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Density
Section 2.3
High population density
(large population per square mile)
Low population density
(small population per square mile)
Density in Science
• Definition: Mass per unit volume• Why do we need to know about density?– Density is a physical property of elements and
compounds.– Density is easy to measure using a balance and a
ruler (for large objects) and a graduated cylinder & water (for small objects)
– Unknown elements and compounds can be identified by finding their density!
Some common densities
Density and Buoyancy• Water has a density of 1.0 g/mL– Any substance with a density greater than
1.0 g/mL will sink in water (all metals, for example)– Any substance with a density less than
1.0 g/mL will float in water (ice, alcohol, all gases)• Mercury has a density of 13.6 g/mL– Any substance with a density greater than
13.6 g/mL will sink in mercury (don’t try this with gold!)
– Any substance with a density less than 13.6 g/mL will float in mercury, including lead!
Density Formulas
• Density = mass ÷ volume
D = m
V
D = m
VVV D
mV
1
D
1
D
=
D
Units of Density
D = m
V
gmL
Density has units of
Sample problem
• An unknown liquid filled up to the 50.1 mL mark on a graduated cylinder has a mass of 40.5 g. What is the density of the substance? What is the unknown liquid?
• D = m/V• D = 40.5 g / 50.1 mL• D = 0.80838 g / mL• D = 0.808 g/mL (only 3 significant digits allowed!)
Some common densities
More densities
• No density: vacuum (0 g/mL)• Water: 1.0 g/mL• Rocks: 2 to 7 g/mL• Copper: 8.92 g/mL• Highest density element: Osmium (22.5 g/mL)• Neutron degenerate matter: 1 x 1015 g/mL (only
found in neutron stars)• Black hole: near-infinite density (5 solar masses
compressed to a point in space)
Group work1. A 20.0 g sample of lead causes the water level in a
graduated cylinder to rise from 50.0 mL to 51.8 mL. Calculate the density of lead.
2. A platinum ring has a mass of 40.0 g and a density of 21.1 g/mL. If placed into a graduated cylinder filled to the 42.0 mL mark with water, to what level will the water rise?
3. A sample of aluminum has a volume of 2.0 L and a density of 2.7 g/mL. Calculate the mass of the sample.
4. An unknown material has a volume of 51,000 mL and a mass of 50,000 g. What is its density? Will it float in water?
5. A stack of 15 nickels has a volume of 4.0 mL and a density of 5.2 g/mL. What is the mass of each nickel?