Answer
The vacuum pump is turned on, and the external pressure over the liquid inside the flask decreases a lot. Now the vapor pressure of the liquid is more than the external pressure, so the water begins to boil. As the water boils, it takes a lot of heat from the remaining water and thereby reduces the temperatures below freezing point inside the flask, so the same water begins to freeze after sometime. Because of very low pressure inside the flask, ice sublimes eventually.
Work Step by Step
The vacuum pump is turned on, and the external pressure over the liquid inside the flask decreases a lot. Now the vapor pressure of the liquid is more than the external pressure, so the water begins to boil. As the water boils, it takes a lot of heat from the remaining water and thereby reduces the temperatures below freezing point inside the flask, so the same water begins to freeze after sometime. Because of very low pressure inside the flask, ice sublimes eventually.
