Chapter 10

Rank the following compounds in order of increasing vapor pressure at \(298 \mathrm{K}\). (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} ;\) (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Explain the difference between sublimation and evaporation.

Can ice be melted merely by applying pressure? Explain your answer.

What phases of a substance are present (a) at its triple point and (b) at its critical point?

Explain how the solid-liquid line in the phase diagram of water differs in character from the solid-liquid line in the phase diagrams of most other substances, such as \(\mathrm{CO}_{2}\).

Which phase of a substance (gas, liquid, or solid) is most likely to be the stable phase: (a) at low temperatures and high pressures; (b) at high temperatures and low pressures?

Freeze-drying is used to preserve food at low temperature with minimal loss of flavor. Freeze-drying works by freezing the food and then lowering the pressure with a vacuum pump to sublime the ice. Must the pressure be lower than the pressure at the triple point of \(\mathrm{H}_{2} \mathrm{O}\) ? Why or why not?

Solid helium cannot be converted directly into the vapor phase. Does the phase diagram of helium have a triple point?

Consult Figures \(10.26,10.28,\) and 10.29. List the steps you would take to convert a 10.0 -g sample of water at \(25^{\circ} \mathrm{C}\) and 1 atm of pressure to water at its triple point.

Explain why a needle floats on the surface of water but sinks in a container of methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\).

Explain why different liquids do not reach the same height in capillary tubes of the same diameter.

Explain why pipes filled with water are in danger of bursting when the temperature drops below \(0^{\circ} \mathrm{C}\).

A hot needle sinks when put on the surface of cold water. Will a cold needle float in hot water? Explain your answer.

The meniscus of water in a glass tube is concave, but that of mercury (Figure \(\mathrm{P} 10.95)\) is convex. Explain why.

The mercury level in a capillary tube placed in a dish of mercury is below the surface of the mercury in the dish. Explain why.

Describe the origin of surface tension at the molecular level.

What is the cause of the high viscosity of molasses?

Describe how the surface tension and viscosity of a liquid are affected by increasing temperature.

Explain how strong intermolecular forces are expected to result in a relatively high surface tension and viscosity of a liquid.

One of two glass capillary tubes of the same diameter is placed in a dish of water and the other in a dish of ethanol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\right) .\) Which liquid will rise higher in its tube? Explain your answer.

A simple viscometer consists of a thick-walled glass tube with a 0.5 -mm bore. The tube has etched marks at one-quarter and three-quarters of its height. The tube is clamped with its lower end dipped in a container of the liquid to be tested. A pipette filler is used to draw liquid up the bore past the upper mark. The pipette filler is removed from the tube, and the time taken for the liquid meniscus to drain between the upper and lower viscometer marks is measured with a stopwatch. Using this viscometer to measure the drain times for two pure liquids \(A\) and \(B\) at the same temperature gives 3.45 seconds for liquid \(\mathrm{A}\) and 4.64 seconds for liquid B. a. Which liquid is more viscous? b. Which liquid has weaker intermolecular forces? c. Would the measured drain times be longer or shorter at a lower experimental liquid temperature?

Why do ethers typically boil at lower temperatures than alcohols with the same molecular formula?

Does the sublimation point of ice increase or decrease with increasing pressure? Explain why.

Does the sublimation point of \(\mathrm{CO}_{2}\) increase or decrease with increasing pressure? Explain why.

Liquid substances are often compared for their physical properties in different applications. Comparison of two liquids \(\mathrm{A}\) and \(\mathrm{B}\) at constant temperature and atmospheric pressure shows that liquid A has higher viscosity and surface tension, a higher boiling point, and lower vapor pressure than liquid B. Are these data all consistent with stronger intermolecular forces in liquid A than in liquid B?

Why is methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\) miscible with water, whereas \(\mathrm{CH}_{4}\) is almost completely insoluble in water?

Explain why water climbs higher in a capillary tube than in a test tube.

Explain why ice floats on water.

Explain why fish in a pond die if water becomes too warm.

Refrigerators have a unit called a compressor that liquefies a gas. The refrigerator is cooled by a continuous cycle of compression of the gas to produce the liquid, followed by evaporation of the liquid to provide the cooling. Ammonia (NH \(_{3}\) ) and sulfur dioxide \(\left(\mathrm{SO}_{2}\right)\) were the gases used originally, and hexafluoroethane \(\left(\mathrm{C}_{2} \mathrm{F}_{6}\right)\) has been used since the 1990 s. What are the intermolecular interactions that characterize these substances?

Disposable wipes used to clean the skin before receiving an immunization shot contain ethanol. After the nurse wipes your arm, your skin feels cold. Why?