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This image shows a pool ball floating on liquid mercury. A pool ball is fairly heavy for its size, but you can see it floats on, or is less dense, than liquid mercury.
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Density Definition Page

Density is a measure of how much mass is contained in a given unit volume (density = mass/volume). It is usually expressed in kg/m^3. Put simply, if mass is a measure of how much ‘stuff’ there is in an object, density is a measure of how tightly that ‘stuff’ is packed together.

One of the ways we can see density illustrated in everyday life is by looking at whether things float or sink in a liquid (water, for example). If an object is less dense than the liquid it is placed in, it will float. If it is more dense, it will sink. This is why an anchor, which is very dense (lots of mass in a relatively small volume) sinks very quickly, while an inner tube (not much mass in a relatively large volume) floats and is very difficult to push underwater.

Some elements are naturally found in very dense forms, and one of the most surprising cases of a less dense object floating on a denser liquid occurs when objects are put in a container of liquid mercury (Hg). This element is a metal that is liquid at room temperature, and it is VERY dense. In fact, it is denser than lead, and metal objects like forks, coins, or even a cannonball will float on top of it!

Last modified September 23, 2002 by Jennifer Bergman.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF