Grades 58 Performance Task
Contributed by: Oregon State Department of Education
Description:
Using the concepts of density and buoyancy to predict which objects
will float or sink in water, students explore why an object floats
or sinks in water. This task introduces and provides demonstration
of density and buoyancy; practice measuring, calculating, and predicting.
The task assesses students' understanding of scientific inquiry
including the following skills: observation, background research,
scientific procedures (including investigation design, measurement
techniques, and error analysis), data collection, data display,
scientific questions, formulating an hypothesis.
This task is designed to take students approximately 3 to 7 hours.
Overall Task Content Area:
 Physical Science
Specific Knowledge Areas:
 Properties of specific substances
Performance Expectations:
 conducting investigations
 using equipment
 gathering, organizing, and representing data
 formulating conclusions from investigational data
National Science Education Standards:
8 A SI 1: Abilities necessary to do scientific
inquiry: Grades 58
1.1 Identify questions that can be answered through scientific
investigations. Students should develop the ability to refine and
refocus broad and illdefined questions. An important aspect of
this ability consists of students' ability to clarify questions
and inquiries and direct them toward objects and phenomena that
can be described, explained, or predicted by scientific investigations.
Students should develop the ability to identify their questions
with scientific ideas, concepts, and quantitative relationships
that guide investigation.
1.2 Design and conduct a scientific investigation. Students
should develop general abilities, such as systematic observation,
making accurate measurements, and identifying and controlling variables.
They should also develop the ability to clarify their ideas that
are influencing and guiding the inquiry, and to understand how those
ideas compare with current scientific knowledge. Students can learn
to formulate questions, design investigations, execute investigations,
interpret data, use evidence to generate explanations, propose alternative
explanations, and critique explanations and procedures.
1.7 Communicate scientific procedures and explanations.
With practice, students should become competent at communicating
experimental methods, following instructions, describing observations,
summarizing the results of other groups, and telling other students
about investigations and explanations.
8 B PS 1: Properties of objects and materials:
Grades 58
1.1 A substance has characteristic properties,
such as density, a boiling point, and solubility, all of which are
independent of the amount of the sample. A mixture of substances
often can be separated into the original substances using one or
more of the characteristic properties.
(Use the "hot" link on the PALS home page
to check the full text of related National Science Education Standards,
if desired.)
National Council of Teachers of Mathematics:
NO1: Understand numbers, ways of representing numbers,
relationships among numbers, and number systems:
Grades 68 q. understand and use ratios and proportions
to represent quantitative relationships
NO2: Understand meanings of operations and how they
relate to one another:
Grades 68 h. understand the meaning and effects of arithmetic
operations with fractions, decimals, and integers
AL4: Analyze change in various contexts:
Grades 68 e. use graphs to analyze the
nature of changes in quantities in linear relationships
MEAS2: Apply appropriate
techniques, tools, and formulas to determine measurements:
Grades 68 k. select and
apply techniques and tools to accurately find length, area, volume,
and angle measures to appropriate levels of precision
Grades 68 o. solve simple
problems involving rates and derived measurements for such attributes
as velocity and density
DAP1: Formulate questions that
can be addressed with data and collect, organize, and display relevant
data to answer them:
Grades 68 i. select, create, and use appropriate
graphical representations of data including histograms, box plots,
and scatter plots
DAP3: Develop and evaluate
inferences and predictions that are based on data:
Grades 68 e. use conjectures to formulate new questions
and plan new studies to answer them
RP2: Make and investigate mathematical conjectures:
Grades 68
CNX3: Recognize and use connections among mathematical
ideas:
Grades 68
REP3: Use representations to model and interpret
physical, social, and mathematical phenomena:
Grades 68
General Instructions to the Teacher:
This task is designed to take students approximately 3 to 7 hours.
Varies according to the preassessed level of students.
 Students unfamiliar with concepts five to seven 45minute
periods or three to four block days.
 Students with prerequisite knowledgethree to four 45minute
periods or two blocks.
Students will be working individually during this exercise.
Students should be ready to work as soon as periods begin. A central
supply area, if needed, should be easily accessible. All supplies
should be clearly labeled.
Materials for "Density and Buoyancy":
The teacher will need:
Density:
 blocks of various materials and sizes (ideally wooden blocks
of different sizes for the demonstration and initial lab)
 clay, butter, candy bars, and other common food substances cut
to size
 bowling ball, volleyball, soccer ball, Nerf ball, and shot put
or softball (2 balls of similar sizes but dissimilar weight.)
 triplebeam balances or scales, and metric rulers
 wideopening graduated cylinders (100  200 ml)
 water displacement canisters or 500 ml beakers
 calculators
Buoyancy:
 35mm film canisters
 various materials to fill canisters (bb's or lead shot, popcorn,
flower, sugar, sand, cork, etc.
 Permanent marking pens or grease pencils.
 masking tape (if canisters are black)
 widemouth jars
 twoliter soda bottles cut in half (use bottom only) or 500ml
beakers
 triplebeam balances or scales
 metric rulers
 calculators
Advance Preparation:
The goal of this activity is to prepare the students to do their
own experiment using what you have taught them in order to discover
that objects with a density greater than 1g/cm^{3} sink,
and less than 1g/cm^{3} float.
 First you must teach them that density is a general property,
that is: it is something that all objects have. Unlike color,
which is a specific property, that is: it describes an individual
characteristic. The property of density describes how many atoms
are packed into a centimeter size cube.
To find an object's density, we have to measure its characteristics
and do a little math. The characteristics we have to measure are
its volume (size) and its weight (mass). The mass we measure by
weighing it to find out how many grams it has. Volume is measured
in centimeters cubed (L x W x H). By dividing the amount of atoms
(grams) into the space, it occupies in cm^{3}, we discover
the objects density. The formula for finding density is D=M/V.
Example: 5cm cube of brick weighs 50 grams.
50g/5cm^{3} = 10g/cm^{3}.
The density of this brick is 10 grams per cubic
centimeter.
In other words, every cubic centimeter of this brick
has
50 grams worth of atoms stuffed into it.
 Once you have explained this concept it would be helpful for
students to see objects of the same material in different sizes,
so they can discover that the density of the object never changes
regardless of size.
Using the brick again: if you used a larger piece
of brick, such as 50cm^{3},
the mass would be 500g, so the density is still
10g/cm^{3}.
Let them calculate the density of 34 blocks of wood. Let them
use a data table like this one:
Block # 
Mass 
Volume 
Density (m/v)

