This activity stands alone as an investigation of density and how it affects the buoyancy of an object. To complete this activity, students will need to be able to use a scale accurately. It is necessary for them to know how to find the volume of an object using water displacement. The methodology is described in this activity. If you would like to practice this skill first with your students, I have a separate investigation of this technique listed as “Volume Two Ways.” To complete this activity, students will also need to be able to use a scale accurately.
For this activity, students first calculate the density of water. This is your chance to discuss metric units and the relationship of the metric system to water. One cubic centimeter of water is also one milliliter of water, and this volume of water weighs one gram. Therefore, if you calculate the density of water in grams per milliliter (or cubic centimeter), you will get a density of 1. Objects with greater mass per volume will have a density greater than 1 and will sink in water. Objects with less mass per volume will have a density less than 1 and will float.
For this activity, you will need some objects to test. It is helpful to have one that floats, and one that sinks. I have used foam rubber balls, wooden blocks, and rocks. As you are getting materials together, know that the objects, even those that float, need to fit in the cup you are using. The floating objects will be pushed down until they are just under the water so that they displace their volume of water out of the cup and into the bowl.
Part of my discussion of this activity includes properties of matter. Before beginning, as a class I will discuss weight versus mass, volume, density, and buoyancy. There are many other properties of matter to be investigated. I have included a list of properties of matter that I discuss with students. One extension of this activity would be to have students each choose a property of matter to research and present to the class with a definition and examples.
Another interesting extension of this investigation is to consider the displacement of water by objects that float. Objects that sink displace their own volume in water. Objects that float displace only their weight in water. Water weighs about 8 pounds per gallon or 3.8 kilograms. A cubic foot of water weighs 62 pounds and 28 kilograms. In metric units 1 liter of water weighs 1 kilogram. If you can calculate the volume of water displaced by a boat or floating object, the weight of that volume of water will equal the weight of the boat or object.