Wikijunior:Big Book of Fun Science Experiments/Test floating

You need

• a tub of water
• salt
• any small, unimportant, inexpensive household items

Experiment

• fill the tub with cold water
• then put the items into the water to see which ones float
• then put in salt (about 5 table spoons for every liter of water in the tank)
• see if any of your sinking objects rise

Principle

The principle here is something called buoyancy. The theory was first created by a Greek Scientist called Archimedes. When he noticed, while getting into a completely filled bathtub, how water splashed over the sides. He realized that the amount of water being splashed out must have the same volume (the space that an object takes up) as his own body. He theorized that, in order for an object to float, it's total weight had to be less than the volume of water that it displaces (or takes up).

This idea has been modeled easily into the metric system by the fact that a liter (a liquid volume measurement) of water weighs a kilogram (a unit of weight). We can calculate the density (a ratio between the volume and the weight of a given object of substance) of water very easily, it is 1. However different liquids have different densities. For example, salt water has a density of 1.2, because the salt makes it heavier.

If any of your sinking objects rise, what can you tell about their density?

You can tell they must have a density between 1 and 1.2.