Viscosity
What do melting
chocolate, pancake
syrup, ketchup, and ice
water have in common?
They are all liquids, and
they all flow.
The property that
describes a liquid’s
thinness or thickness is
called viscosity.
You can easily observe
fluids such as water
flowing out of a tap,
milk being poured into
a glass, or ketchup
being squeezed out of
a bottle.
Your body contains
many fluids, such as
blood and the watery
cytoplasm inside cells.
It is more difficult to
imagine gases flowing,
but they do.
 Take a deep breath. What
happened?
 Some of the air that
surrounds you flowed into
your lungs when your
lungs and your ribcage
expanded.
 Air flows out of your lungs
when you exhale. Like
liquids, gases flow and
take up space. Therefore,
gases and liquids can both
be classified as fluids.
The essentials of life
(food, water, and air),
are examples of
substances that occur in
the three different
states of matter: solid,
liquid, and gas.
Solids have a definite
shape and volume.
Liquids have a definite
volume, but no definite
shape
Gases have neither a
definite volume nor a
definite shape.
 The 5 major points of the particle
theory are:
1. All matter is made up of
very small particles.
2. All particles in a pure
substance are the same. Different
substances are made of different
particles.
3. There is space between
the particles.
4. The particles are always
moving.
5. The particles in a
substance are attracted to one
another. The strength of the
attractive force depends on the
type of particle.
A substance has
a definite
volume, but an
indefinite shape.
Is the substance
a solid, liquid, or
a gas?
• According to the particle
theory, we can think of
solids as being made up of
particles that are tightly
packed together, like bees in
a hive.
• This way of thinking about
the particles of a solid can
explain why solids are
greatly affected by gravity.
• It explains why a solid will
tumble toward the lowest
surface when suspended in
the air and then dropped.
 The particle theory
suggests that the particles
of a solid are so close
together they cannot
move around freely- they
can only vibrate.
 Many solids can be
ground into such small
pieces that they can slip
past each other when
they are poured out of
their containers. Sugar,
salt, flour, dishwasher
detergent, and Kool-Aid
are examples of solids
that can be poured.
 According to the particle
theory, each tiny fragment
of these solids contains
billions of even smaller
particles that are tightly
packed together.
 Each tiny fragment is like a
miniature solid in itself.
This explains why solids
form a pile when they are
poured and why they do
not keep flowing apart
from each other.
Again, according to
particle theory, the
particles that make up
liquids have enough
energy to pull away
from each other and
slide around each
other, while at the
same time vibrating
close together in small
clusters.
 Another way to think
about what is happening
on the level of the
particles is to imagine
groups of people at a
party.
 The guests can move
around by shifting as a
group, or by flowing in
between the other groups
of partygoers.
 Similarly, liquid particles
can slip past each other.
Unlike the particles in
solids, they do not form
rigid structures.
 The particles of a liquid
cannot hold their shape;
instead, they fill a
container and take the
shape of that container.
 As in solids, liquid
particles are so tightly
packed together that they
are easily affected by the
downward pull of gravity.
 Therefore, liquid always
flow to the lowest
possible level.
Liquids form a level
surface when they are
at rest. Some foods,
like chocolate and ice
cream, can be melted
to form a liquid. Many
other substances, like
water, oil, syrup, and
perfume, are liquids
when they are at
room temperature.
 All liquids can be
transformed into their
gaseous state when the
liquids are heated. Many
substances are gases
when they are at room
temperature; for example,
the air around you is a
gas.
 According to particle
theory, gas particles are so
far apart from each other
that there is an enormous
amount of empty space
between them.
 Imagine sitting in a huge
arena. One of your friends
is also in the arena, but
you are as far from one
another as possible.
 This is what gas particles
are like. In fact, the
particle theory explains
that most gases seem
invisible to you because
you are observing mostly
empty space.
 This also explains why gas
particles have no difficulty
moving past one another,
and why they flow very
easily.
 The particle theory
suggests that gas particles
are so free to move that
they do so in every
direction, and they have a
great deal of energy to
move extremely far apart.
 Therefore, gas particles
spread out so much that
in a brief time, they fill up
the space of an entire
container or room.
 Since gases fill an entire
room or container very
quickly, they also take on
the shape of the container
in which they are sealed.
 However, gases do not
flow to the lowest
possible point like liquids
do. Gas particles are not
clustered or packed tightly
together, so the energy of
the particles allows them
to move in all directions,
sometimes even against
gravity.
 For example, water
vapour forms clouds that
float in the sky. Unlike
what happens to liquids,
when the lid is taken off a
container of gas, the gas
particles will start to
spread again, until they
have filled the entire
room or building.
 The particle theory can be
used to explain how gases
always occupy all the
space that they can fillup, down, or sideways.
How could you test
whether or not a
substance is a
fluid?
The process of melting
is an example of a
change of state, which
occurs when the
physical state of a
substance is
transformed into
another state- for
example, when a solid
melts and becomes a
liquid.
 The change from solid to
liquid is called melting,
and the change from
liquid to gas is called
vaporization. These
changes occur when the
substance is heated and
the particles of the
substance gain energy. If
you were to cool the
substance, the opposite
would occur because the
particles lose energy.
 The change from gas to
liquid is called
condensation, and the
change from liquid to solid
is called freezing.
 An unusual change of
state occurs when a solid
turns into its gaseous
state without going
through the liquid state.
