Density Labs
Monday, September 15, 2014
8:31 PM
Slides
Notes
Now lets get in to some labs that really let you dig in
to the concept of density. We will do a series of three
groups of labs.
The first set will explore how you can control if
something sinks or floats.
09 pick up here
The first thing that you must realize is that density is
relative. The object's density compared to the liquid's
density is what matters here.
Water has been set as the standard. When you
compute water's density it conveniently comes out to
be a 1.
Let's take three small balls all with the same volume
but with different masses. That will make them all
have different densities.
The first one we will explore is the one with the lowest
density. If the grams is smaller than the volume, your
object will be less dense than the density of water and
it will float in water.
If the grams are the same as the volume, your object
will be neutrally buoyant in water and it will hover
about in the middle of the container of water.
If the grams are bigger than the volume, your object
will sink because it is more dense than water.
Oil, which has a density of 0.197, is much less dense
than water. That is why you see it float on the water
Italian Salad dressing. All the balls would sink in oil
because all of them would have a density larger than
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because all of them would have a density larger than
0.197.
When you think of something heavy, you might think
of a bowling ball. Will it float or will it sink?
Well, it will depend on the mass to volume ratio, or
density, compared to water. Bowling balls can range
from 6 lbs. up to 16 lbs. and be the same
circumference. If you have a bowling ball at home,
make a hypothesis as to whether it will sink or float
and test it to see if you are correct.
This set of labs was to be focused on controlling
sinking or floating, so we better get to the way you
can control one variable at a time and get that
controlling started.
You have two variables for your object: mass and
volume. However, you also have control over the
liquid too. In this lab, you are going to control sinking
and floating by controlling the density of the liquid.
You can do this by adding salt to the water to increase
its density.
Notice that the computed density of a fresh egg is
very similar to water's density.
If you add salt you can begin to make the density of
the water higher than the density of the egg. This will
make the egg first begin to stand off the bottom of the
beaker and if you add enough salt you can get it to
being neutral in density to the salt water which will
make it hover in the middle. Add more and the egg
will then float. Give this lab a try at home.
In this lab, you will be controlling the mass of the
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In this lab, you will be controlling the mass of the
object which will give you a neat toy called a Cartesian
Diver.
What you do first is set a beaker and a sports drink or
two-liter bottle completely to the top with water and
let it sit for several hour so that it reaches room
temperature.
Next, calibrate your pipette in the beaker of room
temperature water by filling it with water until it
hovers in the middle of the water in the beaker.
Without letting the water drip out, transfer the
pipette in to the drink bottle. It should move to the
middle of the column of water if you calibrated it well
and didn't lose water from the pipette. Add water
from the beaker if needed so that the bottle is
completely full, even overflowing a little. Screw the
cap on tightly.
Now, squeeze the bottle. The pipette will act like a
diver and go up and down as you squeeze it.
Why?
Well, I told you that you were going to change one
variable: mass. Look closely at the diver when you
squeeze the bottle, Can you see the water level rise in
the pipette? The volume cannot change, but the
added water in the pipette makes the mass greater.
This makes it sink. Release the pressure and the extra
water goes back out to the starting amount and it
moves back up to its hovering position.
In this one, the volume changes so that floating and
sinking is controlled. It is a lava lamp.
When the solid is at the bottom near the heater, it
begins to soften and the volume expands. Since its
density is very close to the density of the liquid, it
doesn't take long before the density is lighter than the
liquid and it begins to rise. At the top it cools and the
density becomes greater than the liquid and it begins
to sink. This process can repeat over and over making
a relaxing display of motion.
While we are on floating and sinking, I want you to
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While we are on floating and sinking, I want you to
think about the phenomenon of ice in water. For most
substances, when they become a solid they shrink.
