Question

A closed vessel is half filled with water. There is a hole near the top of the vessel and air is pumped out from this hole.
(a) The water level will rise up in the vessel.
(b) The pressure at the surface of the water will decrease.
(c) The force by the water on the bottom of the vessel will decrease.
(d) The density of the liquid will decrease.

Answer 1

(b) The pressure at the surface of the water will decrease.
(c) The force by the water on the bottom of the vessel will decrease.

EXPLANATION:  

The option (a) is not true because due to gravity the water level will remain the same in the closed vessel.              

Initially, the air over the water surface is at atmospheric pressure so the pressure at the surface of the water is equal to the atmospheric pressure. When air is pumped out the pressure at the surface of the water decreases to zero because there is a vacuum now. Option (b) is true.             

Initially, the force by the water on the bottom of the vessel is equal to the force by the air plus the force by the water. When the air is pumped out the force by the air is absent. Therefore the total force by the water on the bottom of the vessel will decrease. Option (c) is correct.

Since the water is liquid and we assume it to be incompressible there will be hardly any change in the volume. Mass will also remain the same so there will be no change in the density when the air above the water is pumped out. Option (d) is not true.