Why did copper sink in CYCLOHEXANE, but float in water?

I'm guessing that cyclohexane is less dense than copper and water.

Surface tension with the water; you can also float a razor blade.


Sure, cyclohexane is less dense than copper or water, but that's not the point!

Since copper is considerably more dense than either water or cyclohexane, then you had a very thin, and very light strip of copper and it floated on the water because the downward force of gravity on the copper was balanced by the upward force on the copper by the surface tension of water.  

Water has relatively strong hydrogen bonds which provide for a much higher surface tension than cyclohexane has.  All molecules exhibit London dispersion forces (LDF's).  The LDF's for water are less than the hydrogen bonds, another type of intermolecular force, that water exhibits.  Because of your experiment we can say that hydrogen bonding in waer is stronger than the attractions due to London dispersion forces in cyclohexane.  Cyclohexane does not exhibit hydrogen bonding.

Water is different from cyclohexane in that a water molecule has a relatively strong dipole moment which results in strong intermolecular forces, called hydrogen bonding of water molecules. This characteristic force is found greatly on the surface of water as the water molecules tend to line up flatly on the surface due to this dipole-dipole interaction and thereby contributes significant surface tension to hold up a strip of copper metal on its surface.
Whereas, in comparison, cyclohexane is a straight chain aliphatic with no dipole moment, and therefore, cyclohexane has insignificant intermolecular forces in comparison with water, and so cyclohexane does not have a strong film of surface tension, so the copper strip sinks.
The density of the copper metal is insignificant in this case.