1. Atoms, molecules and compounds

Water is abundant on Earth today - as a gas, as a liquid, and as a solid. The physical state of water depends on temperature. If the temperature is high enough, water is a gas. If the temperature is low enough, water freezes to ice. Although the forms of water may

vary, its chemical compositions remains the same. Water has some unusual physical and chemical properties that have had a powerful effect on the evolution of life. A close look at the chemistry of water can provide information about the chemical nature of matter and about the function of biological organisms.

Suppose you found a way to subdivide a drop of water

into smaller and smaller droplets until you could not even see them under a microscope. No matter how small the water droplet, it would still be made of identical units called molecules. Molecules of water

are the smallest units into which water can be subdivided and still have the essential chemical properties of water. When an electric current flows through water under proper conditions, a remarkable change occurs: water becomes two gases. One is the lightest gas, hydrogen, which burns with a very hot flame in air. The other gas is oxygen. Any burning object thrust into

oxygen will continue to burn with a more brilliant flame. Thus, under certain conditions, water molecules can break down into two different substances, hydrogen and oxygen. Neither hydrogen nor oxygen has the appearance or any other property of water.

2. Molecules are made of atoms that have been chemically combined. Molecules may be made from more than one kind (as in water), or they may be made from atoms of the same kind. For example, hydrogen gas consists of hydrogen atoms that exists in combination with each other. A molecule of hydrogen has two hydrogen atoms. The same arrangement is true of an oxygen molecule, which has two oxygen atoms (Oxygen also forms molecules of ozone that contain three atoms of oxygen).

3. A substance made of only one kind of atom is called an element. More than 100 different elements are known today. A substance made of two or more different kinds of atoms, such as water, is called a compound. Elements can combine chemically in many ways to form the millions of compounds that give Earth variety of materials. Chemists have given each element a symbol of letters from the element’s name. H stands for hydrogen, O for oxygen, C for carbon, and N for nitrogen. Iron, however, is Fe, derived from the word ferrum, reflecting that some symbols come from an element’s Latin or Greek name.

Exercise 2. Master the active vocabulary:

hydrogen [´haidr∂d (∂)n] - водород

nitrogen [´naitr∂d (∂)n] - азот

oxygen [´oksid (∂)n] - кислород

carbon [´ka:b(∂)n] - углерод

ozone [´∂ z∂ n] - озон

compound [´kompa nd] - состав, соединение

Exercise 3. Answer the questions:

1) What does the physical state of water depend on?

2) Under what conditions does water freeze to ice?

3) What are molecules?

4) What remarkable change occurs when an electric current flows through water?

5) What does hydrogen gas consist of?

6) What is an element?

7) How many elements are known today?

Exercise 4. Give English equivalents for the following:

жидкость; химические свойства воды; мельчайшие частицы; электрический ток; водород; кислород; соединение; отражать.

Exercise 5. Give Russian equivalents for:

to freeze to ice; to have a powerful effect on; biological organisms; under proper conditions; to derive.