Chapter 7 Notes
Fluids and Viscosity
7.1
A fluid is any form of matter (substance) that flows.
A review of the five major points of the particle theory of matter:
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. As the particles gain energy,
they move faster.
5. The particles in a substance are attracted to one another.
The strength of the attractive force depends on the type of
particle.
Even though a
solid does not appear to move,
the particles are constantly
vibrating in place.
The particles in a liquid are in
contact with each other, but
they can slip and slide past
one another, changing their
position. This slipping and
sliding means liquids take the
shape of their container.
Solids
Shape
Volume
Particle arrangement
Particle movement
Definite shape
Definite volume
Particles close
Vibrate in place
Gas particles have very large
spaces between them. Gases
are quite different from liquids
and solids because the
particles in a gas can move
freely in all directions. This is
why gases always spread out
or diffuse in their container
Liquids
Indefinite shape
Gases
Indefinite shape
Definite volume
Particles close
Free flowing
Indefinite volume
Particles distant
Random movement
Since liquids and gases do not have a definite shape they are able to flow.
Hence by definition, liquids and gases are fluids.
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Examples of fluids in everyday life:
(i) compressed air in tires
(ii) water
(iii) syrup
Other examples include:
- food fluids (syrup, honey, molasses, water, or oil.)
- cleaning fluids (shampoo, liquid detergents, gels, abrasive creams like Vim™)
- bodily fluids (blood, mucus)
- industrial fluids (oils as lubricant, compressed air in tires)
7.2
Viscosity is a measure of a liquid’s resistance to flow.
Liquids like molasses and maple syrup have a high viscosity, while other liquids
like water and milk have a low viscosity. But what makes one fluid run faster or
slower than another? Even though all fluids flow smoothly, they flow at different
rates because they have different viscosities.
The viscosity of a liquid is related to the amount of friction (a force that resists
movement) between particles. The greater the friction or rubbing of particles in
any fluid, the higher the viscosity. This can result from the size and shape of the
liquid particles as well as the attraction between particles.
The viscosity of liquids is an important property that must be measured very
carefully in some industries. Many substances require the proper degree of
viscosity to perform their intended function. For example,. Peanut butter kept in
the fridge is often too viscous to spread.
Examples of viscosity in everyday life:
(i)
motor oil
 in order for motor oils to be effective lubricants, they need to have
the proper viscosity.
(ii)

(iii)


paints
Paints require an appropriate degree of viscosity in order to spread
properly, they cannot be too thin and runny, nor can they be too thick.
foods
Peanut butter kept in the fridge is often too viscous to spread
Canada’s maple syrup industry depends on controlling the viscosity of
a liquid. The sap (from sugar maple trees) is thin, runny, and
unsweetened but when the sap is heated and water evaporates, the
result is a thick, sweet syrup that is highly viscous.
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Comparing Viscosities
Flow rate as the speed at which a fluid flows from one point to another.
How fast a fluid flows can be determined by measuring its flow rate. This is often
used to compare the viscosity of fluids since viscosity itself is a difficult property
to measure directly.
Examples of liquids with different flow rates include:
(i) water
(Low viscosity →fast flow rate)
(ii) dishwashing liquid
(medium flow rate)
(iii) corn syrup
(High viscosity →slow flow rate)
Remember that viscosity is the measure of a liquid’s resistance to flow Fluids that
have a high viscosity, or fluids that are more viscous, flow slowly. Whereas fluids
that have a low viscosity, or fluids that are less viscous, flow freely or faster.
Other examples of how flow rate can be affected:
(i) Pancake batter
If the batter is thick it is more viscous and has a slower flow rate, whereas if the
batter if thin, it has a low viscosity and flows quickly.
(ii) Motor oil
For example, the temperature of engine oil would dictate how quickly it would
drain from an engine during an oil change, or why different types of motor oil are
used for different engines and different seasons.
7.3 Factors that Affect Viscosity
A liquid’s resistance to flow can be explained in terms of the particle theory of
matter:
1) strength of attraction between particles
There is a force of attraction between particles that can be either strong or
weak. There is an attraction of particles within one particular liquid itself
and that there is an attraction between the particles of the liquid and the
particles of the object on which it is flowing.
For example, the droplets of rain that appear on a clothesline after a
rainfall illustrate this. The fact that water particles remain together to form
droplets is an example of attraction of particles within one particular liquid
and the fact that the droplets remain on the clothesline is an example of
an attraction between the particles of the liquid and the particles of the
object on which it is flowing.
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attractive forces strong → the liquid flows slowly
(difficult for the particles to
pull away from each other
and slide past each other)
→high viscosity
→ the liquid flows easily
→ low viscosity
attractive forces weak
(particles pull apart and
slide past each other
easily)
2) temperature
Particles are constantly in motion. As they acquire more energy, they
move faster. As liquids are heated the particles move faster and farther
apart. As a result, the viscosity decreases because there is less friction
and the attractive force between particles is reduced. For example, when
honey or wax is heated it flows faster (viscosity decreases).
On a very cold winter morning in Newfoundland and Labrador, it can
be hard to start your car. One of the problems is that the oil that lubricates
the engine can become very stiff and resistant to flow under cold
conditions.
In general, a liquid’s viscosity decreases as the fluid is heated and
increases as the fluid is cooled.
The effect of temperature on gas particles is opposite to the effect on
liquids. The particle theory states that the particles in gases are already
very far apart. When energy is added, gas particles speed up and
collide with each other more often, causing an increase in internal
friction, and therefore an increase in viscosity.
A gases’ viscosity increases as the fluid is heated and decreases as the
fluid is cooled.
3) concentration
There is empty space between the particles. Concentration refers to the
amount of substance in a given space. If a liquid is concentrated, it means
that there are more particles in a given space. As a result, the viscosity
increases because there is more friction and the attractive force between
particles is increased. For example, skim milk, 1% milk, 2% milk, whole
milk and cream.
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For example, when making gravy, you may thicken the gravy by adding
cornstarch. By increasing the concentration of cornstarch, you are also
increasing the viscosity of the gravy.
Generally, by increasing the concentration of a substance, the viscosity is
also increased.
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