In This Lesson:
Density
(Lesson 5 of 6)
Stuff You Need:
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Today is Tuesday,
February 16th, 2016
Pre-Class:
Choose one:
Why does wood float?
or
Which is heavier, a kilogram
of lead or a kilogram of
hydrogen gas?
http://fineartamerica.com/images-medium/the-schooner-margaret-todd-bar-harbor-maine-bob-orsillo.jpg
Today’s Agenda
• Density!
• Where is this in my book?
– P. 89 and following…
By the end of this lesson…
• You should be able to calculate density of a
given substance.
Density
• The density of an object (that’s one of its
intensive properties, a physical property) is
the relationship between its mass and
volume.
• Density = mass / volume
– Mass = volume * density
– Volume = mass / density
M
D V
Density
• To put it another way, if two objects take up
the same amount of space but have different
masses, one must be more dense than the
other.
• Alternatively, if two objects have the same
mass but different densities, their volumes
must be different.
– I think you get the idea…
Density Facts
• Density is always expressed in terms of
[mass]/[volume].
– For us, it will typically be g/cm3 or g/mL.
– Don’t forget the units!
• Water has a density of 1 g/cm3.
– Funny how water always has nice round numbers,
huh?
• 1 cm3 of water has a mass of 1 g.
Buoyancy
• To see if something is buoyant (can float), you need two
densities:
– The object’s.
– The fluid’s.
• Object density > fluid density? Object sinks.
• Object density < fluid density? Object floats.
• Object density = fluid density? Object “kinda” floats.
– For water, since its density is 1 g/mL, anything higher than 1
will sink; lower will float.
• Dense water? Try the Dead Sea (video).
Buoyancy
Less Dense Than Water
More Dense Than Water
The Costa Concordia, which wrecked off Italy’s coast in January 2012.
http://cdn.theatlantic.com/static/infocus/costa011612/c01_36991800.jpg
http://i.telegraph.co.uk/multimedia/archive/02109/CostaConcordia-WEB_2109037b.jpg
Density and Problem-Solving
• Here’s a challenge question for you:
– Imagine you’re a king (or queen), and you have a royal
crown-maker.
– Problem is, the dude looks really suspicious.
• Has funny eyebrows or something.
• Maybe he’s wearing a bow tie.
– So you suspect that maybe he’s not making your crowns
out of solid gold like you said. Instead, he might be cutting
the gold with something gold-like, but cheaper.
– How do you prove him innocent or guilty without
destroying one of your crowns?
– Discuss!
Density and Problem-Solving
• Turns out this was a real problem faced by this
guy:
• His name was Archimedes and
he solved this problem.
• What’d you come up with?
http://charlespaolino.files.wordpress.com/2010/07/archimedes_16201.jpg
How Archimedes Solved The Problem
• Archimedes noticed the property of displacement
(how much matter is moved) when he got into his
bathtub.
• He realized this is a way to
measure volume.
• He was so excited by his
discovery (early science dork)
that he supposedly jumped out
of his bathtub and ran naked
down the streets of Sicily
shouting “Eureka!” (“I have it!”)
Let’s try some examples ourselves:
• Grab your calculators, grab the whiteboards,
[don’t] grab your partners, and let’s try some
density problems right now.
http://www.algebralab.org/practice/practice.aspx?file=word_density.xml
Density Problem #1
• What is the density of a piece of wood that has a
mass of 25.0 grams and a volume of 29.4 cm3?
• 0.850 g/cm3
Density Problem #2
• A piece of wood that measures 3.0 cm by 6.0 cm
by 4.0 cm has a mass of 80.0 grams. What is the
density of the wood?
• 1.1 g/cm3
• Would it float in water?
– No! It’s more dense than water, so it would sink.
Density Problem #3
• A group of gold-colored beads was measured to
have a mass of 425 grams. By water
displacement, the volume of the beads was
calculated to be 48.0 cm3. Given the following
densities, identify the metal: (1 mL = 1 cm3)
A. Gold: 19.3 g/mL
B. Copper: 8.86 g/mL
C. Bronze: 9.87 g/mL
Density Problem #3
• A group of gold-colored beads was measured to
have a mass of 425 grams. By water
displacement, the volume of the beads was
calculated to be 48.0 cm3. Given the following
densities, identify the metal: (1 mL = 1 cm3)
A. Gold: 19.3 g/mL
B. Copper: 8.86 g/mL
C. Bronze: 9.87 g/mL
Density Problem #4
• If I throw a plastic ball into a pool for my dog to
retrieve, what volume must the ball be to float?
The mass is 125 grams and the density should be
0.500 g/mL.
• 250 mL
• [density = mass/volume]
Density Problem #5
• CHALLENGING: If the ball happens to spring a
leak, how many mL of water can the ball take in
before it sinks? [remember, ball is 125 g and we
designed it to be 250 mL]
– Greater than 125 mL
Density Problem #6
• CHALLENGING: What is the mass of a cylinder of
lead that is 2.50 cm in diameter and 5.50 cm
tall? The density of lead is 11.4 g/mL and the
volume of a cylinder is V=πr2h.
– Volume of the cylinder is 8.59π mL (≈27 mL)
– Mass of cylinder = 308 g
• [Density * Volume = Mass]
Density Problem #7
• The volume of a solution is shown by the
graduated cylinder below. If the mass is 60.75
grams, what is the density?
• 1.350 g/mL
http://www.algebralab.orit to be 250 mL].g/practice/practice.aspx?file=word_density.xml
Density Problem #8
• An ice cube measuring 5.80 cm by 5.80 cm by
5.80 cm has a density of 0.917 g/mL. What is
the mass?
• 179 grams
Density Problem #9
• Gasoline is a non-polar liquid that will float on
water. 450 grams of gasoline is spilled into a
puddle of water. If the density of gasoline is
0.665 g/mL, what volume of gasoline is
spilled?
• 677 mL
Density Problem #10
• The density of aluminum is 2.70 g/mL. If the
mass of a piece of aluminum is 244 grams,
what is the volume of the aluminum?
• 90.4 mL
DENSITY CHALLENGER
• Also known as “The Density Boss.”
• A little aluminum boat (mass of 14.50 g) has a
volume of 450.00 cm3. The boat is placed in a
small pool of water and carefully filled with
pennies. If each penny has a mass of 2.50 g,
how many pennies can be added to the boat
without it sinking?
• 174 pennies
Now it’s your turn to work alone…
• Try the Density Practice Problems worksheet.
So now then…
• Let’s put this to the test in a virtual lab called
Density Laboratory.
• It’s found on ExploreLearning.com.
• Log-in information will be on the next slide or
(recommended) posted on my teacher site on
the Chemistry Links.
• Complete the accompanying worksheet.
• [Log-in Instructions]
Density to Water
• You may have noticed that there is a relationship
between floating/sinking and the density of the
water.
• The specific gravity of a substance is the ratio of
that substance’s density to the density of water.
– Specific gravity greater than 1? It’s more dense than
water.
• Liquids are usually measured with a hydrometer.
Density Lab
• We’re going to finish our lessons on density
with a look into a less electronic exercise.
• The Density Lab!
Closure and Homework
• Tonight for homework, make sure your CocaCola lab is done. Make sure your density
virtual lab is done. Make sure your density
worksheets are done.
• Then read pages 89-92.
• Do questions #85-87 on page 97.