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?

I had assumed that the pump was left on to account for air that would leak back in.  But if the air is pumped out, then the system is sealed, then ravi and Devin are correct that as water evaporates, vapor pressure will equilibrate and no further evaporation or boiling will occur.

But it will definitely boil once the pressure drops below about 20 mm Hg.  At that point vapor pressure all through the liquid water is greater than the air pressure above.  The boiling and/or evaporation will cool the remaining water.  If the pump continues to work, and if it is powerful enough to pump to a near vacuum, then boiling will continue.  I don't know if the continued loss of heat will be enough for the remaining water to freeze at some point.

Posted by Larry on 2005-07-23 13:23:33