Download density lab pictures and explanation

SMS 491/EDW742 – DENSITY LAB
Spring 2007
Follow the instructions for each activity and ADDRESS ALL QUESTIONS IN
YOUR JOURNAL
Density (ρ) is defined as mass divided by volume ( ρ =
m[ M ]
). The SI unit of density
V [ L3 ]
is kg/m3 and the cgs unit is g/cm3.
Density is central to the understanding of many oceanic processes: from the formation of
ocean basins, through ocean circulation to the transport of carbon from ocean surface to
depth. As a property of matter, density is a central topic in secondary school physical
science. While most adults have heard about the concept of density both school-age
students and adults hold a number of misconceptions about density.
Examples of misconceptions include (the list was compiled by the Operation Physics
Elementary/middle school physics education outreach project of the American Institute of
Physics):
-
Large objects sink and small object floats
Objects float in water because they are lighter than water.
Objects sink in water because they are heavier than water.
Wood floats and metal sinks.
All objects containing air float.
Mass/volume/weight/heaviness/size/density may be perceived as equivalent. Note: while
weight is commonly used as an equivalent to mass, in physics it is used to describe a
particular force arising from the gravitational pull between objects (e.g. the Earth and the
mass).
Station 1: Will it float?
Materials:
One cube of balsa (from sciencekit.com)
One cube of lignum vitae wood (from sciencekit.com)
A large hollow metal ball (from sciencekit.com)
Small Delrin® (or other plastic) ball (any hardware store)
A container with water (use a big container for the large ball and a small one for the
wooden cubes and the small ball).
A ruler or caliper
Scale (not shown)
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Materials for Station One
-----------------------------------------------------------------------------------------------------------Experiment:
You have two cubes of wood of the same size and two spheres (a large metal one and a
small plastic one).
1. Predict which of the items will float in water and which one will sink and write
your prediction in your journal. What was the reasoning behind your prediction?
2. Discuss your prediction with your group (and reflect upon it in your journal).
3. Test your predictions with the wooden cubes. Does the observation support your
prediction? What are the similarities between the cubes? What are the differences
between the cubes?
4. Now test your prediction with respect to the balls. Does the observation support
your prediction? If not, how can you explain it.
5. Measure the densities of the cubes and the balls. Can that explain your
observations? (volume of a sphere: V= 4/3πr3)
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Michelle attempts to measure density by hand. Will it float or sink?
EXPLANATION:
This activity can be used to address some of the misconceptions stated above (e.g., large
object sinks, small object floats, wood floats, metal sinks).
There is a wide range in the densities of wood found throughout the world. The range of
density extends from balsa (0.1-0.17 g/cm3) to some tropical hardwoods that have
density that exceeds that of water (1.04-1.37 g/cm3) Lignum vitae has a density of 1.171.29 g/cm3.
The volume of the metal ball is 1023 cm3 and its mass is 144g. Its density is therefore
144/1023= 0.14 g/cm3.
The volume of the Delrin ball is 1.15 cm3 cm and its mass is 1.5 g. Its density is therefore
1.5/1.15= 1.3 g/cm3.
Given that the density of water is ~1 g/cm3 (depends on temperature, see reading
material) the balsa and large metal ball will float and the Lignum vitae cube and
Delrin ball will sink.
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Station 2: Density rods
Materials:
A set of density rods (from sciencekit.com)
A ruler or caliper
Calculator
Graphing paper (or graphing program e.g., Excel)
Materials for Station Two
----------------------------------------------------------------------------------------------------------Experiment:
1. You have a set of 12 black rods of different lengths but the same diameter. Do you
think all the rods are made out of the same material? Feel free to use any sensory
means to help you evaluate the rods. If you don’t think they are all made out of the
same material, can you arrange them in groups according to their material? What
were your category criteria for determining whether a rod belongs to one group or
another?
2. Obtain the mass and the volume of the 12 rods and plot their volume (X) vs. mass
(Y). Do all points fall on the same line? Do you see any pattern in the data?
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(Volume of a cylinder: V=π*r2*h, where r is the radius of the cylinder and h is the
height of the cylinder)
3. What does the slope(s) of the line(s) represent? What does it tell you about the
rods?
