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?
First imagine a jar that the bottom half is filled with water with density 1.0 and the top half is filled with a light oil with density 0.5. Now place a disk with density 0.75 in the jar. It will position itself exactly in the middle - the lower half in the water and the upper half in the oil.
Now decant the oil. The oil is replaced with air, but in this case the density of the air is negligible. The disk will sink so that it is 3/4 down in the water, with only 1/4 exposed. Also, the level of the water will rise slightly as the volume needed to hold the sum of the water and submerged portion of the disk is increased by 1/4 the volume of the disk. However, this increase in the water's surface level will be less than the decrease in the level of the surface of the disk. The net result is the disk sinks down relative to the jar.
Now put the lid on and pump out the air. (Ignore the effect a vacuum has on liquids and assume the water will remain in its liquid state.) Air has a density about 0.001275, and a vacuum has density 0. The same scenario as above happens, just on a much smaller scale - the disk sinks slightly lower.