 # Density Why Do Some Things Float While Others Sink?

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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 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.

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

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!!!!

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.

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

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

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

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

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

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

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

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