It is possible to boil water in a flask. Boiling occurs when the temperature of a liquid reaches its boiling point and the vapor pressure within the liquid becomes equal to the atmospheric pressure. In the case of the flask containing water and water vapor at 100°C, pouring ice water over the outside of the flask causes the water inside to boil. This may seem counterintuitive since the temperature of the water inside the flask is actually decreasing, but there are a few factors at play here.
Firstly, the ice water that is poured over the flask cools down the glass and the water vapor inside. This decrease in temperature causes the water vapor to condense back into liquid water. As the water vapor condenses, it releases heat, which warms up the liquid water inside the flask. This additional heat input can raise the temperature of the water inside the flask to its boiling point.
Secondly, the ice water also cools down the surrounding air. When the air temperature around the flask decreases, the atmospheric pressure also decreases. The boiling point of a liquid is directly related to the pressure exerted on it. As the atmospheric pressure decreases, the boiling point of the water inside the flask decreases as well. This means that even though the water inside the flask is cooling down, it can still reach its boiling point due to the lowered atmospheric pressure.
As a result, when ice water is poured over the outside of the flask, the combined effects of heat released from condensing water vapor and the decreased atmospheric pressure allow the water inside the flask to reach its boiling point and boil.
This phenomenon can be observed in various real-life situations. For example, when camping, one may use a camping stove that utilizes a small flask of liquid fuel. By heating the fuel inside the flask, it can reach its boiling point and vaporize, generating a flame for cooking. Despite the temperature of the fuel being much lower than the boiling point of water, boiling still occurs due to the pressure generated by the fuel vapor.
Boiling can indeed occur in a flask. The combination of heat release from condensing water vapor and the decreased atmospheric pressure caused by cooling can allow the water inside the flask to reach its boiling point and boil, even if the temperature is decreasing.