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 5-8

1.1 Identify questions that can be answered through scientific investigations. Students should develop the ability to refine and refocus broad and ill-defined 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 5-8

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:

MEAS2: Apply appropriate techniques, tools, and formulas to determine measurements:
Grades 6-8 k. select and apply techniques and tools to accurately find length, area, volume, and angle measures to appropriate levels of precision
Grades 6-8 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 6-8 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 6-8 e. use conjectures to formulate new questions and plan new studies to answer them

RP2: Make and investigate mathematical conjectures:
Grades 6-8

CNX3: Recognize and use connections among mathematical ideas:
Grades 6-8

REP3: Use representations to model and interpret physical, social, and mathematical phenomena:
Grades 6-8

General Instructions to the Teacher:

This task is designed to take students approximately 3 to 7 hours.

Varies according to the pre-assessed level of students.

• Students unfamiliar with concepts-- five to seven 45-minute periods or three to four block days.
• Students with prerequisite knowledge--three to four 45-minute 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.)
• triple-beam balances or scales, and metric rulers
• wide-opening 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)
• wide-mouth jars
• two-liter soda bottles cut in half (use bottom only) or 500ml beakers
• triple-beam 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/cm³ sink, and less than 1g/cm³ 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 cm³, we discover the objects density. The formula for finding density is D=M/V.

• 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.

• 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.

• Now is the time for students to practice predicting. Prepare the common food substances. Students should make a data table of predictions

• 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.

• 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 half-full.
• 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.)

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 ²h). If students ask about approximate surface area, the formula is 2(p r²)+(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 results--not whether they were "right."

There are many variables they can choose (density is the hot one). Some are:

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: