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Chapter 10: Problem 3

Explain the following: You add \(100 \mathrm{~mL}\) water to a \(500-\mathrm{mL}\) round-bottom flask and heat the water until it is boiling. You remove the heat and 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

Expert verified
In summary, running cool water over the neck of the stoppered flask reduces the pressure inside, causing the boiling point of the water to decrease. The water inside the flask is still hot enough to boil at this new lower boiling point, hence the boiling resumes. This phenomenon is due to the change in pressure and subsequent lowering of the boiling point, not the act of cooling the water itself.

Step by step solution

01

Understand the initial situation

Initially, we have a round-bottom flask containing 100 mL of water. The flask is heated until the water reaches boiling point (approximately \(100^\circ \mathrm{C}\) at sea level).
02

Observe the change after removing heat

After the heat source is removed and the flask is stoppered, the boiling stops because the energy source (heat) is removed, and the flask is sealed to not allow any more heat or gas exchange from the surroundings.
03

Analyze the effect of running cool water on the neck

When cool water is run over the neck of the stoppered flask, the pressure inside the flask starts decreasing. Cooling the neck of the flask causes the vapor inside to condense back into a liquid as the temperature drops, reducing the gas pressure inside the flask.
04

Understand the relation between pressure and boiling point

The boiling point of water depends on the atmospheric pressure. At a lower pressure, the boiling point of the water decreases, meaning that water boils at a lower temperature than it would at sea level.
05

Relate the reduced pressure in the flask with boiling point

As the pressure inside the flask decreases due to the cooling of the neck, the boiling point of the water inside the flask also decreases. Since the flask still has heat energy stored from the initial boiling, the cool water running outside the neck doesn't significantly affect the temperature of the water inside the flask. In simpler terms, the water inside is still hot enough to boil at the reduced pressure.
06

Explain the observed phenomenon

Now that the pressure inside the flask has been reduced, the boiling point of the water has also decreased, allowing the still-hot water to start boiling at the new lower boiling point. Thus, the phenomenon observed, where boiling seems to begin again while cooling, is actually due to the change in pressure and the resulting decrease in the water's boiling point within the flask.

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