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?
The particle model of matter theory explains that particles have forces that attract one another, this force is called cohesive force. In gases this cohesive force is very weak, while in liquids this force is much stronger.
A stronger cohesive force in liquids indicates a more viscous liquid
Fluids with high viscosity do not flow as easily as fluids with low viscosity
WHY CARE ABOUT VISCOSITY?
Engineers must calculate how long it takes for a fluid to flow, or to fill a reservoir
Physicist must figure out how much energy is needed to pump fluids a processed fluid
Mechanics need to know how oil will flow in an engine to keep it lubricated
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!
FLOW RATE: The amount of a substance moving in a given amount of time Flow Rate (FR) = Volume (ml)/time (s)
Slower flow rate indicates the fluid has a higher viscosity
Faster flow rates indicate the fluid has a lower viscosity
A COUPLE QUESTIONS! What is the flow rate of a substance that takes 14 seconds for 500 mL to flow? What is the flow rate of a substance that takes half a minute for 0.5 L to flow?
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. 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 larger 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).
Recall that concentration is amount of solute in a given amount of solvent, even though it has the same units as density (g/ml) it is not the same as density.
Questions Substance A - Has a mass of 40 g and a volume of 100 ml, what is it’s density? Substance B - Has a mass of 250 g and a volume of 130 ml, what is it’s density? Substance C - Has a mass of 0.2 kg and a volume of 0.2 L, what is it’s density?
HYDROMETER:
An instrument that measures the density of a liquid
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
VISCOSITY CHANGES WITH TEMPERATURE
Temperature increases, viscosity decreases = fluid flows easier
Temperature decreases, viscosity increases = fluid flows slower Think - what flows more melted butter, or room temperature butter?
HOW TO CHANGE DENSITY
Two ways to change density:
Change the size of a substance’s volume (temperature does this!)
Change the amount of particles in that substance
CHANGING THE DENSITY BY ADDING ENERGY
Adding energy to particles makes particles move faster, which makes them move further apart
If particles move farther apart it will increase the volume, but since the number of particles stays the same, the mass stays the same!
This results in the density decreasing.
What would happen if you were to take away energy from particles?
CHANGING THE DENSITY BY CHANGING CONCENTRATION
Adding more solute to a solution would increase the number of particles which would then increase the mass.
Increasing the mass would increase the density
What would happen if you were to reduce the amount of solute in a solution?