1 



2 



3 



4 



By the third set of measurements, they should be close to discovering
that they do not have to measure to figure out the density of the
objects anymore, it does not change! Some students will not make
this connection. (Check for understanding.) Students may make mistakes
in their measurements (use inches instead of cm's, misread the ruler
or scale, or reverse the formula), so have them check their work
carefully.
 The next step is teaching them about buoyancy. For the purpose
of this lab, we will focus on a narrow definition of buoyancy.
Buoyancy is an object’s ability to float. Gravity pulls everything,
even water, towards the center of the earth. An object in water
is still pulled down by gravity. However, water molecules like
to hold onto each other and they push against anything that is
trying to fall down (sink) through it.
You float, no matter how big you are. If you balance your mass
and volume numbers, so water can hold you up (making your overall
density less than 1). If you do not balance those numbers and your
overall mass is not countered by the right amount of volume, you
sink (overall density greater than 1).
 Now is the time for students to practice predicting. Prepare
the common food substances. Students should make a data table
of predictions
Object 
Prediction: Sink
or Float 
Actual: Sink or
Float 
Snickers 


Three musketeers 


Marshmallow 


Baby Ruth 


Clay 


Butter 


Ask kids to make their predictions  then place the objects in
the water, so they complete the "Actual" column. Ask students to
think about this question: What is it about an object's density
that courses it to float or sink in water? Tell them you would like
them to answer that question for you later on.
 Now they are going to do a lab on buoyancy. In advance, make
two film canisters for the next demo. Follow the students directions,
prepare one that partially submerges and one that submerges fully,
but does not sink. Before placing them in the water, let some
students hold them and ask them to predict  which one sinks and
which one floats? When they both float, the kids will be surprised.
Anticipatory Experiment:
 Choose one type of material to work with.
 Mark the sides of your canisters from bottom to top by centimeters.
 Fill the canisters with three different weights of material
 making sure that your heaviest canister is no more than halffull.
 Write the weight in grams on the canister lid.
 Place canisters in water and note number of centimeters it submerged.
 Complete data table.
 Create graph (weigh vs. centimeters submerged) and plot points.
 Refill canisters with three heavier weights than the first trial
 the heaviest should be completely full. Write weight in grams
on canister lid.
 Make predictions about the number of centimeters the canisters
will sink and place your predictions on the graph.
 Write results.
 Graph your results weight vs. centimeters submerged.
 The students should have created a graph that shows a rising
diagonal straight line. (It will slope if they plotted centimeters
vs. weight.)
Discussion of content and framing scientific questions:
Now it's time for them to recall the earlier question: "What is
it about an object's density that causes it to float or sink in
water?" Some students may ask what the volume of the canister is,
so they can calculate its overall density. If they do not ask, tell
them it is the next step. If you feel it is appropriate, provide
the volume of your canister. If you feel it is appropriate for students
to do the math formula for calculating the approximate volume of
canister is (V=p ^{2}h). If students
ask about approximate surface area, the formula is 2(p
r^{2})+(2p
r)h. Surface area is not required for this activity, but
some students may ask about it.
Now they have data from their density lab and the buoyancy lab.
Their job is to come up with a question, as a class, that will help
explain the relationship between buoyancy and density. It is important
that they spend a proper amount of time thinking of an answerable
question. They should base the answer to their question on the data
they have gathered. Reinforce the idea that the answer can start
with, "Yes, we were right," or "No, we were wrong," and still be
correct. The test is on the design of the experiment and their ability
to communicate their resultsnot whether they were "right."
There are many variables they can choose (density is the hot one).
Some are:

container size or shape 
solutions (salt or sugar water)

type of liquid (oil, molasses,
etc). 
temperature of water (ice water,
cold, warm, or hot) 
temperature of object 
type of materials (rocks, bb's,
popcorn, flower, etc.) 
amount of materials (20g, 40g,
60g) 
Safety:
 Be careful.
 Teachers and students should always exercise appropriate safety
precautions and utilize appropriate laboratory safety procedures
and equipment when working on science performance tasks.
Extensions/modifications:
Helpful resources:
De Vito, Alfred, Recycling 35mm Canisters for the Teaching
of Science, Creative Ventures, Inc., West Lafayett, Indiana,
1993, Second Edition, p.15, 2425.
Windschitl, Mark, "What is Inquiry?" Paper presented at the National
Science Teachers Association meeting, 10/29/98 [mwind@u.washington.edu)].
Bailer, Jill; Ramig, Joyce E.; Ramsey, John, M., Teaching
Science Process Skills, Good Apple: Grand Junction, CO,
1995, p.111.
Math On Call, Great Source Education Group, Wilmington,
MA, 1998, p.40941
Teaching Physics With TOYS, Terrific Science Press:
Miami University Middletown, OH, 1995, p.233.
Matter: Building Blocks of the Universe, Teacher's
Edition, Prentice Hall, New Jersey, 1993, p. 1011, 2223.
To target the specific NCTM standard(s) to be measured, ask students
to explore the proportional relationship of mass and volume.
