A jar filled partially with water has an object floating in it. The jar is open. Now, if the jar is closed and a vacuum pump is used to pump the air out, what will happen to the floating object? will it rise up more or sink or ...? and why?
My first impression is that the previous posters are overcomplicating the question a bit. With regard to the air pressure removed, the force on both the object AND the water is removed, and this in itself does not affect the position of the object, except infinitesimally by the change in water pressure (since water is almost, but not quite, incompressible).
Secondly, I agree the water would evaporate, but why boil? It is much easier for the evaporating water to leave from the surface than from somewhere inside, so that's what will happen (boiling would only occur if differential heat were applied to the bottom, or something like that).
Last, I read the question as: the vacuum pump is used to remove the air, but is not left on;
so enough water would evaporate to bring the vapour pressure to an equilibrium, then evaporation would cease.
To summarize: The object would first sink very very slightly from the reduction in water pressure; water would then start to evaporate, dropping the level of the whole system, but not of the object with respect to the water level; and at some point the evaporation would equilibrate, and then nothing else would happen.