DensityDefinition - the relationship between mass and volumeReminder:Mass = the amount of matter inside of an objectVolume = the amount of space it occupies

Describes how tightly packed the molecules are (how close they are together)

VS.VS.

Not all solids will have the same density!

Because Lead has Because Lead has a lot of atoms a lot of atoms squeezed together, squeezed together, it has a:it has a:High densityHigh density

Because Wax has Because Wax has fewer molecules fewer molecules squeezed together, squeezed together, it has a:it has a:Low densityLow density

For a solids that are For a solids that are Pure Pure SubstancesSubstances or or Homogeneous Homogeneous MixturesMixtures, density will stay , density will stay the the samesame no matter how large or no matter how large or small the samplesmall the sample

For example, the density of a tree is the same as the density of a cut piece of the same tree

ExamplesExamples

The density of a copper penny

the density of a copper pot.is the is the

same same as as

The density of a steel paper clip

the density of a steel screw

For solids that are For solids that are Heterogeneous Mixtures, they Heterogeneous Mixtures, they will will notnot always have the same always have the same densitydensity

Not all pieces of the same chocolate chip cookie are going to have the same density

Are the densities of these pieces of cookie the same?

Which piece has the greatest density?

11

22

33

We can also examine how tightly packed the atoms are in Fluids.

Liquids and Gases have density based on their mass and volume.

The Mass The Mass willwill notnot change with a phase change with a phase changechange

A phase change from a Solid A phase change from a Solid → → Liquid Liquid oror Liquid →Gas will cause Liquid →Gas will cause

density to change. (the opposite density to change. (the opposite is true as wellis true as well))

HOWEVERHOWEVER……Volume Volume doesdoes changechange

This is why:o As As heatheat energy is added, the energy is added, the particles inside a substance particles inside a substance gain energy, spread out, and gain energy, spread out, and move around more. move around more.

o Liquids and Gases tend to Liquids and Gases tend to take up take up moremore spacespace, because , because they can slide around!they can slide around!

In general, an increase in volume In general, an increase in volume will result in a will result in a lowerlower densitydensity

ExceptionExceptionHH22OO is different!! In is different!! In this case the this case the solidsolid (Ice) (Ice) isis lessless densedense

than the than the liquidliquid (water)(water) because of because of hydrogenhydrogen bondingbonding..

H2O is unique because it expands when it freezes.

This expansion gives you a larger volume when liquid water changes to ice.

This is why ice floats!The same number of particles The same number of particles occupying a larger space is a occupying a larger space is a decreasedecrease inin densitydensity

SolidSolid

LiquiLiquidd

D = .9167 g/cmD = .9167 g/cm33

D = 1 g/cmD = 1 g/cm33

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Ice would sink to bottoms of lakes, rivers and oceans

It wouldn’t melt in the summer at the bottom.

What would happen if ice What would happen if ice was more dense than water?was more dense than water?