Density of Objects – Critical Thinking in Action

Sinking and Floating Problems Reinforce the Science Process Skills

Aug 28, 2008
David R. Wetzel

Density - Queens University

Two investigations are used for students to question why objects sink or float. They make predictions and inferences about density of objects based on observations.

Studying the density of objects in science is a common topic in the physical sciences. Investigations which foster student curiosity and questioning skills are the best kind of action to help students retain new information.

Using objects that obviously sink or float prevent students from using or developing critical thinking skills. Simple investigations which require students to make predictions before testing if the object will sink or float are good; however the why is always the challenge.

Although the students will use all basic science process skills in these investigations, prediction and inference are the two important process skills being used. Additional information on the science process skills can be found by visiting Problem Solving and Science Process Skills.

These two investigations are:

Will a Change in Temperature Cause Water to Move?

Materials – one large clear container of room temperature water (small aquarium or similar) and 4 ice cubes dyed dark blue with food coloring.

Ask students to make a prediction if they think water temperature causes water to move or not. A simple yes or no does not provoke critical thinking. Their answers must explain why or why not.
Record their answers and reasons on the board, white board, or overhead projector
Now drop an ice cube into the water.
Let them observe what happens for a few minutes – they are making inferences of what they thinking is happening.
Now drop a second ice cube in the water and let them observe.
The next step is for students to explain what they have observed and why.
As you ask questions, drop the two remaining ice cubes in the water.

What the students have observed is ice cubes float. As they melt, columns of dark blue water stream to the bottom of the tank.

As the dark blue water nears the bottom of the tank it will begin to spread out. This demonstrates temperature does cause water to move, because cold water is denser than warm water.

Do Soda (Pop) Cans Sink or Float?

Materials – one large clear container of room temperature water (small aquarium or similar), six soda (pop) cans – three regular and three diet (different brands), and one small scale that displays grams.

Ask students to make predictions if the soda/pop cans are denser than water – will the cans sink or float?
Record their answers in a data table on the board, smart board, or overhead. Again their answers should be more than simple yes /no, answers need to include why or why not.
Now have a student select and place one of the soda/pop cans in the water.
Students should then explain their observations making inferences of why it sank or floated.
Do the same for the remaining five cans.

The students will observe that regular soda/pop cans sink and diet cans float.

Now have them explain why they think this occurred.
Have students read the ingredients to search for clues (if none ask).
Have them measure the mass of the cans with the scale.
Let them conduct any other measure they want.

The students will quickly discover the ingredients differ due to sugar in the regular soda/pop. They will also see diet cans are a few grams lighter than regular soda/pop.

These two activities cause students to use critical thinking skills and the results cause them to internalize new information. This also helps them develop a better understanding of why objects sink or float.