Why is it so that when a CD is depositted on the surface of water it
floats, but if it's pushed down it will sink?

Hi tesla:

Thanks for the fascinating question. The reason that the CD floats
when placed gently on the water's surface is because of *surface
tension*. The following site explains this phenomenon well.

URL: http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html
Quote: "The cohesive forces between liquid molecules are responsible
for the phenomenon known as surface tension. The molecules at the
surface do not have other like molecules on all sides of them and
consequently they cohere more strongly to those directly associated
with them on the surface. This forms a surface "film" which makes it
more difficult to move an object through the surface than to move it
when it is completely submersed.
Surface tension is typically measured in dynes/cm, the force in dynes
required to break a film of length 1 cm. Equivalently, it can be
stated as surface energy in ergs per square centimeter. Water at 20°C
has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl
alcohol and 465 for mercury."

Another variable that comes into play is the nature of the material
being floated. Metal objects tend to be hydrophobic and a CD, as you
know, is very light for its surface area. See below for more
information.

The General Chemistry Demo Lab
URL: http://www.ilpi.com/genchem/demo/tension/
Quote: "If a solid material more dense than water is placed on the
surface of water, what happens next depends on the nature of the
material. If the material is hydrophilic ('water loving') it has a
surface to which water is attracted. The adhesion of water to the
surface of this material coats the surface of the object with water,
reduces the surface tension, and causes the object to sink.
If the solid object is hydrophobic ('water fearing'),the unfavorable
interactions between the water surface and the object make it
difficult to wet the surface. Two forces now come into play -- the
energy it would take to overcome this repulsion and the force of
gravity. If the force of gravity is strong enough, it will prevail and
the object will sink (assuming that the object has a density greater
than water). If the gravitational force is less than the surface
tension then the object will float on the surface of the water."

I hope this information is of help to you.           
          
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Thank you very much. I thought it had to do with that, but I didn't
know enough to be sure. Your answer is what I wanted.