Viscosity - That's a new word


One property of fluid is how they move or flow.  Why?

Viscosity :

Liquid’s internal resistance or friction that keeps it from flowing.

  • If a liquid has a high internal resistance (pours slowly) it's considered to have a 'thick' viscosity 
  • If a liquid has a low internal resistance (pours quickly) it's considered to have a 'thin' viscosity

How does this happen? 

  • Particle theory explains that the particles in a liquid slide around each other. In gas, they move more easily.
  • The greater the friction or rubbing between particles in any fluid, the higher the viscosity.
  • Fluids with high viscosity do not flow as easily as fluids with low viscosity.

Understanding Viscosity and Temperature



  • As the temperature of a liquid increases, its viscosity decreases. (flows faster)
  • As the temperature of a liquid decreases, its viscosity increases. (flows slower)
  • Particles can slide more quickly when there is extra energy from heat.

Measuring Viscosity

Ramp method

  • Pouring a fluid down a ramp and timing how long it takes to get to the bottom
  • Able to test different fluids, or same fluids at different temperature, or both!

Density of Fluids

Density :

Is the amount of matter in a given volume or mass per unit of volume; calculated by dividing the mass of a substance by its volume.

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Not all substances have the same density, because different substances are made of different particles (particle theory).

This results in different densities due to the different particles!


  • If an object has a bigger density than a solution, it will sink.
  • If an object has a smaller density than a solution, it will float.

Calculating Density


Density (D)= Mass (m)/ Volume (v)  or D= m/V


  • Units are in grams per millilitre (g/mL) or kilograms per litre (kg/L) for liquids.
  • For gases it is grams per centimeter cubed (g/cm³).




An instrument that measures the density of a liquid

A Recap on Density & Temperature

Viscosity changes with temperature

  • Temperature increases, viscosity decreases = fluid flows easier
  • Temperature decreases, viscosity increases = fluid flows slower

Remember! In a substance, the number of particles in a given volume remains the same unless you add or delete (then the amount of particles change)

  • A denser object will sink. A less dense object will float.
  • Density does not change as long as the temperature stays the same!!
  • Particles in a substance move more quickly when energy is added



The Particle Model of Matter as an Explanation for Density Changes

Changing the Density by Concentration

  • By adding more solute you increase the number of particles and thus increase the mass and the density.

Changing the Density by adding energy

  • As the particles move faster, they move further apart
  • This increases the volume but since the number of particles stays the same, the mass stays the same    
  • This results in the density decreasing.

Galileo’s Thermometer

  • A hydrometer is a device for measuring the density of liquids
  • Each temperature bulb acts like a hydrometer and floats to the top when the water’s density is greater than that of the bulb

Buoyancy :

Tendency of an object to float when placed in a fluid

  • When an object is in a liquid, the force of gravity pulls it down.
  • The liquid however exerts an opposite force that pushes the object upward
  • This is called the buoyant force!

Buoyant Force :

Upward force that a fluid exerts on an object; opposite to the pull of gravity on an object in a fluid.

  • If the buoyant force of an object is greater than the force of gravity, the object will float.
  • In other words, if the density of the liquid is greater than the density of the object, the object floats. 
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Neutral buoyancy :

Is when the force of gravity is equal to the buoyant force

Heavier object will sink…Duh


Gravity pushes an object down.  The more mass the object, the more it will be pulled down.

•    When you dive in water, you’re able to go downward because the force of gravity, and force of leg movement is greater than the buoyant force of the water

  •  To move upward, the buoyant force of water and leg movement is greater than the force of gravity!

Buoyancy in the Real World

  • A fully loaded boat that crosses the Atlantic ocean and enters the fresh water of the St. Lawrence river will sink lower because the density of the salt water is greater than the density of the fresh water.
  • Same thing occurs from cold water in the North to warm water in the south…the ship sinks. Why? Temperature has an effect on density!
  • Because of density variations, cargo ships have a Plimsoll line (Shows how heavy a ship can be loaded in different water conditions)
  • Hot Air Balloons use buoyancy as well.


