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?