The history of the Earth is primarily the history of water. It has continuously changed and still goes on changing the face of our planet.

   Water is the greatest chemist in the world. No natural process takes place without it - be it the formation of a new rock, a new mineral, or a highly complicated biochemical reaction taking place if the organism of a plant or an animal.

   Chemists could hardly do anything in their laboratories without water. In studying the properties of substances, their transformations, in producing new compounds they can get along without water only in rare cases. Water is one of the best solvents known. And many substances must be dissolved before they can enter into reactions.

   What happens to a substance when it dissolves? The forces acting between the molecules and atoms at its surface are weakened many hundredfold in water, as a result of which they break away from the surface and pass into the water. In a glass of tea a piece of sugar breaks down into separate molecules.

   Table salt decomposes into charged particles known as sodium and chloride ions. Owing to its peculiar structure the water molecule possesses a great ability to attract the atoms and molecules of a dissolving body. Many other solvents are inferior to water in this respect.

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