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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
When you heat the water in the flask and stopper it, the vapor pressure inside remains high. Running cool water over the neck of the flask lowers the internal pressure as the air and vapor molecules lose energy. This lower internal pressure creates a pressure difference, with the vapor pressure now greater than the internal pressure, causing the water to boil again. The phenomenon appears as if you are boiling water by cooling it, but it's actually due to the pressure difference created by cooling the neck of the flask.

Step by step solution

01

Understanding Boiling and Vapor Pressure

Boiling occurs when the vapor pressure of a liquid becomes equal to the atmospheric pressure. In this case, the liquid is water. Vapor pressure is the pressure exerted by the vapor molecules on the surface of the liquid when they are in equilibrium with the liquid molecules. When the vapor pressure equals atmospheric pressure, bubbles of vapor form throughout the liquid, and we observe boiling.
02

Heating the Water and Trapping the Vapor

When you heat the water in the round-bottom flask, you increase its temperature and consequently its vapor pressure. By reaching the boiling point, you have created a condition in which the water's vapor pressure equals the atmospheric pressure outside the flask. When you remove the heat, the boiling stops, but the vapor pressure inside the flask remains relatively high. Upon stoppering the flask, you effectively seal the vapor pressure inside.
03

Cooling the Neck of the Flask and Lowering the Internal Pressure

By running cool water over the neck of the flask, you cause the air and vapor molecules inside to lose energy and move slower. This results in a decrease in pressure inside the flask. The cooler temperature at the neck area ensures that vapor near the neck condenses into liquid, allowing the closed space to maintain an equilibrium without continuous boiling.
04

Boiling by Cooling - A Pressure Difference

With the lower internal pressure, the required vapor pressure for boiling is now lower, too. From Step 3, the internal pressure of the flask has already decreased to a level where it is lower than the vapor pressure of water at the current temperature inside. Since the vapor pressure is now greater than the internal pressure, boiling starts again. The phenomenon appears as if you are boiling water by cooling it. In reality, it's caused by the pressure difference created due to the cooling of the neck of the flask.

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Most popular questions from this chapter

Consider the following melting point data: $$ \begin{array}{|llllllll|} \hline \text { Compound: } & \mathrm{NaCl} & \mathrm{MgCl}_{2} & \mathrm{AlCl}_{3} & \mathrm{SiCl}_{4} & \mathrm{PCl}_{3} & \mathrm{SCl}_{2} & \mathrm{Cl}_{2} \\ \operatorname{mp}\left({ }^{\circ} \mathrm{C}\right): & 801 & 708 & 190 & -70 & -91 & -78 & -101 \\ \text { Compound: } & \mathrm{NaF} & \mathrm{MgF}_{2} & \mathrm{AlF}_{3} & \mathrm{SiF}_{4} & \mathrm{PF}_{5} & \mathrm{SF}_{6} & \mathrm{~F}_{2} \\ \operatorname{mp}\left({ }^{\circ} \mathrm{C}\right): & 997 & 1396 & 1040 & -90 & -94 & -56 & -220 \\ \hline \end{array} $$ Account for the trends in melting points in terms of interparticle forces.

When wet laundry is hung on a clothesline on a cold winter day, it will freeze but eventually dry. Explain.

Cake mixes and other packaged foods that require cooking often contain special directions for use at high elevations. Typically these directions indicate that the food should be cooked longer above \(5000 \mathrm{ft}\). Explain why it takes longer to cook something at higher elevations.

The molar enthalpy of vaporization of water at \(373 \mathrm{~K}\) and \(1.00\) atm is \(40.7 \mathrm{~kJ} / \mathrm{mol}\). What fraction of this energy is used to change the internal energy of the water, and what fraction is used to do work against the atmosphere? (Hint: Assume that water vapor is an ideal gas.)

In each of the following groups of substances, pick the one that has the given property. Justify each answer. a. highest boiling point: \(\mathrm{CCl}_{4}, \mathrm{CF}_{4}, \mathrm{CBr}_{4}\) b. lowest freezing point: \(\mathrm{LiF}, \mathrm{F}_{2}, \mathrm{HCl}\) c. smallest vapor pressure at \(25^{\circ} \mathrm{C}: \mathrm{CH}_{3} \mathrm{OCH}_{3}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\), \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) d. greatest viscosity: \(\mathrm{H}_{2} \mathrm{~S}, \mathrm{HF}, \mathrm{H}_{2} \mathrm{O}_{2}\) e. greatest heat of vaporization: \(\mathrm{H}_{2} \mathrm{CO}, \mathrm{CH}_{3} \mathrm{CH}_{3}, \mathrm{CH}_{4}\) f. smallest enthalpy of fusion: \(\mathrm{I}_{2}, \mathrm{CsBr}, \mathrm{CaO}\)
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