Chemistry_Grade_10-12 (1).pdf

376 chapter 20 the hydrosphere introduction as far as

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Chapter 20 The Hydrosphere - Grade 10 20.1 Introduction As far as we know, the Earth we live on is the only planet that is able to support life. Among other things, Earth is just the right distance from the sun to have temperatures that are suitable for life to exist. Also, the Earth’s atmosphere has exactly the right type of gases in the right amounts for life to survive. Our planet also has water on its surface, which is something very unique. In fact, Earth is often called the ’Blue Planet’ because most of it is covered in water. This water is made up of freshwater in rivers and lakes, the saltwater of the oceans and estuaries, groundwater and water vapour . Together, all these water bodies are called the hydrosphere . 20.2 Interactions of the hydrosphere It is important to realise that the hydrosphere interacts with other global systems, including the atmosphere , lithosphere and biosphere . Atmosphere When water is heated (e.g. by energy from the sun), it evaporates and forms water vapour. When water vapour cools again, it condenses to form liquid water which eventually returns to the surface by precipitation e.g. rain or snow. This cycle of water moving through the atmosphere, and the energy changes that accompany it, is what drives weather patterns on earth. Lithosphere In the lithosphere (the ocean and continental crust at the Earth’s surface), water is an important weathering agent, which means that it helps to break rock down into rock fragments and then soil. These fragments may then be transported by water to another place, where they are deposited. This is called erosion . These two process i.e. weathering and erosion, help to shape the earth’s surface. You can see this for example in rivers. In the upper streams, rocks are eroded and sediments are transported down the river and deposited on the wide flood plains lower down. On a bigger scale, river valleys in mountains have been carved out by the action of water, and cliffs and caves on rocky beach coastlines, are also the result of weathering and erosion by water. Biosphere In the biosphere, land plants absorb water through their roots and then transport this through their vascular (transport) system to stems and leaves. This water is needed in photosynthesis , the food production process in plants. Transpiration (evaporation of water from the leaf surface) then returns water back to the atmosphere. 377
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20.3 CHAPTER 20. THE HYDROSPHERE - GRADE 10 20.3 Exploring the Hydrosphere The large amount of water on our planet is something quite unique. In fact, about 71% of the earth is covered by water. Of this, almost 97% is found in the oceans as saltwater, about 2.2% occurs as a solid in ice sheets, while the remaining amount (less than 1%) is available as freshwater. So from a human perspective, despite the vast amount of water on the planet, only a very small amount is actually available for human consumption (e.g. drinking water). Before we go on to look more closely at the chemistry of the hydrosphere, we are going to spend some time
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