Прочитайте текст Valency is the reason for the varying weights of the same element in different comp...: ответ на тест 256499 - Английский язык

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The Latin word for valency is valere, which means “to be strong”. The “strength” divides the elements into classes. The elements of class one have a valency of one: hydrogen and chlorine belong here. One packet of each can join to make hydrochloric acid. But to combine with one packet of oxygen which is in valency class two, there must be two packets of hydrogen (H2 + O  H2O - water).
Valency does not depend on size or weight of the element. It is its “strength of joining” (combining capacity).
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The “strength” divides the elements _________________.

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How many types of atom does carbon dioxide have?
Aluminium (Al), water (H2O), oxygen (O2) and carbon dioxide (CO2) are substances. Some substances are elements and some substances are compounds. Elements have only one type of atom whereas compounds have more than one type.
Aluminium is an element: it has only one type of atom (i.e. it has only aluminium atoms). Oxygen has only one type of atom, so it is also an element.
Carbon dioxide has two types of atom (carbon and oxygen), so it is a compound. Sulphuric acid (H2SO4) is also a compound: it has three types of atom (hydrogen, sulphur and oxygen). Water is also a compound.

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Radiation affects materials in a variety of ways because of the diversity of types of radiation and the difference in materials. Radiation effects may be either detrimental or beneficial to the desirable properties of a material. Some effects of radiation have been known for some time (e. g. discoloration of glass after long exposure to light; exposure of photographic film to electromagnetic radiation), while others have been recently observed with the advent of nuclear reactors and accelerators (very high-energy radiation). Radiation may be divided into two general groups:
1. electromagnetic radiation
2. radiation that is particulate in nature.
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Radiation effects may be either detrimental or beneficial to the desirable properties of _________________.

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A body possesses energy when due to its position or condition it is capable of doing work and the work it can do is a measure of its energy.
Energy is of two forms: potential energy and kinetic energy. The potential energy of a body is the energy possessed by the body thanks to its position or configuration. For example, a lifted weight has potential energy due to the higher position into which it has been lifted since it can do work as it falls under the action of gravity. The potential energy of the weight in its highest position is the same as its kinetic energy after it has fallen to its lowest position.
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The potential energy of a body is the energy possessed by the body thanks to its __________________.

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There exist many sources of energy in the world both potential and kinetic. One source consists in water falling from a high level such as an upland lake. Another source is wind or moving air. Others consist in tides in the sea, heat from subterranean sources and coal deposits and oil wells yielding mineral oil.
But all these sources and stores of energy are not equally useful to mankind. Moreover some stores of energy such as coal and oil can never be replaced by us when once used up. On the other hand stores of high level water are continually being replaced by rain and wind and tides will not, as far as we know, ever cease to exist. A very important matter is the conversion of energy from one form to another. The form most required by us is mechanical rotational energy.
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Stores of high level water are continually being replaced by rain and wind and tides will not ever ________________.

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Mechanical rotational energy is required to rotate shafts in a factory for driving various machines and also for driving the wheels of vehicles or automobiles or locomotives. The energy of water power is very unequally distributed, some countries such as Norway and Switzerland being rich in it and some such as England poor. England is rich in the potential energy of coal but has little or no oil wells.
Appliances for converting energy from one form to another are called engines. Thus a heat-engine is a machine for converting heat energy into mechanical energy of rotation by the combustion of coal or oil. A turbine or water engine can convert the kinetic energy of falling water into mechanical rotational energy.
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The energy of water power is ___________________.