III. Control text

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Radiation exerts two opposing effects on polymers. On the one hand, it breaks up the polymer molecules into smaller pieces. On the other, it causes liberation of a hydrogen atom from each of the two adjoining molecules with formation of a link between the two molecules (cross-linking). The existence if cross-links in a polymer makes the material tougher and higher melting and is very desirable for certain applications.

The cross-linking of polymers by radiation has been much studied. The irradiation of any organic compounds results in breaking of CH bonds, leaving free bonds on the carbon atoms while the hydrogen atoms go off together in pairs to form hydrogen gas. In a liquid the resulting free radicals can diffuse as a whole through the solution and eventually meet together and combine. In a solid polymer it is not clear how these centers get together. One proposed mechanism is that a hydrogen atom from a neighbouring carbon will pop into the vacated hydrogen space, producing a new free bond on the atom adjacent to the original free bond position. This process will continue, with the free bond flowing up and down the chain, until the free bond happens to find itself next to a free bond formed on the adjacent molecule which is likewise traveling up and down. Another mechanism, possible perhaps only with amorphous polymer, is that the long-chain molecules as a whole may move with respect to one another until the free bonds find themselves in proximity.

Whatever the mechanism of cross-linking may be, the result is of commercial value.

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Test 5

1. The nuclei of hydrogen and oxygen are absolutely…

Large

Strong

Stable

Weak

2. The Curies discovered the … radioactivity

1. Artificial

2. Nuclear

3. Natural

4. Uranium

3. The discovery of radioactivity … а number of questions.

Rise

Raised

Rose

Raise

The discovery of natural radioactivity … by Henry Becquerel

1. Was being made

2. Was made

3. Has been made

4. Had been made

The field of application of radioactive nuclides has rapidly…

1. Increased

2. Improved

3. Discovered

4. Expanded

Uranium and radium are … into other chemical elements

1. Transformed

2. Emitted

3. Based

4. Done

Blackening a photographic plate is done by the … of light

1. Ways

2. Atoms

3. Rays

4. Movement

Radiation exerts two opposing effects on …

1. Elements

2. People

3. Atoms

4. Polymers

The irradiation of organic compound … in breaking of CH bonds

1. Leads

2. Continue

3. Results

4. Makes

The range of application of radioactive nuclides … all branches of research work

1. Includes

2. Comprises

3. Consists

4. Makes

Unit 6

I. Прочитайте и переведите текст

There is a tradition among physical chemists to regard the properties of liquids as essentially invariant. When comparing them with gases we can see that they are only slightly com­pressible.

Having carried out a number of investigations the scien­tists made a very interesting discovery, i.e. a new stable form of water having the density almost one and a half times that of ordinary water was obtained. This water was named Water II. Having examined its properties they found out that its index of refraction was equal to about 1.48 to 1.49. But when Water II was diluted with ordinary water (Water I) this value dropped to the value for pure water, i.e. 1.33. Having contin­ued their work the scientists stated that the density of Water II was equal to that of pure water. Then it was very important to learn the stability of the molecules of Water II and the forces responsible for their stability.

Some scientists suggested the possibility that on Venus wat­er exists mainly in the form of Water II. This question attracted the attention of many scientists and will be discussed in future.

Water is hydrogen oxide, a compound of hydrogen and oxy­gen. It can be made if hydrogen or a hydrogen-containing substance are burnt in air or oxygen.

Most of the world's water is liquid, but an important frac­tion is solid as ice and snow.

Many mineral substances contain water of crystallization (e.g. copper sulphate) and in the atmosphere there are millions of tons of water vapour. Clouds consist of minute droplets of water or crystals of ice.

Water dissolves a very large number of substances and is the most important solvent. It does not dissolve greasy, fatty substances or most plastics.

Having found the composition of water, the scientists could Investigate its properties. It was stated that ordinary water is impure, it usually contains dissolved salts and dis­solved gases, and sometimes organic matter.

For chemical work water is to be purified by distillation. Pure water is colourless, tasteless and odourless. Rain water formed by the condensation of water in the air is nearly pure water, containing only small propoftions of the dust and of dissolved gases.

Having examined the properties of water, the chemists found that physical properties of water can be used to define many physical constants and units.

The freezing point of water (saturated with air at 1 atm pressure) is taken as 0°C and the boiling point of water at 1 atm is taken as 100°C.

The unit of volume in the metric system is chosen so that 1 ml of water at 3.98°C (the temperature of its maximum density) weighs 1.000 g/cm³.

So water is one of the most important of all chemical sub­stances. It is a major constituent of living matter and of the environment in which we live.