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Name: Kathryn M.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: Saturday, October 12, 2002


Question:
Today in my science class, my teacher boiled water in a stoppered beaker. He placed the beaker with the boiling water in ice cold water. It continued to boil in the ice water. Why did this happen?


Replies:
Kathryn,

He very likely used a flask -- not a beaker. When he stoppered the flask containing boiling water and then placed it in the ice cold water, the steam inside the flask (which drove out the air) condensed into liquid water. This temporarily created a vacuum inside the flask. Because the water was still rather hot, it boiled because the confining pressure inside the flask was less than the atmospheric pressure outside it.

He could have made ordinary (room temperature) tap water boil had he placed the water in an evacuated chamber. I have done such a demonstration for my own students and they were astounded to discover that the water (after being subjected to "boiling" in the vacuum chamber) was actually colder than when it was first placed in the chamber.

The temperature at which water boils is dependent on the (atmospheric) pressure that confines it to its container. At standard atmospheric pressure (14.7 pounds per square inch) water boils at 212 F. If the pressure is lowered, the boiling point is likewise lowered. That's why it is so very difficult to boil (cook) an egg atop Mt. Everest -- just cannot get the water hot enough.

Regards,
ProfHoff 500


He did not explain why??? Glass is a poor conductor of heat, and water has a large heat capacity (about 1cal/gm). So when the boiling water is placed in a cold bath, it takes a while -- a matter of some seconds depending upon how thick the glass is, and whether or not the glass has been heated several degrees above the boiling point of water, even though the hot water temperature will still be essentially 100 C. Unless he has arranged some "magic" experiment, and something else is going on here, the water will stop boiling in a matter of seconds and the beaker will begin to cool.

Vince Calder


Kathryn,

I suspect you did not get all the facts. Are you sure the beaker was actually stoppered when the water was boiled initially? I think it was probably only loosely covered with the stopper. In this case the air in the flask would be displaced by the water vapor being generated by the boiling action. When all the air had been displaced with water vapor the teacher probably then securely stoppered the flask shortly after removing it from the heat so that no air could get back into the flask as it cooled down.

As the water vapor cools it condenses out until the pressure inside the flask reaches the vapor pressure of water. At this point the water in the bottom of the flask is at equilibrium with the water vapor.

Now the flask is immersed in ice water. Very quickly the sides of the flask cool off at the top and the vapor starts to condense, reducing the vapor pressure in the flask (to the vapor pressure at 0 C). But the liquid water is still warm and has a relatively high vapor pressure, higher than the pressure over it. Since it is at a temperature such that it's vapor pressure is higher the pressure of the vapor above it, it boils. It will continue to boil until the liquid water cools to nearly the same temperature as the gas above it.

Greg Bradburn



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