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Published byAriel Rice Modified over 6 years ago

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Density Why Do Some Things Float While Others Sink?

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What is Density? Density is how heavy something is for its size Density is mass divided by volume or Density = Mass = m Volume V Units:grams/cm 3, grams/mL or kg/L

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Mass vs. Volume Graph Mass and volume have a direct relationship This means it will produce a straight line graph with a positive slope As mass increases, so does the volume. The Slope of the line is the density and it remains constant.

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Density is a Physical Property It can be used to identify unknown pure substances. It also allows us to convert from mass to volume or vice versa. It is a conversion factor!!! Densities of some materials: Water1.0 g/mL Cork0.24 g/mL Lead11.35 g/mL Gasoline0.67 g/mL

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Density of Water = 1 g/ml MEMORIZE THIS!!! More dense materials sink in water Less dense materials will float or rise in water. Liquids form layers if they have different densities. Like oil and water. – Water is more dense than oil, so it settles on the bottom, and oil on top. Density varies with temperature – Usually density decreases with an increase in temperature. – Ice is the exception!!!!

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Density of Solutions The density of water will increase when it has something dissolved in it. The concentration of the solution will determine the density. The unknown concentration of a solution can be predicted by its density.

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Measuring density in the lab Take the mass on the balance Measure volume (2 ways to do this) – Regular shaped objects (cubes, cylinders) – measure the dimensions and use volume formulas Cube Volume = (L)(W)(H) or Cylinder = πr 2 h

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Volume Displacement For irregular shaped objects The object will displace an equal volume of water to its volume.

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Example #1 A block with dimensions 2.9cm x 3.5cm x 10.0cm has a mass of 615.0grams. What is it’s density? Calculate volume of block using dimensions. Make sure all given dimensions have the same unit!!!! V = (2.9cm)x(3.5cm)x(10.0cm) = 101.5 cm 3 D = 615.0g/101.5cm 3 = 6.059 g/cm 3 **round at the end based on sig. Figs D = 6.1 g/cm 3

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Example #2 A metal rock has a mass of 33.42grams. It is placed into a graduated cylinder filled with water and the water level raise from 12.7mls to 21.6mls. What is the density of the rock? Water Displacement gives us volume. 21.6mls – 12.7mls = 8.9mls D = 33.42g/8.9mls = 3.8 g/ml

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Example #3 Diamond has a density of 3.26 g/cm3. What mass of diamond has a volume of 0.35 cm 3 ? Two ways to solve. Using formula D = m/v 3.26 g/cm 3 = mass 0.35 cm 3 Mass = (3.26g/cm 3 )(0.35cm 3 ) = 1.141 g Round to proper sig figs Mass = 1.1 g

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Solving using Dimensional Analysis We use density as a conversion factor. 0.35 cm 3 3.26 g = 1.1 g 1 cm 3

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Example #4 Mercury has a density of 13.6 g/cm 3. What volume of mercury, in Liters, does a 76.2 gram sample of mercury occupy? Formula D = m/v 13.6 g/cm 3 = 76.2g V Cross multiply (13.6) V = 76.2 V = 76.2 = 5.60 cm 3 13.6 5.6 cm 3 1mL.001 L = 5.60 x 10 -3 L 1cm 3 1mL

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Using Dimensional Analysis Again using Density as a conversion factor. We can solve this in one set up. 76.2 g 1cm 3 1mL 0.001 L 13.6g 1cm 3 1 mL = 5.60 x 10 -3 Liters

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