Physics Lab?

Question

In a lab where we boiled a flask of water, pluged it, tipped it over then set a bag of ice on the bottom(now the top), why does the water boil?
Please include as much detail as possible.

Thank you

Answer 1

Your experiment was a demonstration of the ideal gas law, which establishes a specific relationship between the temperature, pressure, and volume of a gas. Let me explain:

1) When you boiled the flask of water and then plugged the top, the temperature of the gas trapped in the top of the flask would have been approximately the normal boiling point of water, 100 degrees C.

2) By plugging the top of the flask, you are, in effect, creating a small system inside the flask where the volume of the gas is held constant -- no gas goes in or out. In this situation, according to the ideal gas law, the pressure and the temperature of the gas will increase or decrease together. So, when you flip the flask over and place the bag of ice on its surface, the ice lowers the temperature of the gas inside, which also lowers the pressure of the gas, according to the law.

3) As the pressure of the gas decreases, this in turn decreases the boiling point of the water inside. Why? Because a liquid boils when its 'vaporization pressure' equals the pressure of the atmosphere around it. In other words, as a liquid gets hotter, the individual atoms or molecules move around with increasing force, and at some point they begin to break free of the liquid -- this tendency for particles to break free is the vaporization pressure, and obviously it increases as the temperature of the liquid goes up. At the boiling point, the vaporization pressure matches the pressure that the surrounding atmosphere is exerting on the liquid, and at this point particles begin to break free and vaporize all throughout the liquid.

In your experiment, the water in the flask had cooled a few degrees below its boiling point under normal atmospheric pressure (100 C). However, when the ice lowers both the temperature and the pressure of the gas (the atmosphere) surrounding the water, all of a sudden the atmospheric pressure drops below the vaporization pressure of the water (which is still very high, since water cools slowly), and so the water boils again, at a temperature less than 100 C. Interesting stuff.

Hope this answer isn't too long. I wasn't sure how much you'd like explained.

Answer 2

You boiled the water before plugging the flask. This means that the gas over the water is now at the same temperature as the water. Heat increases the air pressure of a gas. Now adding the ice on top of the flask lowers the temperature of the air, lowering the air pressure, reducing the boiling point of the water. This is the same phenomenon as water boiling at a lower temperature at high altitudes, where the air pressure is lower.