Theory: Liquid water reaches its maximum density at about 4 C. As the temperature is raised the density decreases. A density ball is a hollow copper sphere partly filled with steel shot so that the ball has the same density of warm water (as of 5/21/96, water at 42 C). In cooler (i.e., more dense) water, the ball floats; in warmer, less dense, water, the ball sinks. This is because of Archimedes' Principle: an object displaces a volume of fluid with a weight equal to its own. The mass of the density ball is fixed, so when the water is cold and more dense the ball needs to displace less water in order to float. As the water temperature is raised and its density drops, the ball displaces a larger volume of water to remain floating. When the volume of water required exceeds the volume of the ball, the ball sinks.
Description: The simplest way to do this demonstration is to start with the ball floating in a beaker of cool water. Then slowly add hot water from the kettle, monitoring the temperature, until the ball sinks. If you don't get the water too hot, it shouldn't take long for the water to cool enough to float the ball again.
Setup time: 5 minutes.