Sink or Float - An Introduction to Buoyancy and Density

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Students will sort objects by whether they sink or float by completing a simple investigation.

Big Idea

Students will be introduced to the basics of buoyancy in this hands-on, interactive lesson.


5 minutes

The students have had some exposure to the concept of "sink or float" from a story that we read in reading curriculum called Sink or Swim by Valeri Coulman.  Click here to find this book on Amazon.  In this story, two cows are trying to learn how to swim.  The cows are unsuccessful at the beginning of the story and make several adjustments in their quest to float. It is a humorous story and a great way to introduce the lesson content. 

I remind them of the book and ask them some questions to set the stage for our learning.

Do you remember our story that we read called Sink or Swim?  Who was trying to learn how to swim in this story?  That's right the cows?  What happened when they first tried to swim, did they sink or float?  That's right.  What does it mean if something sinks?  It goes under the surface or top of the water.  The cows in our story kept sinking and they tried different ways to help them float.

What does it mean to float? That right.  It means that something stays on top of the water.  The cows finally figured out how to float at the end of the story.  They are able to stay on top of the water. 

Today, we are going to be exploring sinking and floating with an investigation.  Let's get started!



20 minutes

Materials Needed:

Large container filled with water for each work group.  A clear container is a little more effective for observation purposes.

Investigation recording sheet for each student, included as a PDF with the lesson.

The following objects for the investigation (one set per work group):

a small block

a marble

a sponge

a key

an eraser cap

a  marshmallow

a penny

a cotton ball


I distribute the recording sheet to the students and have them put their name on top.  I tell the students, We are going to be conducting an investigation today to learn more about sinking and floating.  We will be using what we learn to help us understand why things sink or float.

It is important to set out expectations about behavior during the investigation so the students can have a successful experience.  I tell the students, It is important that we follow some rules to make sure that we have a successful investigation.  So, we are going to listen to directions and not touch anything without being told.  We are using water, so it could get messy if we play in it, so hands do not go in the water.

I want the students to always view themselves as scientists, so I make sure to include a reminder to my students to think like a scientist.  I say to the students, When a scientist wants to learn more about something, they do an investigation.  Scientists will make predictions when they conduct an investigation.  Do you remember what a prediction is?  That's right.  It is a a really good guess, using the information that we know.  Today, we are going to test some different objects to see if they sink or float.  Before we test each one, you are going to make a prediction.  I want you to take the block and pass it around the table.  After you have a chance to hold the block, I want you to make a prediction.  Do you think the block will float or sink?  Find the picture of the block on your sheet and circle your prediction. 

We then test the block  (The students will take turns testing each item so everyone gets an opportunity to drop an item into the water).  I have the students record what happened on the recording sheet.  We then continue predicting and testing each item until all are tested.  See Student Work Sample Prediction Sheet  We leave the experiment materials on the tables and move to our Smartboard spots. 

Direct Instruction

15 minutes

For this portion of the lesson, I use my SmartBoard.  If you have a SmartBoard, the file Intro to Density Smartboard File easily be downloaded and opened.  If you have a different type of interactive whiteboard, you can still use this lesson by opening the file in Smart Notebook Express.  Click here to download. There is also a PDF of the slides so you can recreate this part of the lesson.  Click here to access them: Intro to Density PDF of Smartboard Slides.

The students gather at the Smartboard.  Summarizing data is an important scientific skill, so I begin our direct instruction time by having the students complete a T-Chart to organize the data from our experiment. This T-Chart is the first slide in the file.  I ask the students to share their results and I record the information on the chart. 

The second slide (SmartBoard Slide 1) has the lesson objective written in "student friendly" terms.  There is a content objective and a language objective to help focus on vocabulary expansion for my English Learners (ELs) to be congruent with SIOP instructional techniques (Click here to learn more about SIOP).

The content objective helps me focus my instruction..."What do I really want my students to know when I am done with this lesson?"  The objective should never be too large and it should be assessed, formally or informally at some point during the lesson.  It also helps the students to focus on what is really important during the lesson.  The language objective helps me identify vocabulary that is important for all my students to master and holds me accountable for giving my students an opportunity to practice and use their language during the lesson.

I read these objectives aloud for my students:

Content Objective
I can talk about density and how it effects whether something floats or sink.

Language Objective
I can tell a friend a prediction about whether an object will sink or float and why.

Slide 3:  Why do some things float and other things sink?

Slide 4:  Whether an object sinks or floats depends on the objects density.
All non-living things are made up of tiny pieces called molecules.  Density is a measure of how close together these pieces are.  If they are close together, the object is dense.  If the pieces are far apart, the object is less dense.

Slide 5:  An object with a low density has more space for air.  The air in the object helps it to float.

Slide 6:  An object with a low density will float.  An object with a high density will sink. 

Slide 7:  Now, let's go back to our t-chart and think about the objects we tested and their results and then let's think about  their density.  We go back to the first slide and talk through each object.  I talk about the density of each object, asking the questions:  If you were to cut this object in half, what do you think it will look like?  Will it be solid?  The more solid an object is the greater the density.  We talk about why the cotton ball did not float.  The students immediately identify that the material is different and that it absorbed the water.  The density increased because of the added water. 

We return to the final slide.  It is Turn and Talk time.  Turn and Talk allows my students to practice their academic language while applying their content knowledge to answer a question.  The students all have an assigned Turn and Talk partner.  The students quickly turn toward their partners when they hear it is Turn and Talk time.  I pose the following question for them: 
Which one do you think will float?  Why?  I explain to the students what each item is, just in case they do not recognize the item from the picture.  It is not a test of whether they can identify the item, but rather an opportunity to practice language and apply concepts.  The students are given time to talk.  When they are done, I call on different students to answer the question.  Because they have had so many of these types of questions posed for them, they know they need to share the "why" behind their answer.  I am impressed by their knowledge and application of the concepts learned during the lesson. 

Independent Practice and Informal Assessment

10 minutes

After we finish with the direct instruction portion of the lesson, I send the students back to their seats to work on the independent practice portion of the lesson.  For this part of the lesson, you will need the Did It Sink Or Float Summary Sheet  included as a PDF with this lesson.

I say to the students, as scientists, It is important that you can communicate your results about your experiment.  You are going to make a sink and float diagram.  Cut out the pieces and place them in the container on the sheet the way they look in the water.  If something is on the bottom of the container, glue that picture on the bottom of the container on your diagram.  If something is floating, glue it on the water at the top of the container.  I want you to look carefully at the items that are floating.  Are they sitting right on top of the water or is part of them under the water line?  Make sure your diagram shows where your objects really are. 

The students begin working and place the pictures of the objects where they are in the containers.  I circulate around the room and check their work, pointing out how the objects on the top are floating so the students can accurately depict them on their diagram. See Work Sample

To close the lesson, I say to the students, When you get home, I want you to share with your families what you  learned during our lesson.  What are some of the things that you could tell your parents about our science exploration. Turn to a friend and share with them what you are going to tell your parents about our exploration. The students are given time to talk with their neighbor and we then clean up experiment.