In the particle model, there are spaces between each state -- the amount of space depends on what state!


A  ball can compress…Why?

Air is inside the ball. It has lots of space between the particles, therefore can be compressed the most! As a result, when the ball hits the ground, the ball’s shape deforms.

Liquids and solids have very little space between particles and are considered incompressible.

Compressibility :

The extent to which a substance (solid, liquid, or gas) can be compressed; objects under compression tend to deform in shape (ex. Soccer ball)

  • More compression takes place in gas than liquid because there is more space between the gas molecules than the liquid molecules. (fig. 3.17)

Incompressible :

Not capable of being compressed; ex. Liquids).

Pressure in Fluids- Pascal’s Law


Blaise Pascal, a mathematician, l was curious how force is applied to a fluid

o   Discovered a relationship between water pressure and depth



Pressure :

Measure of the amount of force applied to a given area; measured in Pascals (Pa).

Pressure = Force/ Area or P= F/A

Units:  1N/m²    =  1 Pa

1000 Pa=  1 kPa

1 Pa is very small. It’s the weight of your loose leaf paper on your desk!

The greater the depth, the greater the pressure because the weight at the top puts more pressure on the bottom.

Pascal’s Law :

When pressure is applied to a liquid in a container, the pressure and force is transmitted equally and undiminished throughout the liquid; enclosed liquid transmits pressure equally in all directions.


Barometer : 

An instrument which measures the pressure of air in the atmosphere

To Summarize Pascal’s work:

·      Pressure is a force pushing on a surface (can be solid, liquid or gas)

·      The greater the depth of a fluid, the greater the pressure

·      Force is spread out equally in all directions in a closed system

Systems that use Pressure


Hydraulic System :

A system that uses a liquid (usually water or oil) under pressure to move loads; device that use liquids in a confined space to transfer forces; works according to Pascal’s Law (Fig 3.20).

A) The small piston is the input disk

B) The big piston is the output disk (to lift something)

C) Which piston has more force if the same pressure is applied?? (p=f/a)


Pneumatic System :

A system that uses gas under pressure to move loads; device that uses gases in a confined space to transfer forces; works according to Pascal’s Law.

Technologies Based on Solubility

Detergent is a substance that can remove dirt from a fabric with the help of surfactants which are particles that attach themselves to the dirt, separating them from the fabric (Pg 63, Fig 4.1)      


Nitty Gritty about Dirt

  1. Water has a funny shape to it
  2. As a result it has a attracted end, and a not-attracted end, like a magnet! 
  3. The attracted end attracts detergent, the other attracts other particles of water.
  4. Results in dirt being pulled off your clothes and washed away!



Diving and Decompression

SCUBA – Self Contained Underwater Breathing Apparatus

  • At greater pressure, nitrogen gas dissolves in our blood at higher concentrations.
  • If a diver ascends too quickly and the pressure decreases rapidly, the dissolved nitrogen bubbles out fast and collects in other body parts causing great pain. This is called “the bends.”
  • This is why a diver needs to come up to the surface slowly; for the extra gas leaving the body slowly as water pressure decreases

Technologies Based on Flow Rates and Moving Fluids

Pump :

Device that moves a fluid through or into something

3 common kinds of pumps:

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  • piston pumps (bicycle)

  • diaphragm pumps (aquarium air pump)

  • Archimedes screws (irrigation, not really used now)

The heart is a natural pump!


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Valves :

A device that controls the flow of fluids.

Types of valves:

–      One way (ex. Like in your heart and veins)

–      Ball valves

–      Stop valves

–      Butterfly

Designing a Working Model of a Fluid-Using Device

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How a Submarine Works (Pg 71, Fig 4.14)

  • To float, the submarine fills the ballast tanks with air which decreases the overall density on the submarine to less than that of water.
  • To dive, air is released from the ballast and they are then filled with water; the density of the submarine is now greater than that of the water.
  • To resurface, compressed air is forced into the ballast tanks and that pushes the water out, and again decreases the density so the submarine can surface.