3 Part One. The Predicate

Liquid-Vapour Equilibrium
A liquid of relatively low boiling temperature, when placed in a container open to the atmosphere, will eventually evaporate entirely. Remembering that molecules in the liquid are “bound" by attractive forces to their neighbours, We might ask why some are able to overcome these forces and leave the liquid spontaneously. The answer lies in a considerationof the possible magnitudes of molecular kinetic energies, for these, as We have already mentioned, range from very low to very high values, and are distributed according to the MaxWell-Boltzmann law. Therefore, even if the average potential energy which binds the molecules to the liquid is substantial, there are always some molecules which have enough kinetic energy to overcome the binding forces and enter the vapour. According to the MaxWell-Boltzmann law, the fraction of the molecules which have kinetic energies greater than some minimum value e, the value required for the molecules to leave the liquid, is proportional to the Boltzmann factor, e^lkT. Therefore, the temperature remaining constant, the fraction of liquid molecules with enough kinetic energy to evaporate remains the same, and evaporation continues. The vessel being open to the atmosphere, vapour molecules are swept away, and evaporation continues until no liquid is left.

Now, let us analyse what happens when a liquid is placed in a closed evacuated container. Immediately, the liquid starts to evaporate at a rate which is primarily determined by the fraction of molecules which have enough kinetic energy to overcome attractive forces and leave the surfacc.

Initially, the rate of condensation is zero, there being no molecules in the vapour. As long as the temperature stays constant, evaporation continues at a constant rate, and the number of molecules in the vapour phase increases. Consequently, the rate of condensation starts to increase, for as the pressure of the vapour grows, the number of gas molecules which collidc with and reenter tlic liquid surfacc also increases.

The time dcpcndcncc of the evaporation and condensation rates is worth considering. While growing, the condensation rate eventually becomes equal to the rate of evaporation. At this time, the number of molecules which enter and which leave the vapour per unit time is the same, and, consequently, the pressure of the vapour stops increasing and remains constant. If the system is left undisturbed at a fixed temperature, evaporation and condensation continue at equal rates, and the pressure of the vapour remains unchanged. This, then, is a situation of equilibrium between the two phases. Note particularly that at equilibrium, evaporation and condensation do not stop, but that the constancy of the equilibrium vapour pressure is a conscqucncc of these opposing processes proceeding at equalrates. Thus We say that phase equilibrium is dynamicin nature.

Words and Word-Combinations to Be Memorized

analyse, attractive, bind, constant, dependence, initial, magnitude, note, primarily, proportional, situation, spontaneous, substantial, it was not until... that, not until, vessel, zero

Ex. 5. Give the Uzbek equivalents Гof the following:

evaporate, entirely, overcome the forces, leave the liquid, the magnitude of the energy, range from... to, substantial, overcome the binding forces, be proportional to, remain the same, start immediately, the dependence isworth considering, become equal to, per unit time, consequently, at a fixed temperature, remain unchanged, opposing processes

Ex. 6. Give the English equivalents for the following:

nisbatan past harorat, idishga joylashtiring, tortish kuchini yengib chiqing, o'z-o'zidan, o'rtacha potentsial quvvat, ochiq idish

Ex. 7. Fill in the blanks with articles where necessary.

1. Let us analyse what happens when ... liquid is placed in ... closed evacuated container. 2. As long as ... temperature remains constant, ... evaporation continues a t... constant rate. 3. As ... pressure o f ... vapour grows,... number o f ... gas molecules returning to ... liquid also increases.4 time dependence o f... evaporation and condensation rates should beconsidered. S. At equilibrium,... evaporation and condensation do not stop.

Ex. 8. Give synonyms for the following:

magnitude, mention, substantial, fraction, remain, place, primarily, initially, increase, equal

Ex. 9. Give antonyms for the following:

low, open, leave, minimum, the same, continue, container, immediately, start, also, consequently

Ex. 11. Translate the sentences into English without using a dictionary.

1. Agar past qaynash nuqtasi bo'lgan suyuqlik ochiq idishda bo'lsa, u oxir-oqibat to'liq bug'lanadi. 2. Suyuq molekulalar qo'shni molekulalar bilan bog'lanadi. 3. Ba'zi molekulalar bog'lovchi kuchlarni yengib, bug'ga o'tish uchun yetarli kinetik energiyaga ega. 4. Agar harorat o'zgarmasa, bug'lanish doimiy tezlikda davom etadi. 5. Agar suyuqlik yopiq idishda bo'lsa, unda kondensatsiya jarayoni ko'rib chiqilishi kerak.

Ex. 12. Answer the following questions:

1. What happens with a liquid if it is placed in an open container? 2. What forccs act between the molecules in a liquid? 3. Under what conditions can some molecules leave the liquid? 4. What happens if a liquid is placed in a closed evacuated container? 5. What does the rate of condensation depend upon? 6. What situation is called equilibrium between the liquid and vapour phases?