Students use sensory means to help evaluate differences among the density rods
EXPLANATION:
Table 1 summarizes the mass, volume and density for each of the rods.
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3
Volume (cm )
Length of rod (cm)
Table 1
Mass (g)
1.16
1.42
1.17
1.42
1.15
1.40
1.40
1.40
1.40
1.41
1.15
1.41
3.7
5.4
5.2
7.2
6.6
8.9
9.8
10.7
11.6
12.5
11
14.3
3.2
3.8
4.4
5.1
5.7
6.4
7.0
7.6
8.3
8.9
9.5
10.2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
Density (g/cm3)
16
y = 1.3983x + 0.0551
R2 = 0.9999
14
12
Mass (g)
10
y = 1.1467x + 0.0679
R2 = 0.9999
8
6
4
2
0
0
2
4
6
8
10
12
Volume (cm^3)
The data points are arranged along two linear lines, where the slope of each line provides
the density (1.4 and 1.15 g/cm3. Compare the slopes with the range of calculated densities
in the table. While the derived densities are not identical the small difference within each
group is not significant given the uncertainties in the measurements). The data suggest
that the rods are made of two different materials. (Caution: if two items have the same
density it does not necessarily mean that they are made of the exact same material,
however, density can be used as a method to distinguish between different materials).
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Station 3: Effects of temperature on density
Materials:
Two glass beakers: one with cold water (~6-15oC) and one with warm water ~40oC
A set of reverse density rods (From Arbor Scientific): 1 Aluminum rod, 1 plastic rod
Thermometer
Ice (+ ice cooler to store ice)
Hot plate (but hot tap water will work fine)
Materials for Station Three
-----------------------------------------------------------------------------------------------------------Experiment:
1. Which rod do you think will float/sink in the beaker with the cold water? What is
the reasoning for your prediction?
2. Place the rods in the beaker with cold water. Make sure that no air bubbles are stuck
to the rods. Record your initial observation in your journal. Does your observation
meet your prediction?
3. Observe the rods for a few minutes. What is happening here?
4. Repeat this experiment, this time using the beaker with the hot water.
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5. How would you explain the different behaviors of the rods in cold and warm water?
With your group discuss possible explanations for what you have observed and
record them in your journal.
EXPLANATION:
In this activity, the rods are used to demonstrate the effect of temperature on density of
liquids and solids and the activity can be used as a discrepant event.
One rod is made out of aluminum and the other is made out of PVC.
When you place the rods in cold water both rods initially float because their density is
lower than that of the cold water. Over time, as the PVC rod gets colder it contracts and
as a result its density changes (volume shrinks but its mass remains the same). When the
density of the rod exceeds that of the water the PVC rod begins to sink. Aluminum has a
much lower temperature expansion coefficient and hence its density is less affected by
temperature and it remains floating. When you place the rods in hot water, the density of
the water is now lower than that of the aluminum rod ands it will sink. The PVC rod is
initially denser than the water and sinks but as it warms up it begins to expand. As a
results its density changes (again, the mass remains constant) and when its density
becomes lower than that of the water in begins to float.
Station 4: Coca-Cola (Note: we used Coke but the success of the activity depends on
the temperature of the water in the container. We later found out that Mountain
Dew works better because the difference in mass between regular and diet mountain
Dew is larger compared to that of Coke and diet Coke)
Materials:
Cans of Coke and diet Coke
A fish tank or a large container with water.
Paper towels
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Materials for Station Four
-----------------------------------------------------------------------------------------------------------Experiment:
1. Examine the two cans. List as many similarities and differences between the 2 cans.
2. Now place the two cans in the tank. How would you explain this observation?
3. Wipe each can well and obtain the density of each cans (how can you determine
it?). Is the density of coke smaller/greater than that of tap water? Is the density of
diet coke smaller/greater than that of tap water?
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Morgan and Michelle discover the difference between adding sugar or NutraSweet to a
beverage
EXPLANATION:
Density of Coke- 1.01 g/cm3 (volume=376 ml (1 ml= 1 cm3), mass=380.8 g.)
Density of Diet Coke- 0.97 g/cm3 (volume=376 ml (1 ml= 1 cm3), mass=366.6 g.)
