Density

What is Density?

Density is defined as mass per unit volume.

Density = MassVolume

Density describes how matter is packed into space.

Density is a derived unit. It is calculated from other

measurements.

Density is a physical physical constantconstant.

• A physical constant a physical physical propertyproperty that can be described with a number and a unit.

• A physical constant is the same for allall samples of a substance regardless of sample size or shape.

1 drop of pure water has the same density as 1 liter of pure water.

Density does not change with sample size.

Physical Constants

• Some other physical constants include melting point, boiling point, heat capacity, heat of vaporization, and heat of fusion.

• We use physical properties such as density to identifyidentify substances.

All samples of pure gold have the same density.

What are the units of density?

•The most common units of density are grams per cubic centimeter (g/cm3) or grams per milliliter (g/mL).

•By the way, 1 cm3 = 1 mL.

Density and Phase

• For mostmost substances, the solid is the densest phase and the gas is the least dense.

S S L L G G• Water is Weird!Water is Weird! Water is at its

densest at 4.0C, when it’s a liquid.

L L S S G G

• The density of water is 1.0 g/mL (At 4.0°C).

• The relative density of any substance can be determined by adding it to water.

• If the substance floats, it’s density is less than the density of water:d < 1.0 g/mL

• If the substance sinks, it’s density is greater than the density of water:d > 1.0 g/mL

1.0 g/mL1.0 g/mL

0.80 g/mL0.80 g/mL

1.2 g/mL1.2 g/mL

source

Density Column

Density of Gases

• Balloonx.mov• Balloony.mov

• Flow of CO2

What is the density of …?SubstancSubstanc

eeDensity Density (g/cm(g/cm33))

SubstanceSubstance DensityDensity (g/cm(g/cm33))

waterwater 1.001.00 seawaterseawater 1.0251.025

iceice .92.92 aluminumaluminum 2.702.70

gasolinegasoline .70.70 oiloil 0.900.90

leadlead 11.3011.30 goldgold 19.3019.30

white white pinepine

.50.50 rubberrubber 1.101.10

corkcork .24.24 bonebone 1.801.80

ironiron 7.907.90 human fathuman fat .92.92

mercurymercury 13.6013.60 heliumhelium .000178.000178

source

What is the density of some selected elements?

Can you explain this?

Picture 1 Insert V. 2, CCA

What variables affect density?

• Temperature, T• Pressure, P

Liquid or solidLiquid or solid: Even large changes in P & T small changes in density.

GasGas: Even small changes in P & T large change in density.

How does Temperature affect Density?

• Most materials expandexpand when they are heated.

• Mass stays the same (it’s constantconstant), but volume increases. So the matter isn’t packed as tightly.

• Density must decrease. (Hot air balloons!)

When the material expands, the density decreases.

Temperatue & Density

Source: http://www.cogifts.com/products/execgifts/galileo.jpg

As T , liquid becomes less dense. Spheres sink.As T , liquid becomes more dense. Spheres rise.

How does Pressure affect Density?

• If you the pressure on an object, the volume .

• If the volume , the matter must be packed more tightly & density must .

When you apply pressure to an object, the density increases!

Remember: Water is weird!

Ice Bomb Insert V. 2, CCA

Density ProblemsDensity Problems

• Algebra-type problems.• The density equation has 3

variables. • In any given problem, you are

given information about 2 of them and you solve for the 3rd.

• You might have to rearrange the rearrange the equationequation.

Problem-solving Rubric

• GGiven: Identify the given info.• UUnknown: Identify the unknown.• EEquation: Identify the correct

equation and rearrange it as necessary.

• SSubstitute: substitute numerical values in for the letters.

• SSolve: Do the math!

You can’t ignore reciprocals!

1 can’t magically become V.V

D =

V =

MV

MD

or M = D X V

Algebra Helper

• You might remember the density triangle from earth science:

DD

MM

VV

Density Problems

•An aluminum block 1.25 cm X 1.25 cm X 1.25 cm has a mass of 5.27 g. What is its density?

2.70 g/cm2.70 g/cm33

Find the volume!Find the volume!

Density Problems

• Mercury has a density of 13.60 g/ml. What is the mass of 25 ml of mercury?

340 g340 g

Density Problems

• Silver has a density of 10.5 g/cm3. What is the volume of 210 g?

20 cm20 cm33

Density Measurements

• Mass – Use a balance.

• Volume of regularly-shapedregularly-shaped objects:

• direct measurement of length, width, & height.

• Volume of irregularly-shapedirregularly-shaped objects: use water displacement.

Mass is measured on a balance.

Water displacement is used to find the volume of oddly shaped objects.

Direct measurement is used to find the volume of simple objects.