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Открытая библиотека для школьников и студентов. Лекции, конспекты и учебные материалы по всем научным направлениям.


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Solids are usually described as hard, at least compared with liquids, and as having a definite shape. A powder does not appear to have a definite shape; we can make it into a pointed heap or a flat heap, but in a heap of powder we are looking at quantities of grains each of which has a definite shape. On examining a little piece of common salt under a microscope it will be seen to consist of tiny cubes, some of them damaged, but there will be some perfect ones.

All these substances having a fixed shape are called crystals. Thus we can say that a solid is a crystalline substance. It is not always easy to see the crystals, even through a microscope and at one time it was thought that a powder such as chalk was amorphous, a word which means without shape. However, we now know that chalk is a crystalline substance and even metals are made of exceedingly tiny crystals packed very close together. Now, we know that a crystal of common salt is composed of millions of sodium ions and chlorine ions holding together by electrical attraction, and since chemists have been able to "look" inside a very tiny crystal by means of X-rays, it has been discovered that sodium and chlorine ions are always arranged in a very regular pattern in relation to one another.

inside - всœередині compare - порівнювати packed - складений consist of – складатися з будь-чого point- точка damage- пошкоджувати, псувати powder - порошок exceedingly - надзвичайно regular- вірний, нормальний flat- плаский grain- зерно relation- відношення heap- куча, маса chalk - крейда

1. Has powder a definite shape?

2. What do we see if we examine common salt under a microscope?

3. What do we call crystals?

4. Is solid a crystalline substance?

5. What can you say about chalk?

6. What is a crystal of common salt composed of?

7. How are sodium and chlorine ions arranged in a crystal?

2. Give synonyms:

quantity, to call, several, to approach, some, to reach, various, to show, to name, different, to indicate, shape, form, characteristic, number, property, to use, value, generally, ordinarily, to apply, amount.

3. Translate the following sentences:

1. Since a little piece of salt has been examined under a microscope, it was established that it consisted of tiny cubes. 2. Many experiments with different solids were carried out in Middle Ages, since then a great many experimental methods have been developed. 3. Radioactivity is a natural process that has been investigated since the end of the last century. 4. The sum of the charges of proton and electron is equal to that of neutron. 5. The free energy of the reactants is equal to that of the products obtained. 6. Common salt consists of tiny cubes, some of them being perfect ones. 7. A mechanical method was followed by an electrical one. 8. The behaviour of aluminium is similar to that of zinc. 9. The values of the boiling-points are those carefully determined by us, and those of the molecular volumes have been taken from Lassen's data.

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