Questions on density
What is heavier, a kilogram of
bricks, or a kilogram of
feathers?
Answer: They weigh the same
This is true, but which one will
take up more space?
The kilogram of feathers. Why?
Questions on density
Then answer has to do with the
density of each substance
The kilogram of bricks will take
up less space than the feathers
because the bricks are denser
than the feathers
Density
What is the definition of
density in words?
The amount of matter present
in a given volume of a
substance
Density Formula?
d = mass (g) / volume (ml)
Density
If you know the mass and volume of a substance
you can calculate the density
ex: a pure gold ring weighs 4g and has a volume
of 2.07 x 10^-1 mL. What is the density of gold?
Answer: d = 19.32g/mL
Check page 44 in book to make sure this value is
consistent with the density of Au
Density
What if you know the density of a substance, but
it has an odd shape, can you find the volume?
Ex: An irregular piece of aluminum has a mass
of 20 kg. You can find the density of aluminum in
the book. Density of aluminum = 2.70 g/ cm^3
Find the volume of this piece of aluminum
Density
This is a good time to tell you that 1mL = 1cm^3
So what is the density of the aluminum in g/mL?
2kg x 1000g/1kg = 2000g/volume = 2.70g/mL
Solve for volume. 2000/2.70 = volume
Volume = 740.7mL = 740.7cm^3 = 7 x 10^2 mL
Density
What if you know the volume, but not the mass?
Not to worry, you can find the mass using
density and volume!
Ex: Object of pure lead has a has a volume of
300 L and lead has a density of 11.34 g/cm^3
Solve for mass
Density
300L x 1000mL/1L = 300,000mL = volume
300,000mL / 11.34g/mL = 26, 455g = 26.455kg =
2.6 x 10^1 kg = 3 x 10^1 kg
Nice Job!
This smart guy named
Archimedes
What if you know the mass, but don’t know
the density and the object is irregularly
shaped?
A long time ago (around 250 B.C.) a brilliant
man named Archimedes came up with a
solution to this problem
Archimedes Principles
A kind asked Archimedes to examine a
gold crown to see if it was pure gold.
Archimedes knew the density of pure
gold and the weight of the crown, but
not the volume.
So……..He was thinking about the
problem while in a bath when he came
up with the answer and screamed
“Eureka!”
Archimedes Principle
Archimedes knew that placing the
crown in water would displace a
certain volume of water = to the
volume of the crown.
He ran and told the king and was
rewarded for his brilliance!!!
Archimedes Principle
Mass of crown = 3500g
Volume of water before crown = 100L
Volume of water after crown was added = 100.18112L
Volume of crown = 100.18112L-100L = 0.18112L x 1000ml/1L
Volume = 181.12 mL
Density = 3500g / 181.12 mL = 19.32g/mL !!!Great Job!!!!
Density
This only works because
Au is denser than H2O
If you try this with an
object that is not as dense
as liquid water it will not
work
Ex: icebergs
Cool things about density
There is a lake at the bottom of the ocean.
How can this be true?
The water there is super saline, meaning
super salty. This makes it more dense
than the surround water so it sinks.