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Check Your Understanding Why does snow often remain even when daytime temperatures are higher than the freezing temperature?

Snow is formed from ice crystals and thus is the solid phase of water. Because enormous heat is necessary for phase changes, it takes a certain amount of time for this heat to be transferred from the air, even if the air is above 0 ° C .

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Summary

  • Most substances have three distinct phases (under ordinary conditions on Earth), and they depend on temperature and pressure.
  • Two phases coexist (i.e., they are in thermal equilibrium) at a set of pressures and temperatures.
  • Phase changes occur at fixed temperatures for a given substance at a given pressure, and these temperatures are called boiling, freezing (or melting), and sublimation points.

Conceptual questions

A pressure cooker contains water and steam in equilibrium at a pressure greater than atmospheric pressure. How does this greater pressure increase cooking speed?

It raises the boiling point, so the water, which the food gains heat from, is at a higher temperature.

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As shown below, which is the phase diagram for carbon dioxide, what is the vapor pressure of solid carbon dioxide (dry ice) at −78.5 ° C ? (Note that the axes in the figure are nonlinear and the graph is not to scale.)

Figure shows a graph of pressure in atmosphere versus temperature in degrees Celsius for carbon dioxide. The curve goes up and right to reach the triple point, which is at 5.11 atmosphere and minus 56.6 degrees Celsius. From here, the curve branches. One branch goes up almost vertically, the other goes up and right to the critical point. This is at 73 atmosphere and 31 degrees Celsius. The area left of the vertical branch is solid, the area between two branches is liquid and that to the right of the right branch is vapor.
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Can carbon dioxide be liquefied at room temperature ( 20 ° C )? If so, how? If not, why not? (See the phase diagram in the preceding problem.)

Yes, by raising the pressure above 56 atm.

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What is the distinction between gas and vapor?

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Heat transfer can cause temperature and phase changes. What else can cause these changes?

work

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How does the latent heat of fusion of water help slow the decrease of air temperatures, perhaps preventing temperatures from falling significantly below 0 ° C, in the vicinity of large bodies of water?

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What is the temperature of ice right after it is formed by freezing water?

0 ° C (at or near atmospheric pressure)

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If you place 0 ° C ice into 0 ° C water in an insulated container, what will the net result be? Will there be less ice and more liquid water, or more ice and less liquid water, or will the amounts stay the same?

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What effect does condensation on a glass of ice water have on the rate at which the ice melts? Will the condensation speed up the melting process or slow it down?

Condensation releases heat, so it speeds up the melting.

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In Miami, Florida, which has a very humid climate and numerous bodies of water nearby, it is unusual for temperatures to rise above about 38 ° C ( 100 ° F ). In the desert climate of Phoenix, Arizona, however, temperatures rise above that almost every day in July and August. Explain how the evaporation of water helps limit high temperatures in humid climates.

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In winter, it is often warmer in San Francisco than in Sacramento, 150 km inland. In summer, it is nearly always hotter in Sacramento. Explain how the bodies of water surrounding San Francisco moderate its extreme temperatures.

Because of water’s high specific heat, it changes temperature less than land. Also, evaporation reduces temperature rises. The air tends to stay close to equilibrium with the water, so its temperature does not change much where there’s a lot of water around, as in San Francisco but not Sacramento.

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Questions & Answers

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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