DENSITY
Density is a ratio of mass to volume.
Mathematically speaking, D=m/v
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TERM
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DEFINITION
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measured in the unit...
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mass
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amount of matter in a sample
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gram
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volume
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space occupied by a sample
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mL or cm3
(these equal each other)
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density
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ratio of a sample's mass to its volume
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g/mL or g/cm3 (these
equal each other)
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How should you understand density?
Since students are always familiar with speed, I tell
them that density is similar to speed. Speed is also a ratio. It
is a ratio of distance to time.
Here are densities for 3 substances:
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SUBSTANCE
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DENSITY
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gold
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19.3g/mL
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water
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1g/mL
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styrofoam
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.1g/mL
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The density number means this: one milliliter of
pure gold has mass = 19.3g
If you throw gold into water, since 19.3>1, it sinks.
Likewise, styrofoam floats because .1<1.
You can use the idea of density when you swim.
If you inhale and hold a bunch of air, it is easier to float because air
has a very low density.
Maybe this will help... below are two pieces of bread.
They both have the same mass, but I squished one so its volume is
smaller.
What happened when I threw them into water?
the big one floated
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and the little one sank
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Fish use a similar tactic. They have an air bladder
inside, and when they compress it (volume goes down) they sink more easily.
When they expand it (volume goes up) they float more easily.
SOME MATH
Since D=m/v, then m=D*v, and v=D/m
You cam use these equations when you know two variables
and need to find the third.
For example, if D=.5g/mL and m=4g, then what is the volume
of this sample?
v = D/m = (.5g/mL)/(4g) = this
could be something like styrofoam)(notice the "g" cancels)