Density

• Is a physical property of matter• Every pure substance has its own

density• Is important for the identification of a

sample of matter• Is an intensive property of matter

Density

• Density can be described as the concentration of mass in an area of space OR

is described as the mass of a given substance per unit volume

• The more concentrated the mass is in an object, the greater is its density.

Density• Since Density is defined as mass per unit

volume.

• The relationship can be written mathematically as:

densit y = mass

volumeOR

d = mv

units: solids [ g / cm3 ]

liquids[ g / mL] gases[ g / L]

Comparison of Densities

Each cube represents a volume of 1.00 cm3. The mass of each sample increases while the volume remains constant.

Determining the Volume

• In order to figure the density we need to determine the volume:

• For regularly shaped objects we use direct measurement:

• Block I

• Mass = 79.4 grams

• Volume=29.8 cubic cm.

• D = 79.4 g ÷ 29.8 cm3 = 2.66443

= 2.66 g/ cm3

• Block II

• Mass= 25.4 grams

• Volume=29.8 cubic cm

• D = 25.4 g ÷ 29.8 cm3 = .85235

= .852 g/ cm3

Which block is more dense?

• Cylinder 1 • Radius = 3.5cm• Length = 7.25 cm• Mass = 32.3 g• What is the volume?

V = π X radius2 X heightπ = 3.14

What is the density?

Sphere• Diameter = 7.0cm• Radius = 3.5cm• Mass = 32.3 g• What is the volume?

V = 4/3 π X radius3

π = 3.14

What is the density?

Determining Volume for an Irregular object

• Done by displacement

• We did this in the Scientific Measurement lab earlier in the quarter.

Question for you- Why does ice float?

• Solid H2O (ice) is less dense than liquid H2O (water).

• Rivers never become completely frozen because as soon as water freezes, it floats up to the surface.

• Fish live in the unfrozen, denser liquid water below the layer of ice.

• Example: Calculate the volume of a 100. g ice cube with a density of 0.92 g /mL.

• Solution: The density translated as a conversion factor is:

0.92 g = 1 mL

 100. g x 1.0 mL = 108.7 mL = 110 mL

0.92 g

 •  

• Comparison of:

Liquid water  Ice

Mass = 100 g   Mass = 100 g

Volume = 100 mL  Volume = 110 mL

Density = 1.0 g/mL Density = 0.92 g/mL

Density

• Can also be used as a means of separating components of matter

• Ex: if 2 solids of a different density are mixed together, the solid with the greater density will gravitate to the bottom

• When a solid object is placed in a liquid that has a great density, the solid object will float.

Density Gradient

THE DENSITY CONCEPT

• The picture illustrates a density gradient.

• Each item (solid or liquid) is arranged in the cylinder according to its density.

• The samples are arranged with the greater densities toward the bottom and the lowest density on the top.

The Concept of Density for Liquids and Solids

Which kind of matter is more dense?

Density - Review

• The concentration of mass in an area of space

OR• The mass of a given

substance per unit volume

• The more concentrated the mass is in an object, the greater is its density.

densit y = mass

volume