Floating foil density

This activity is part of our Science Lab series. Check out the ANU Science Lab for more experiments! In this activity, join Ziang as they experiment with aluminium foil balls of varying sizes and densities in fresh and salt water to explore buoyancy, mass, volume, and density. Watch the video above to see how it is done.

Aluminium foil density experiment materials

For this activity:

• Aluminium foil sheets (cut into equal square sizes)
• A large clear glass bowl or deep basin
• Tap water
• Table salt
• A spoon for stirring
• A ruler (to measure ball diameters)

How to test aluminium foil density: Step-by-step instructions

Step 1: Roll aluminium foil balls of different sizes

1. Take two identical sheets of aluminium foil of equal mass.
2. Roll the first sheet loosely into a larger, hollow ball containing trapped air.
3. Crush the second sheet as tightly as possible into a compact ball with a much smaller diameter.
4. Prepare additional balls using multiple sheets of foil pressed together to test how increasing mass alters volume and sinking depth.

Step 2: Float the aluminium balls in tap water

1. Fill your clear basin or bowl with fresh tap water.
2. Gently place your aluminium balls into the water side by side.
3. Observe how much of each ball sits above versus below the water level. Notice that the smaller, tightly packed ball sits deeper under the water surface because it has a higher mass per unit volume.

Step 3: Test buoyancy in salt water

1. Remove the foil balls from the water.
2. Add several generous tablespoons of table salt to the basin and stir thoroughly until the salt completely dissolves.
3. Place the exact same aluminium balls back into the salt water solution.
4. Compare how high the balls float in salt water compared to how they floated in normal tap water.

Aluminium foil density: The science explained

This fluid dynamics demonstration illustrates mass, volume, density, and Archimedes' principle:

• Density is defined as mass per unit volume. Crumpling the same sheet of aluminium foil tighter reduces its overall volume while trapping less air, making the tightly rolled ball much denser than the loosely rolled ball.
• An object floats when the upward buoyant force equals the weight of the fluid it displaces. A smaller, tightly packed aluminium ball traps less air and displaces less water relative to its mass, causing more of the ball to submerge under the water surface.
• Adding table salt to tap water increases the overall density of the liquid because dissolved salt ions add mass without significantly increasing the water volume. Denser liquids exert a greater buoyant force, allowing objects to float higher in salt water than in fresh tap water.

Science fair projects & taking it further: Experimenting with variables

• Test how varying amounts of salt (such as 1 tablespoon vs 3 tablespoons) incrementally affect how high the aluminium balls float in the bowl.
• Apply the same fluid density principle using a fresh egg or a potato in tap water versus salt water to observe buoyant force changes in action. Do these items have the same results as the foil?
• Compare a flat aluminium boat shape against a crumpled ball made from the same sheet size to test how overall shape impacts displaced volume and floating capability.

Enjoyed this experiment? Explore more hands-on activities on the Science Lab ANU YouTube channel.