When iodine crystals arc heated to 114°C, they melt forming liquid iodine. The temperature at which the crystals and the liquid arc in equilibrium — that is, at which crystals have no tendency to melt or the liquid has no tendency to freeze—is called the melting point of the crystals, and the freezing point of the liquid. This temperature is 114°С for iodine.
Liquid iodine differs from solid iodine (crystals) mainly in its fluidity. Like the solid, and unlike the gas, it has a definite volume (1 g occupics about 0.2 cm3), but it docs not have a definite shape: instead, it fits itself to the shape of the bottom part of its container.
From the molccular viewpoint the proccss of melting can be described in the following way. As a crystal is heated, its molecules are increasingly agitated, and move about more and more vigorously, but at lower temperature, this thermal agitation docs not carry any one molcculc any significant distance away from the position fixed for it by the arrangement of its neighbours in the crystal. At the melting point the agitation finally becomes so great that it causcs the molecules to slip by one another and to change somewhat their location relative to one anothcr. Thcy continue to stay close together, but do not continue to retain a regular fixed arrangement. Instead, the grouping of molcculcs around a given molcculc changes continually, sometimes being much like the close packing of the crystal, in which cach iodine molcculc has twelve near neighbours, and sometimes considerably different, the molecule has only ten or nine or eight near neighbours. Thus, a liquid, like a crystal, is a condensed phase, as contrasted with a gas, the molcculcs being piled rather closcly together; but whereas a crystal is characterized by regularity of atomic or molecular arrangement, a liquid is characterized by randomness of structure. The randomness of structure is usually the reason why the density of a liquid is somewhat less than that of the corresponding crystal.
Words and Word-Combinations to Be Memorized
away, bottom, characterize, considerably, container, contrast, differ from, differ in, distance, finally, fit, fix, freeze, increasingly, instead, mainly, melt, melting point, phase point, melting, random, regular, relative, shape, significant, sometimes, somewhat, stay, that is, thermal, viewpoint
Ex. 5. Give the Russian equivalents Tor the following:
be in equilibrium, the tendency for crystals to melt, freeze the liquid, differ in properties, unlike the liquid, have a definite volume, instead, fit itself to the shape of, in the following way, carry away, change somewhat, stay close together, the close packing of the crystal, considerably different, a randqm structure, the density of a liquid
Ex. 6. Give the English equivalents for the following:
нагреть до, точка плавления вещества, отличаться от, иметь определенную форму сосуда, с точки зрения, изменить местоположение, группировка молекул вокруг, ближний сосед, плотная упаковка, характеризоваться чём-л., произвольное расположение молекул, несколько меньше, каждая молекула йода, относительно друг друга, отличаться чём-л., в отличие от газа
Ex. 7. Fill in the blanks with prepositons where necessary.
1. Crystals ... iodine melt when they arc heated ... 114°C. 2. Liquid iodine differs ... solid iodine ... its fluidity. 3. Usually, liquid fits itself... the shape ... the bottom space ... its container. 4. The proccss ... melting can be described ... the following way. 5. This proccss is intcrcsing... the molecular viewpoint. 6. Regularity... atomic or molecular arrangement is characteristic ... a crystal. 7. A liquid is characterized ... randomness ... structure. 8. A solid occupies ... a definite volume.
Ex. 8. Translate the sentences into Russian, paying attention to different functions of the words ending in -ing.
1. Crystals melt, forming liquid iodine. 2. The temperature at which the crystals and the liquid arc in equilibrium is called the melting point. 3. Freezing of a liquid is vciy interesting to observe. 4. Heating mu§t be done in the following way. 5. The grouping of molcculcs around a given molecule changcs by raising the temperature. 6. What arc you going to do now? 7. Attempts arc being made to synthesize new elements in laboratory.
Ex. 9. Translate the sentences into Russian.
1. Who is the discovcrcr of the periodic law is well known not only to specialists but practically to everybody. 2. That solid, liquid and gas arc the three main states of a substance is a matter of common knowledge. 3. Whether these substances will react depends on the conditions of the reaction. 4. Whether a solution is acidic or not may be easily shown using litmus paper. 5. That a particular substance contains one clement or another may be determined by qualitative analysis. 6. That the vapour pressure of a crystal depends on its size is well illustrated by many experiments. 7. Whether water is a compound or not may be shown by the reaction between water and some metals. 8. What ancient scientists thought was based on what they could observe around them. 9. That the alchemical period was very important in the history of chemistry is unquestionable. 10. That chain molcculcs can be many thousands of atoms in length aficcts the behaviour of these substances. 11. What number of protons is there in the nucleus of the clement is known from its atomic number. 12. That water will dissolve a great number of different substances is an interesting fact. 13. That at the early stages of its development chemistry was mostly descriptive in character was quite natural. 14. Who the discoverer of nitrogen was is well known, it was isolated by Rutherford. 15. That students of chcmistry should be able to write chemical equations readily and accurately is highly important. 16. That it was A. M. Butlcrov who introduced the term "chemical structure" should not be foigotten. 17. That melting is a physical proccss must be quite clear to the student of chemistry. 18. Whether the hydrogen bond will be broken depends on the amount of energy. 19. Who will examine us in general chemistry is not known yet. 20. That the aim of scicncc is to explain things is a popular belief. 21. When the proccss started was not registered by the instrument. 22. What pressure should be applied should be decided before the experiment.
Ex. 10. Translate the sentences into English.
1. Как происходит переход из одного физического состояния в другое, должно быть хорошо понятно каждому студенту. 2. Каждое вещество имеет свою собственную точку плавления при одинаковом давлении. 3. Точка плавления твердого вещества — это в то же время и точка замерзания жидкости. 4. Известно, что жидкость принимает форму сосуда, в котором она содержится, но сс объем не зависит от объема сосуда. 5. В отличие от газа, у кристаллов и жидкостей молекулы располагаются довольно близко друг к другу. 6. То что структура является одной из важнейших характеристик состояния вещества, необходимо помнить всегда. 7. Кристаллы имеют упорядоченную структуру, а структура жидкости произвольна, поэтому плотность жидкости несколько меньше, чем у соответствующего кристалла.
Ex. 11. Answer the following questions:
1. What temperature is callcd the melting point? 2. What is the difference between liquid and crystalline iodine? 3. In what way is it possible to explain the change from the solid to the liquid? 4. What is the principal difference between solid and liquid from the structural viewpoint? 5. What does the molecular motion depend on?
Scction II
Упр. 1. Назовите значения следующих интернациональных слов:
attractive, motion, total, diameter, extremely, distance, characteristic, operate, gram, atmosphere, temperature, ordinary
Упр. 2. Определите значения выделенных слов по контекст};
1. How many times have you been to Moscow? 2. The specimen was analysed and it bccamc clcar that it contains copper. 3. What size arc your boots? 4. On the one hand, the proccss is rather simple, on the other hand, there is some difficulty because the temperature must be very high.
Слова к тексту:
negligibly — ничтожно; apart — отдельно, в стороны друг от друга; accordingly — соответственно