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Density

Density. Why Do Some Things Float While Others Sink?. 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. Mass vs. Volume Graph.

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Density

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  1. Density Why Do Some Things Float While Others Sink?

  2. 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/cm3 , grams/mL or kg/L

  3. 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 .

  4. 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: • Water 1.0 g/mL • Cork 0.24 g/mL • Lead 11.35 g/mL • Gasoline 0.67 g/mL

  5. Density of Water = 1 g/mlMEMORIZE 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!!!!

  6. 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.

  7. 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 = πr2h

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

  9. 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 cm3 • D = 615.0g/101.5cm3 = 6.059 g/cm3 • **round at the end based on sig. Figs • D = 6.1 g/cm3

  10. 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

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

  12. Solving using Dimensional Analysis We use density as a conversion factor. 0.35 cm3 3.26 g = 1.1 g 1 cm3

  13. Example #4 • Mercury has a density of 13.6 g/cm3. What volume of mercury, in Liters, does a 76.2 gram sample of mercury occupy? • Formula D = m/v • 13.6 g/cm3 = 76.2g • V • Cross multiply (13.6) V = 76.2 • V = 76.2 = 5.60 cm3 • 13.6 • 5.6 cm3 1mL .001 L = 5.60 x 10-3 L • 1cm3 1mL

  14. Using Dimensional Analysis • Again using Density as a conversion factor. • We can solve this in one set up. • 76.2 g 1cm3 1mL 0.001 L • 13.6g 1cm3 1 mL • = 5.60 x 10-3 Liters

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