The two cans have the same volume but they differ in their sugar content and hence in
their mass. There are 39 grams of sugar in a 12 oz Coke can (see label) vs. approximately
100 mg of Nutra sweet in a 12 oz diet Coke can.
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Station 5: Density of rocks
(This lab is based on an activity designed by Donald F. Collins, Warren-Wilson College
http://www.warrenwilson.edu/~physics/EarthLightSky/Activities/Density/Density.html)
Materials:
Rock samples of Basalt (typical of oceanic crust) and Granite (typical of continental
crust)
A graduate cylinder or a graduated beaker
A container with a spout
Scale
Materials for Station Five
-----------------------------------------------------------------------------------------------------------Experiment:
1. Given what you know about density (ρ = M/V), obtain the densities of the rocks
you have (to your aid, you have a graduate cylinder, a scale and a container with a
spout). How do the granite and basalt compare to each other? How would you use it
to explain the topography and bathymetry of the earth?
2. Based on the information you have, can you predict the average density of earth?
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3. Given that earth’s mass is 5.9742 × 1024 kilograms (a brain teaser: how this was
determined?) and that earth’s radius is 6,378 km, calculate the average density of
earth. How does the average earth’s density compare to the density of the rocks?
What does it tells you about the structure of earth?
Students examine the volume of water displaced by a sample of granite
EXPLANATION:
Density of basalt sample- 2.8 g/cm3 (volume=45 ml (1 ml= 1 cm3), mass=125.5 g.)
Density of granite sample- 2.6 g/cm3 (volume=53 ml (1 ml= 1 cm3), mass=136.9 g.)
The Earth crust is made of two types of crusts: continental and oceanic. Continental crust
is composed mostly of granite while oceanic crust is mostly composed of basalt. Oceanic
crust is thinner and denser than continental crust (textbook values: 2.9-3.0 vs. 2.7-2.8
g/cm3 , respectively; remember that you have measured the density of only one sample of
each type of rock). Both overlie the earth’s mantle which is denser (3.3 g/cm3), with the
continental crust “floating “ higher. Understanding the differences between oceanic and
continental crust is crucial for understanding plate tectonics and the formation of ocean
basins. We will discuss this in more details on February 14.
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On average, earth’s density is 5.5 g/cm3. That implies that the earth must be composed of
additional layer/s of higher density then the rocks within its crust. In fact, the crust is the
least dense layer. The estimated density of the core mantle is 5.5 g/cm3, the estimated
density of the outer core is 10-12 g/cm3 and the estimated density of the core is 12-13
g/cm3.
The density of the Earth can be computed using Newton’s laws:
(1) Two bodies attract each other with a force (F) that is directly proportional to the
product of their masses (m1,m2) and inversely proportional to the square of the distance
between them (r)
F=Gm1m2/r2, where G is the gravitational constant. G=6.7x10-11N m2 kg-2
(2) The force attracting a body to earth if its weight (not to confuse with mass) - mg,
where g is the gravitational acceleration (g=980 cm/s2): F=mg
Thus, Gme/r2 = g
Æme =g*r2/G~6x1024Kg. Dividing by the volume of the Earth (4/3πR3) we obtain the
Earth’s density (5461Kg/m3= 5.5 g/cm3)
Station 6: Density of fluids
Materials:
A rectangular tank with a divider
A salt solution or salt
Food coloring
Ice
2 beakers
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Materials for Station Six
-----------------------------------------------------------------------------------------------------------Experiment:
1. In one beaker prepare a salt solution. Fill the other beaker with cold tap water and
add ice so the water remains cold.
2. Place tap water in one compartment of the tank and salt water in the other. Add
food coloring to one compartment.
3. What do you predict will happen when you remove the divider between the
compartments? Explain in your journal.
4. Test your prediction.
5. Measure the densities of the tap water and the salt solution. Do the densities you
obtained support your observations?
6. Now repeat the experiments with hot and cold water.
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What happens when I combine two fluids of differing
densities?
Amazing!! One floats on top of the other!
Station 6: Density of fluids (EXPLANATION)
Fluids arrange into layers according to their density. When the divider is removed the
denser water (salt water or cold water) sink to the bottom of the container and the less
dense water (fresh water or warm water) float above. See reading material.
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