Density and Floating
Why do ice cubes float in water?
Why doesn't the Ivory soap sink?
Density. Ice is less dense than water and they add air to Ivory soap so it
also is less dense and floats in water.
When an object sinks into the water, the same volume of water has to be
pushed up. If a quart of steel goes into a lake, a quart of water has to
come up to the surface. It has to move out of the way. For most solid
objects, this is no problem. Some materials, however, are not as heavy as
water. A pound of ivory soap going into the water is only heavy enough to
lift a pound of water out of the way. A pound of water is just as heavy as
a pound of ivory soap, but it is not a large. Once a pound of water has
been pushed up, the soap isn't heavy enough to push any deeper. The soap
floats. Because a cup of soap weighs less than a cup of water, the soap is
less "dense" than the water. Ice is less dense than water, too. These
objects float. A cup of steel weighs more than a cup of water: steel is
more dense than water. Steel does not float.
The upward force on an object in a fluid is equal to the weight of the
fluid it displaces. If this force is greater than the downward force of
gravity, it will push the object up until it reaches the point where
gravity and the "buoyant" force are equal.
Ice cubes float higher in salt water... it is more dense... than fresh
Ice cubes sink in alcohol. Can you figure out why?
You might want to read up on density and specific gravity.
Ice floats because ice has a lower density [specific gravity] than liquid
water at temperatures equal to or less than 0 C. the melting point of ice.
Ivory soap floats because it is, as advertised, "99.44% pure". The 0.56%
is air intentionally whipped into the molten soap during the manufacture to
give the overall product a density slightly less than water. The marketing
ploy has served them well for decades! Although its only benefit is to help
find the soap or to let babies make soap boats in the tub.
Click here to return to the Physics Archives
Update: June 2012