Please explain to me?

Question

an insulated beaker containing water is placed in a vessel. explain what happen to the water when the gas in the vassel is evacuated using a vacuum pump

Answer 1

The water will start to boil.

A liquid boils when its vapor pressure at a given temperature exceeds the ambient pressure.  Normally, to boil water, one increases the temperature, increasing the vapor pressure.  In this case, we decrease the ambient pressure until it is less that the vapor pressure of the water at the ambient temperature.

The boiling of the water will remove heat from the liquid water (because the container is insulated, not much heat will enter from the environment), so the liquid water remaining will cool down (and its vapor pressure will drop).  If the water is allowed to cool enough, it will actually freeze!