermal Expansion,
Evaporation, Condensation
• Higher energy and higher temperature go
together.
• Higher temperature “stuff” vibrates more.
• The result of increased vibration is thermal
expansion unless:
– The vibration is strong enough to break the
rigid bonds in a solid, resulting in melting.
– The vibration is strong enough to break the
surface tension resulting in evaporation.

• Temperature is a way of measuring mean
energy of particles.
Fractionof
moleculesinthe
sample
Energy of the
molecules
for a particular Temp.

How the distribution of molecular energies
changes with temperature
from www.gs68.de/tutorials/ plasma/node7.html
Fractionofatomsinthe
sample
Energy of the atoms

But above a certain energy
particles escape
• What happens to the temperature of the
remaining liquid? (Recall Evaporation)
• So what will the temperature of the liquid do as it
boils off?
Energy required to
escape liquid
surface.

Temperature

Temperature
• How do we measure temperature? =
How do we assign numbers to a
measurements of hotness?
– Thermocouples
– Liquid crystals
– Thermometers--Conventional Temperature
Scales
• Fahrenheit
• Celsius (or Centigrade)

Shortcomings of conventional
Temperature Scales
• Suppose that it is 10ºC (50ºF) outside. What
temperature is twice as hot?
– Double the Celsius temperature?: 20ºC (68ºF);
– Double the Fahrenheit temperature?: 100ºF (38ºC)
• Can “twice as hot” depend on which
temperature scale that you are using?
• If one car is twice as expensive as another
car, does this depend upon whether you are
buying the car with dollars, pesos, or euros?

Shortcomings of conventional Temperature Scales (continued)
• Consider that you and
Shaquille O’Neill are standing
behind a counter.
– The top of Shaquille’s head is 3
ft above the top of the counter.
– The top of your head is 1.5 ft
above the top of the counter.
– Is it correct to say that Shaquille
is twice as tall as you are? Why
or why not?
1.5
ft
3
ft

Shortcomings of conventional Temperature Scales (continued).
• Consider that you and Shaquille
O’Neill are standing behind a
different counter.
– The top of Shaquille’s head is 4 1/2
ft above the top of the counter.
– The top of your head is 3 ft above
the top of the counter.
– Is it correct to say that Shaquille is
50% taller than you are? Why or
why not?

• Can how many times taller someone is
compared to someone else depend
upon which counter they are standing
behind?
• What is the appropriate, reasonable way
to compare how tall two people are?
• How does this relate to measuring
temperature? Comparing with how we
usually measure height, what point is it
reasonable to measure temperature
from?

A new temperature scale
• We’d like a temperature scale that:
– starts with “as cold as it can get” and goes up
from there.
– correlates with molecular activity; that is, twice the
temperature means that the molecules are twice
as energetic
– has the size of one degree on the new scale be
the same as 1 degree on the common (Celsius)
scale.
• Folks came up with the KELVIN temperature
scale, which meets all of these requirements.

The Kelvin Temperature scale
• Starts at absolute zero (0 K or –273ºC); as cold as it
can get.
• Each degree is the same size as a Celsius degree
• Ice melts at 273 K
• Water boils at 373 K
What does absolute zero mean?