Chemistry_Grade_10-12 (1).pdf

Important it is important to realise that the water

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Important: It is important to realise that the water cycle is all about energy exchanges . The sun is the original energy source . Energy from the sun heats the water and causes evaporation. This energy is stored in water vapour as latent heat . When the water vapour condenses again, the latent heat is released, and helps to drive circulation in the atmosphere. The liquid water falls to earth, and will evaporate again at a later stage. The atmospheric circulation patterns that occur because of these exchanges of heat are very important in influencing climate patterns. Activity :: Experiment : The Water Cycle Materials: Tile or piece of plastic (e.g. lid of ice-cream container) to make a hill slope; glass fish tank with a lid; beaker with ice cubes; lamp; water Set up a model of the water cycle as follows: lamp ice cubes water slope glass tank 1. Lean the plastic against one side so that it creates a ’hill slope’ as shown in the diagram. 2. Pour water into the bottom of the tank until about a quarter of the hill slope is covered. 3. Close the fish tank lid. 4. Place the beaker with ice on the lid directly above the hill slope. 5. Turn the lamp on and position it so that it shines over the water. 6. Leave the model like this for 20-30 minutes and then observe what happens. Make sure that you don’t touch the lamp as it will be very hot! Observation questions: 1. Which parts of the water cycle can you see taking place in the model? 2. Which parts of the water cycle are not represented in the model? 358
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CHAPTER 18. THE WATER CYCLE - GRADE 10 18.4 3. Can you think of how those parts that are not shown could be represented? 4. What is the energy source in the model? What would the energy source be in reality? 5. What do you think the function of the ice is in the beaker? 18.4 The microscopic structure of water In many ways, water behaves very differently from other liquids. These properties are directly related to the microscopic structure of water, and more specifically to the shape of the molecule and its polar nature , and to the bonds that hold water molecules together. 18.4.1 The polar nature of water Every water molecule is made up of one oxygen atom that is bonded to two hydrogen atoms. When atoms bond, the nucleus of each atom has an attractive force on the electrons of the other atoms. This ’pull’ is stronger in some atoms than in others and is called the electronegativity of the atom. In a water molecule, the oxygen atom has a higher electronegativty than the hydrogen atoms and therefore attracts the electrons more strongly. The result is that the oxygen atom has a slightly negative charge and the two hydrogen atoms each have a slightly positive charge. The water molecule is said to be polar because the electrical charge is not evenly distributed in the molecule. One part of the molecule has a different charge to other parts. You will learn more about this in chapter 4.
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