Buoyancy, sinking and floating - free experimentation

Freely experiment with materials that sink and float in water. Extend to discussion about fish, boats or how air in fur/feathers help animals float.
Science content
Chemistry: States of Matter, Properties of Materials (K-7)
Chemistry: Indigenous People’s Materials/Separation methods (1, 6)
Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6)
  • tray of water, a deeper tub is best for Challenge 2
  • materials that float/sink/can change shape to float or sink in water e.g. sponges, ping-pong balls, straws, modeling clay, tin foil, wood, styrofoam, nails, paper cups, paper towels, marbles

This activity has been run in several ways:

Free experimentation with the collection of materials, asking students to explore floating and sinking. See the resource for the Play-Debrief-Replay method of teaching free play.

Challenge 1:
Make a flat piece of foil sink or float. Make a crumpled piece of foil sink or float (air gets trapped in crumpled foil unless it is crumpled under water, or pressed together tightly). Shape a piece of modelling clay so that it floats.
After making boat shapes from foil or modelling clay, add cargo of marbles/coins to see how much it can carry before sinking. In each case ask students to explain what is happening in terms of weight/density/displacement/buoyancy.

Challenge 1 as an activity on boats and how they float:
After using tin foil to make shapes that can carry cargo, test how many marbles/rocks/seeds they can carry before sinking. Discuss how boats are constructed to carry a lot without sinking. Then add a little flow to the water by moving a finger or stick through it to see if the boat and cargo can still float, and relate to how some boats are designed to carry cargo on rivers or oceans.
Pacific Northwest indigenous canoes are made from Western Red Cedar. The wood is strong, but lightweight. The oils make it buoyant and resistant to rot. Freight canoes travelling over open water are made large with high prows and sterns, so that they can ride the ocean waves without sinking.
Students can try experimenting with their foil boat shape and cargo to see how much water turbulence they can withstand.

Challenge 2:
Start with a 2x2x2cm piece of styrofoam. Add nails, paper clips and modelling clay to give it neutral buoyancy, so that it floats half way down the water (natural buoyancy). For this challenge, make sure the water is deeper. If not possible to achieve neutral buoyancy (hard), try and make the assembly sink as slowly as possible. At neutral density the combined density of the materials are the same as water density.

Calculate density using challenge 2:
To measure the density of their foam/nail/paper clip/modelling clay sculpture which floats as close to natural buoyancy as possible, students can use the mass/volume formula. Weigh the sculpture on a kitchen scale (in grams). Measure the volume of the sculpture: use a graduated cylinder (more accurate) or beaker (less accurate) that the sculpture can fit into, add water to the cylinder/beaker and read off the volume, then immerse the sculpture in the water and calculate the volume increase of the water (in ml). Divide the mass of the sculpture by the volume increase, to find its density (in g/ml). If the sculpture floated at near-neutral buoyancy its density will be close to that of water - 1g/ml. (Using a beaker to measure volume, we arrived at sculpture densities ranging from 0.7g/ml to 1.1g/ml - similar to the density of water. The lower numbers would be sculptures that slowly rise in the water and the higher numbers would slowly sink.)

Challenge 2 as an activity on fish movement in water:
Fish are able to swim at different levels in water - near the surface or deep, so that they can move to find food or hide from predators. Fish hold varying amounts of air in their swim bladder to change what level they are swimming at.
The activity using styrofoam piece with heavy items added to it models how fish float at different levels in water. The styrofoam holds air so that it floats (like the fish swim bladder) and the heavy loads pull it down in the water (like the fish body). Just as students balance the styrofoam with the weight of the objects added to achieve neutral buoyancy, fish can balance the amount of air in their swim bladder with their body mass to float at the level they need. This allows them to use their energy for moving back and forth, with no need for energy to stay at a certain depth
(Note: it might be a little confusing to students that fish regulate the amount of air, whereas this activity regulates the amount of mass, but the net effect of floating at varying levels is the same.)

Free experimentation exploring how air in fur or feathers makes animals buoyant
Give students objects with air in them that float (e.g. dry cloth, ping pong balls, styrofoam, popsicle sticks or wood pieces) and objects that sink (e.g. marbles, paperclips, pipe cleaners, wet cloth) to experiment with. Discuss that some objects float because they have air in them. Similarly, animals that live in the water trap air in their fur (e.g. otter) or feathers (e.g. ducks and water birds), to help them float.

Age-dependent concepts on sinking, floating and buoyancy that might be useful:
Heaviness, lightness, density of an object: Things that sink are “heavier” than things that float, or more specifically, they have a greater “density” (more mass for their volume; more particles packed into the same space). If an object has a greater density than the water it will sink - hence solids tend to sink (unless they have air in them); and gases float.
Weight, buoyancy, displacement: When an object is placed in water, its weight (the force of gravity pulling on its mass) pushes down on the water. The water pushes back up on it, called the force of “buoyancy” (or “upthrust”). The object rests at a level where these forces are balanced. The force of buoyancy equals the weight of the water displaced, so if the object is denser than water, the force of its weight will be greater than the force of buoyancy and it will sink.
Surface tension: forces between the surface molecules that come into play if the object is small enough, and can make things float.

Grades taught
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 6
Gr 7