Density

The concentrationof matter in an object is known as density.

Computing Density

Density = mass (g) volume (cm3)

DETERMINE VOLUME:One way to determine volume is to use a

graduated cylinder to determine the displacement of water achieved by placing the object in a predetermined volume of water.

DETERMINE MASS:Use a triple beam balance to

determine the mass of the object.

RT = Pg. 1

Computing Density

DETERMINE VOLUME:Another way to determine

volume is to measure the height, length and width of a solid that is cubic or rectangular in design. The formula shown will determine the volume of the object.

Sample Density problem #1

A student determines the mass of a rock sample to be 156.3 g. The volume of the same rock sample is 51.3 cm3. What is the density of the rock sample?

Density = mass (g) volume (cm3)

Density = 156.3 g 51.3 cm3

Density = 3.0 g/cm3

Sample Density problem #2

Given the information provided, determine the density of the toy?

Mass of Toy Dinosaur = 10 g

Volume of Toy Dinosaur= ?

Density = mass (g) volume (cm3)

Density = 10 g = 12.5 g/cm3

0.8 cm3

Determining Relative Density

The densities of objects in gases and fluids can be determined by observing the “flotation” of the object in the fluid or gas

Density of helium gas = 0.2 g/cm3

Density of air= 1.2 g/cm3

Objects with lower densities than the fluid or gas they are in will float above objects with higher densities.

Liquid water has a density of 1.0 g/mL. Determine the relative densities of the objects in the glasses of water.

Determining Relative Density

The object has a relative

density less than water because it

floats.

The object has a relative

density more than water because it

sinks.

Physical Changes That Affect Density

If temperature of a gasincreases while pressure is constant…

-- molecules move faster-- molecules will move farther apart

(expansion)-- less mass per unit volume

DENSITY WILL DECREASE

Changes in temperature and pressure can change the densities of substances …. especially gases.

Example: Air that is heated is less dense

than air that is cooled.

Therefore, heated air will

rise above cooled air.

Physical Changes That Affect DensityChanges in temperature and pressure can change the densities of substances ….

especially gases.

If temperature of a gasdecreases while pressure is

constant…-- molecules move slower

-- molecules will move closer together (compression)

-- larger mass per unit volume

DENSITY WILL INCREASE

Example: Air that is cooled is more dense than air that is

heated. Therefore,

cooled air will sink below heated air.

Physical Changes That Affect DensityChanges in temperature and pressure can change the densities of substances ….

especially gases.

If the pressure on a gasincreases while the temperature

is constant…

-- molecules will move closer together (compression)

-- larger mass per unit volume

DENSITY WILL INCREASE

PressurePressure

Phases of Matter and DensityMatter on Earth exists in

three phases: solid, liquid, gas.

Most substances will increases in density as the substance changes phase from gas to liquid and from liquid to solid.

Water’s highest density exists in its liquid phase (3.98 C).

Liquid Iron = 7.2 g/cm3

Solid Iron = 7.8 g/cm3

Ice = 0.97 g/cm3Liquid Water

= 1.0 g/cm3

Density vs. Object Size and Shape

The density of an object will NOT change should an object change its size and/or shape.

Quartz crystal:Mass = 10.9 g

Quartz globe:Mass = 88..9 g

Density of quartz

2.6 g/cm3