Question

Explain the following: You add 100 mL of water to a 500-mL round-bottom flask and heat the water until it is boiling. You remove the heat, stopper the flask, and the boiling stops. You then run cool water over the neck of the flask, and the boiling begins again. It seems as though you are boiling water by cooling it.

Short answer

100 mL of water has been added to a 500-mL round-bottom flask and heated the water until it boils. Now if we add stopper on the flask, it no longer is in contact with the surroundings. Therefore, the pressure inside the flask will change accordingly with the system. Now if the boiling water flask has been placed underneath cool boiling begins again. This is because the reduced pressure will reduce the boiling point of the liquid until equilibrium is attained

Step-by-step solution

Boiling point

Boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure exerted by the surroundings on liquid. All liquids have some sort of vapor pressure where the surface of the liquid is in equilibrium with turning into the gas phase, and that gas is re condensing into the liquid phase and at the higher the temperature, the more water vapor will be produced.

Explanation

In case of water the vapour pressure equals the pressure surrounding at 100⁰ C. The water vapor pressure will equal the pressure, which is one atmosphere or 760 millimeters of mercury. When the vapour pressure and surrounding pressure equals, the liquid changes into vapor. Therefore, it completely evaporates and boils. Now if we add stopper on the flask, it no longer is in contact with the surroundings. Therefore, the pressure inside the flask will change accordingly with the system. Now if the that boiling water flask has been placed underneath cool boiling begins again.

It can be explained by Gay Lussac's law of pressure. It has been stated that pressure is directly proportional to temperature at constant volume. If the content is kept at equilibrium, when the cool water is poured on the bottle, it takes heat from the gas plus the liquid also, reducing the temperature. The pressure inside the closed container is reduced also. Then the liquid will boil again as reduced pressure will reduce the boiling point of the liquid until equilibrium is attained.