3. Properties of seawater

   The oceans and seas contain 23 grams of salt (sodium chloride) per 1000 grams of water.

   The dissolved salt content of a body is called salinity. Cold seas are usually less salty than warm seas.

   The water temperature depends on the depth. In the superficial layers, it is similar to the Earth. But from 200 meters depth, temperature remains unchanged about 2 degrees above zero. In the polar zones, a big layer of ice is formed on the sea surface.

    4. Movements of Seawater

   Seawaters move forming waves, tides and currents.

   - The waves are wave motions of the sea surface. Some of them are caused by wind and others are caused by coastal earthquakes or by submarines. In the latter case, it is formed a large and isolated wave that sweeps along what appears in its path and produces intense damages.

   - Tides are periodic rises and falls of sea water level caused by the attraction of the Moon and the Sun. Each day two rises of sea level occurs (high tide) and two falls of sea level also occur (low tide). The Most high tide occurs when the Sun, Earth and Moon are in the same direction. The attraction of the moon is more intense than the Sun, which is farther from the Earth.

   - The currents are like rivers within the ocean. The currents are caused by the wind, the temperature differences (from the coldest to the warmest seas), the salinity (from the more salty to less salty sea), the movement of rotation of the earth, etc.

   5. Choose one of these words and answer: waves, tides and currents:

    6. The coast

   The contact area between the sea and the continent is called coast. When it is flat and sandy, it forms the beach and it is called flat coast. When it is mountainous is called cliffed coast.

   The islands are pieces of land surrounded by water. For example: Mallorca.

   The archipelago is a group of nearby islands. For example: the Balearic Islands in Spain.

   The cape is a pronounced projection of the littoral. For example: Cape Horn in Argentina.

   The gulf is an inlet of the sea on the littoral. For example: the Gulf of Guinea in Africa.

   The bay is a small gulf. For example: the bay of Cadiz in Spain.

   The strait separates two continents or two islands. For example: the Strait of Gibraltar between Africa and Europe.

   8. Answer with the right word: island, gulf, strait, archipelago, cape or bay

   7. Rivers

   It is a water course originated in the mountains and goes towards another river, lake or sea. It runs through a channel. The tributaries are rivers that flow into the main river.
   Watershed is the land area whose water discharged into the same river. All the rivers that flow into the same sea belong to the same slope.
   The river has several parts:
   - The high course, near the source, when it goes over mountainous areas with great force and speed, it drags soil and stones and causes severe erosion.
   - The middle course, the slope is smaller, the water descends more slowly. The river sweeps along the materials resulting from in the high course.
   - the low course, the river flows slowly, producing meanders (waves of the course) and the materials are deposited in the mouth. It can produce a river delta.

   9. Indicate the course of the river: high, middle or low

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