 This change of state is
known as sublimation. Dry
ice is an example of
sublimation- the particles
of the ice gain energy and
give off a fog.
 What is the difference
between a gas and a
vapour?
 A substance is called a gas
if it exists as a gas at room
temperature (for example,
oxygen or carbon dioxide).
 The same substance is
called a vapour if it
normally exists as a solid
or a liquid at room
temperature (for example,
water vapour).
Evaporation is slow
vaporization. It occurs
over a wide range of
temperatures.
A wet towel will dry
even if the air
temperature is not
high. On a cool day it
will simply take longer
for the water to
evaporate from the
towel.
 Boiling is rapid
vaporization. It occurs at a
specific temperature,
called the boiling point.
The boiling point of water
is 100°C at sea level.
 Every substance has its
own freezing point and
melting point. The
freezing point of water, for
example, is 0°C at sea
level.
 This is the temperature at
which liquid water
freezes. It is also the
temperature at which ice
melts.
 Classify the following items as
either fluids (f), or non fluids (nf)
 Shampoo
 Balloon
 Thread
 Nail polish
 Perfume
 Blood
 Sap
 Smoke
 Sugar
 Air
 Ash
 Honey
 Natural gas
 Snow
 Some liquids can flow
faster than others. Orange
juice flows easily from a
container, but honey is
much slower.
 If you wanted to know
how fast you could run,
you would time yourself
running a specific
distance. In a similar way,
you can measure how fast
a fluid “runs”. You would
measure the time it takes
for the fluid to flow from
one point to another.
 This characteristic is called
the fluid’s flow rate.
 Why might it be important
to know how to
determine the flow rate,
and therefore the
viscosity of a liquid?
 The viscosity of liquids is
an important property
that must be measured
precisely in some
industries.
For example, the
viscosity of paints,
varnishes and similar
household products are
closely regulated so that
the paints and varnishes
can be applied smoothly
and evenly with a brush
or a roller.
The viscosity of some
medicines, such as
cough syrup, are
regulated as well. Cough
syrup has a high
viscosity (but it’s still
drinkable) so it will coat
and soothe the throat.
 Your mouth is highly
sensitive to viscosity, so
food manufacturers
ensure that ice cream
toppings, pasta sauces,
soups, gravies, salad
dressings, and other
products are just the right
consistency (thickness) to
suit consumers’ tastes.
List two substances
that have a low
viscosity and two
substances that have a
high viscosity.
What is the
relationship between
the viscosity of a
liquid and its flow
rate?
 Imagine that you and a
group of friends are
moving through a
crowded shopping mall.
Could you make your way
through the crowd more
quickly if your group were
small rather than large?
 Small groups can move
through a crowd more
quickly than large groups
because they can fit into
the empty spaces
between other groups
more easily.
 In a similar way, particle
theory suggests that small
particles can move past
each other more easily
than large particles can.
Large particles take up
more space.
 Now imagine that you are
back in the mall, but this
time, everyone has
shopping bags, bookbags,
purses, and other parcels.
It would be even more
difficult to squeeze
through the crowd
because everyone would
be taking up even more
space.
Similarly, some
particles are bigger
than others because
of their shape. For
example, oil particles
are bulkier than water
particles.
For this reason, oil is
more viscous than
water.
 Now, imagine you’re back
at the mall, but this time,
everyone is wearing shoes
made of Velcro. Every
time you would try to
walk by someone, your
shoes would stick to
theirs. With every step,
you would have to stop
and pull your foot away
from another person’s
foot. No matter how large
or small the people in the
crowd, it would take a
long time to walk through
the mall.
 Particle theory tells us
that all particles attract
each other, just like
people wearing Velcro
shoes. However, some
particles attract and hold
on to each other more
tightly than other types.
 It is very difficult for these
particles to flow past each
other, so they do so very
slowly. The strength of the
attraction that particles
have for each other is the
most important factor in
determining a fluid’s
viscosity.
Finally, imagine that
everyone in the
shopping mall is
moving very, very
slowly and won’t get
out of the way when
you and your friends
try to go by.
This is similar to what
happens to particles
when they are cooledthey slow down.
 Even though all fluids flow
smoothly, they flow at
different rates because
they have different
viscosities.
 Another way to define
viscosity is “resistance to
flow”.
 This means the particles
can get around, but it may
be difficult for them to
pass one another; this
resistance creates internal
friction.
A liquid’s ability to flow
also depends on the
energy that the particles
have to move around.
The warmer the liquid
becomes, the more
energy the particles
have to move out of the
way and make room for
other particles to pass.
However, as the
temperature drops, the
particles have less
energy to move around,
so the empty spaces
between them get
smaller and smaller.
In general, a
fluid’s viscosity
decreases as the
fluid is heated
and increases as
the fluid is
cooled.
 Although gases, in
general, flow much more
easily than liquids, the
viscosity of gases can vary
too.
 The size and shape of gas
particles and temperature
are factors that affect the
viscosity of gases as well
as the viscosity of liquids.
 While an increase in
temperature reduces
viscosity in liquids, the
opposite is true for gases.
 Gas particles do not
depend on an increase in
energy (a rise in
temperature) to move
father apart, as is the case
for liquids. The particle
theory suggests that gas
particles are already very
far apart.
Extra energy increases
the internal friction of
gas particles because
the particles speed up
and collide with each
other more frequently.
Cooler temperatures
in gases keep internal
friction (and viscosity)
low.