Not water though. Water is the most dense at about 4
degrees Celsius. The molecules pack up tightly. When
they get that close though, the positively and
negatively charged sides of the molecule begins to line
up somewhat like you see when you have magnets
with north and south pole line up. This makes the
molecules create a crystal lattice with space between
the molecules. The ice expands which makes the
volume larger yet the mass stays the same. That drops
the density and the ice floats.
Now, lets look at some of the fun we can have with
layering liquids because of density.
Fill an aquarium tube about half full with water. Add
some food coloring to make it easy to see if you like.
Now change the height of the ends of the tube. Do not
close the ends with your thumbs. What happens to
the water as you make one side higher and then the
other? Do they stay the same?
Now add oil to one open end. Repeat the height
changes that you did a few moments ago. Does the
water and oil act the same as the water alone as far as
the height of the liquid?
Why?
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This one is a pretty and fun whay to play around with
density and liquids.
You will start out by creating some liquids with
different densities and adding food coloring of
different colors to each one. You can see the liquids
we selected and they worked great.
Stick a clear straw in to an apple half. Warning, don't
use plastilin class as a base if one of your liquids is
alcohol. The alcohol will start to melt your Plastilin
clay.
Now, fill up a clear straw with the liquid adding in the
densest to the least dense one by one gently. A small
funnel or pipette is a great tool for this.
Enjoy your rainbow straw by displaying it.
You have probably heard that water and oil don't mix
and you see it in Italian salad dressing. You have to
shake the bottle to get them to mix and it doesn't take
long sitting still and they are separated again.
You can make them mix and stay mixed though. Just
add an emulsifier. Egg white is a great one. If you look
at the ingredients list for real mayonnaise, you will
discover is is oil, water, and egg white.
Soap works like an emulsifier too and is why you can
wash something oily and get it clean again. Try that
without soap in just water. It doesn't work so well.
Next up: What is This?
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Next up: What is This?
You can learn a lot about a substance by finding ways
to make the density more obvious.
For instance, we took several cans of different cans of
soda and placed them in the water. They each would
float at different levels. Here is the data arranged by
cans that floated the highest to the ones than sunk
the deepest.
Can you hypothesize why some would float and some
would sink based on density just by what you see in
the labelling of the sodas?
When plastic gets recycled, density can be used to
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When plastic gets recycled, density can be used to
identify the type of plastic it is based on density.
We created a column of liquid with three different
densities as shown. We then could add in the various
types of plastic chips and get a pretty good estimate
of what the density was. Then we could compare it to
a chart of densities for the common types of plastic.
We were able to identify nearly all the chips we had.
So far, we have focused a lot of attention on liquids.
What about gasses and solids? Can you control sinking
and floating with those?
Well, gasses sink and float all the time and give us
some wild weather. Wen cold fronts of air which are
dense collide with warn fronts, the warm front air
which carries a lot of water vapor ends up going up
and over the denser cold air. The air is colder the
higher that air goes and that makes the water vapor
start to condense into droplets. We end up with rain,
snow, sleet, or ice. Often, all the motion can cause
thunderstorms too.
Okay, well then what about solids. I used density and
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Okay, well then what about solids. I used density and
solids to play a little trick on my boys. I told them I had
a jar of magic M&M's that when you shae it a surprise
would appear.
I had hidden a plastic egg in the M&Ms. It was less
dense than the M&Ms so it gradually moved upward
as the jar was shaken.
Let's close our study with a super stumper.
Did Indiana Jones fail at density?
[play video]
In the escape, Indy tosses the statue about 6 feet to
an unscrupulous man who leaves him stranded and
takes the statue for himself.
Also, the sand in the bad is supposed to be equal in
weight to the gold statue to prevent triggering the
trap.
My question:
Does that look like the right amount of sand and could
he have really tossed the statue?
video
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Work out how much sand in volume Indy really would
have had to bring in there with him to replace the
statue with. To make the answer something you can
connect with, convert the volume in to how many 2liter soda bottles of sand it is.
Did he put enough sand in the bag and could he have
tossed the statue that easily?
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