Experimental measurements show that the equilibrium vapour pressure of a liquid increases as the temperature increases. In the temperature range in which the vapour pressure is small, it is relatively insensitive to the temperature, but the vapour pressure grows at an increasing rate as the temperature is raised. The temperature at which the equilibrium vapour pressure bccomcs equal to 1 atm is callcd the normal boiling temperature, or the boiling point. In the boiling process, bubbles of vapour form throughout the bulk of the liquid. In other words, evaporation occurs anywhere in the liquid, not just at the upper surface. The reason that this occurs only when the vapour pressure equals the atmospheric pressure is easy to understand. In order for a bubble to form and grow the pressure of the vapour inside the bubble must be at least equal to the pressure exerted on it by the liquid. This, in turn, is equal to the pressure of the atmosphere plus the very small pressure due to the weight of the liquid above the bubble. Therefore, bubble formation and boiling occur only when the vapour pressure of the liquid is equal to the pressure of the atmosphere.
The initiation of a bubble in the bulk of a pure liquid is a very difficult proccss, sincc it requires that many molcculcs with kinctic energies greater than that required for vaporization must be closc to one another. Hence, the fact that the liquid reaches the boiling temperature is no guarantee that boiling will occur. If it docs not, continued addition of heat will causc the liquid to bccomc superheated, that is, to reach a temperature greater than its boiling point. When finally occurring in a superheated liquid, the bubble formation proceeds with almost explosive violence, bccausc the vapour pressure in any bubble formed greatly exceeds atmospheric pressure, the bubbles tending to expand rapidly. Such violent boiling can be avoided introducing agents which initiate bubbles in the liquid as soon as the boiling temperature is rcachcd. Porous picccs of ceramic material which evolve small bubbles of air into which evaporation can occur serve very well in this application.
Упр. 3. Передайте основное содержание текста в нескольких предложениях.
Упр. 4. Какие слова в тексте означают «в свою очередь»?
Упр. 5. Найдите в тексте ответы иа следующие вопросы:
1. In what way docs the vapour pressure changc at equilibrium? 2. What is the normal boiling temperature? 3. Under what conditions does the initiation of a bubble in the bulk of a pure liquid occur? 4. When does the liquid bccomc superheated? 5. What arc porous picccs of ccramic material used for?
Упр. 6. Закончите следующие предложения:
1. The vapour pressure grows at an increasing rate... 2. The boiling point of a liquid is... 3. Bubble formation and boiling occur only when... 4. The fact that the liquid reaches the boiling temperature is no guarantee that... 5. Violent boiling can be avoided...
Section III
Ex. 1. Make up short dialogues according to the example. Example:
— What kind of equipment do students learn to use in your lab?
— First of all, we learn to handle test-tubes, beakers, funnels, burners and so on.
— It's quite natural. You can't do without all these things if you area chcmist.
Ex. 2. Respond to the following statements:
1. The processes of evaporation in an open container and in a closed one arc not the same. 2. It is interesting to consider the time dependence of the evaporation and condensation rates, isn't it? 3. One can easily describe what happens in a liquid when it begins boiling.
Ex. 3. Give detailed answers to the questions.
1. In what way docs vaporization procccd? 2. What is liquid-vapour equilibrium? 3. How docs temperature influence the vapour pressure?
Ex. 4. Discuss the following topics:
1. The Vaporization Proccss.
2. The Phase Equilibrium.
3. Factors Influencing the Vapour Pressure.