1. Density describes the amount of mass in a given
Science 8 Unit 3 - Fluids
Chapter 8
Density describes the amount of mass in a given
volume of a substance.
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2. Defining Density
Density is a measure of the mass contained in a given volume.
Substances with a lower density will float on substances with a higher density.
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3. Density and the Particle Theory
According to the particle theory, different substances have different-sized particles.
The size, shape, and mass
of the particles determine
how many particles and how
much mass can “fit into” a
given space.
Therefore, each substance has its own unique density, based on particle size, shape, and mass.
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4. Density and the Particle Theory
Generally the greater the mass compared to volume, the more tightly packed the particles are and hence the greater the density.
Which of the following would have the higher density?
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5. Density and the Particle Theory
The particle theory states that there are spaces among the particles of matter.
Q. In which state of matter are the spaces among particles the least? Greatest?
Generally the space between particles is greatest in gases and least in solids.
Q. What relationship can be made between space between particles and density?
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6. Density and the Particle Theory
Will the rock pictured fall more quickly through water or air? Explain based upon the density of the particles.
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7. Density and the Particle Theory
In general, gases are less dense than liquids and liquids are less dense than solids.
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8. The Relationship Between Mass, Volume and Density
Generally, matter with the highest density has the greatest mass making it heavier.
Which of the liquids in the diagram has the greatest density?
Which has the Least?
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9. The Relationship Between Mass, Volume and Density
Which is heavier?
Which is heavier a pound of feathers or a pound of lead?
The answer is neither, because they both weigh a pound.
This riddle reveals a confusion that some people have regarding weight versus density.
Density involves weight but that alone is not enough.
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10. The Relationship Between Mass, Volume and Density
Which is heavier if the volume is constant?
Which is heavier a cubic centimeter of feathers or a cubic centimeter of lead?
When we specify a certain volume, we are less fooled. We realize that for the same volume, lead will be a lot heavier. In other words, lead is more dense than feathers.
Density is a measure of the mass AND the volume that mass occupies.
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11. The Relationship Between Mass, Volume and Density
Objects may have the same mass but not have the same density.
You can look at these and tell that the smaller red box has a higher density than the grey box. It has the same mass but packed into a smaller volume.
The Red Box is More Dense
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12. The Relationship Between Mass, Volume and Density
Remember
At constant temperature and pressure matter has a constant mass to volume ratio
When temperature and pressure remain the same, as the volume of a type of matter increases so to does its mass by the same factor
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13. The Relationship Between Mass, Volume and Density
Therefore the ratio of mass to volume is a constant value. This constant value is called density.
Where
D = Density (g/cm3)
M = Mass (g)
V = Volume (cm3)
Note for water: 1 cm3 = 1 ml = 1 g
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14. The Relationship Between Mass, Volume and Density
Determining Mass and Volume
Mass can be determined using a balance scale,
Volume can be determined geometrically or for irregularly shaped ojects using Displacement. Displacement is the amount of space that an object takes up when placed in a fluid.
In the diagram, the volume of water displaced equals to the volume of the rock, that is 2cm3.    
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15. The Relationship Between Mass, Volume and Density
Sample Problem 1
A student determines that a piece of an unknown material has a mass of 5.8 g and a volume of 7.5 cm3.  What is the density of the material?
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16. The Relationship Between Mass, Volume and Density
Sample Problem 2
Iron has a known density of 7.87 g/cm3.  What would be the mass of a 250 cm3 piece of iron?
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17. The Relationship Between Mass, Volume and Density
Sample Problem 3
Mercury has a density of 13.5 g/cm3.  How much space (Volume) would 50.0 g of mercury occupy?
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18. The Relationship Between Mass, Volume and Density
Practice: Complete density calculations for the following table.
Complete Practice problems on page 312, 313 and 314 of your text.
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19. Changes in Density
The particle theory states that the particles of a substance spread out as they gain energy when heated.
Since they spread out, the particles take up more space as they are heated, which means that the density of a substance decreases.
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20. The Less Dense heated air rises above the cooler air outside.
Changes in Density
This Hot Air Balloon is able to float due to the differences in density between the heated air inside the balloon and the unheated air outside.
The Less Dense heated air rises above the cooler air outside.
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21. Changes in Density Tire Pressure is affected by temperature.
The pictures on the right represent the same tire at different times of the year. Although both tires contain the same mass of air the density of the winter tire is much less. Can you explain why?
Winter
Summer
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22. Changes in Density
Although most substances are denser in their solid form,
water is an exception to this rule. When water freezes, the
particles move slightly farther apart as they become fixed in
position. This means that ice is actually less dense than liquid water, so it floats.
Water is most dense at 4 oC. Consequently the water at the bottom of lakes and oceans is usually around this temperature especially during winter months
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23. Natural Changes in Density
Have you ever noticed that wood is much lighter after it has dried than when it was first cut…
Once a tree has been cut and is left to dry, the water particles in the wood evaporate, and are replaced with air. Because air is much less dense than water, the dry wood is less dense than the moist wood
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24. Natural Changes in Density
Recall our work from the first unit..
Explain using the concept of density why it is easier to float in salt water than fresh water.
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