density is a physical property of matter every pure substance has its own density is important for...
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Density
• Is a physical property of matter• Every pure substance has its own
density• Is important for the identification of a
sample of matter• Is an intensive property of matter
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Density
• Density can be described as the concentration of mass in an area of space OR
is described as the mass of a given substance per unit volume
• The more concentrated the mass is in an object, the greater is its density.
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Density• Since Density is defined as mass per unit
volume.
• The relationship can be written mathematically as:
densit y = mass
volumeOR
d = mv
units: solids [ g / cm3 ]
liquids[ g / mL] gases[ g / L]
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Comparison of Densities
Each cube represents a volume of 1.00 cm3. The mass of each sample increases while the volume remains constant.
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Determining the Volume
• In order to figure the density we need to determine the volume:
• For regularly shaped objects we use direct measurement:
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• Block I
• Mass = 79.4 grams
• Volume=29.8 cubic cm.
• D = 79.4 g ÷ 29.8 cm3 = 2.66443
= 2.66 g/ cm3
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• Block II
• Mass= 25.4 grams
• Volume=29.8 cubic cm
• D = 25.4 g ÷ 29.8 cm3 = .85235
= .852 g/ cm3
Which block is more dense?
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• Cylinder 1 • Radius = 3.5cm• Length = 7.25 cm• Mass = 32.3 g• What is the volume?
V = π X radius2 X heightπ = 3.14
What is the density?
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Sphere• Diameter = 7.0cm• Radius = 3.5cm• Mass = 32.3 g• What is the volume?
V = 4/3 π X radius3
π = 3.14
What is the density?
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Determining Volume for an Irregular object
• Done by displacement
• We did this in the Scientific Measurement lab earlier in the quarter.
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Question for you- Why does ice float?
• Solid H2O (ice) is less dense than liquid H2O (water).
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• Rivers never become completely frozen because as soon as water freezes, it floats up to the surface.
• Fish live in the unfrozen, denser liquid water below the layer of ice.
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• Example: Calculate the volume of a 100. g ice cube with a density of 0.92 g /mL.
• Solution: The density translated as a conversion factor is:
0.92 g = 1 mL
100. g x 1.0 mL = 108.7 mL = 110 mL
0.92 g
•
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• Comparison of:
Liquid water Ice
Mass = 100 g Mass = 100 g
Volume = 100 mL Volume = 110 mL
Density = 1.0 g/mL Density = 0.92 g/mL
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Density
• Can also be used as a means of separating components of matter
• Ex: if 2 solids of a different density are mixed together, the solid with the greater density will gravitate to the bottom
• When a solid object is placed in a liquid that has a great density, the solid object will float.
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Density Gradient
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THE DENSITY CONCEPT
• The picture illustrates a density gradient.
• Each item (solid or liquid) is arranged in the cylinder according to its density.
• The samples are arranged with the greater densities toward the bottom and the lowest density on the top.
The Concept of Density for Liquids and Solids
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Which kind of matter is more dense?
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Density - Review
• The concentration of mass in an area of space
OR• The mass of a given
substance per unit volume
• The more concentrated the mass is in an object, the greater is its density.
densit y = mass
volume