Montessori Science
Materials That Sink or Float: Density and Buoyancy Worksheets for Elementary
Quick science lesson and four free worksheets on density, buoyancy, and objects that sink or float.
Have you ever wondered why some things float and some things sink in water?
- To float means to stay above the surface of the water.
- To sink means to fall to the bottom of the water.
Density
All things are made up of very tiny particles called molecules. Density is how loosely or tightly packed the molecules in an object are. It refers to the degree of compactness of an object.
Objects will sink if it is more dense — that is, if its molecules are more tightly packed — than the liquid it is put in.
Objects will float if it is less dense — if its molecules are less tightly packed — than the liquid it is placed in.
Some things float at first but then sink as they absorb water or take in water through holes (and thus become denser).
Buoyancy / Upthrust
Buoyancy, also known as upthrust, is the force that pushes an object up and makes it seem to lose weight when immersed in a liquid.
When people talk about the buoyancy of an object, they usually mean the tendency of that object to float in liquid or in air.
In more scientific terms, buoyancy is the upward force that a fluid exerts on an object. It is the force that opposes the weight of a partially or fully immersed object.
An object floats when the buoyant force or upthrust becomes large enough to counter its weight.
Shape
Changing the object’s shape will also change its ability to float because it will hold less (or more) air and upthrust will have less (or more) space to push up on the object.
But although an object’s shape can affect its ability to float, some materials — such as styrofoam and cork — float no matter what their shape is.
Sink or Float Worksheets
Note on the Worksheets
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Sink and Float Worksheet for Grade 2 and Grade 3 (Experiment)
Will the objects below sink or float? Make your predictions, then conduct an experiment, and record your findings.
Objects you will need: (1) empty plastic bottle with the cap on, (2) full plastic bottle with the cap on, (3) empty glass bottle with the cap on, (4) full glass bottle with the cap on, (5) small rubber ball, (6) big rubber ball, and (7) marble.
You can get a printable copy of this worksheet here: Sink and Float Worksheet for Grade 2 (PDF)
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Sink and Float Worksheet for Grade 2 and Grade 3 (Experiment - Fill In Your Own Objects)
In this worksheet, you get to choose what objects you will include in your experiment. Try to choose things made of different materials and of different shapes.
You can get a printable copy of this worksheet here: Sink and Float Worksheet for Grade 2 (PDF)
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Density: Float or Sink Worksheet for Grade 4
Sort the items in the worksheet below according to whether they would sink or float on water. If you have them in your house, try to check your answers by conducting an experiment.
You can get a printable copy of this worksheet here: Density: Float or Sink Worksheet for Grade 4 (PDF)
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Density and Buoyancy Quiz: Float and Sink Worksheet for Grade 4
This quiz lets you assess what you’ve learned in this lesson about density, buoyancy, and why objects sink or float. Fill in the blanks before the number with the letter of the correct answer.
You can get a printable copy of this worksheet here: Density and Buoyancy Quiz: Float and Sink Worksheet for Grade 4 (PDF)
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