Chemistry_Grade_10-12 (1).pdf

3 transpiration transpiration is the evaporation of

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rivers and dams, into the atmosphere. 3. Transpiration Transpiration is the evaporation of water from the aerial parts of plants, especially the leaves but also from the stems, flowers and fruits. This is another way that liquid water can enter the atmosphere as a gas. 4. Condensation When evaporation takes place, water vapour rises in the atmosphere and cools as the altitude (height above the ground) increases. As the temperature drops, the energy of the water vapour molecules also decreases, until the molecules don’t have enough energy to stay in the gas phase. At this point, condensation occurs. Condensation is the change of water from water vapour (gas) into liquid water droplets in the air. Clouds , fog and mist are all examples of condensation. A cloud is actually a collection of lots and lots of tiny water droplets. This mostly takes place in the upper atmosphere but can also take place close to the ground if there is a significant temperature change. Interesting Fact erest Fact Have you ever tried breathing out on a very cold day? It looks as though you are breathing out smoke! The moist air that you breathe out is much warmer than the air outside your body. As this warm, moist air comes into 356
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CHAPTER 18. THE WATER CYCLE - GRADE 10 18.3 SUN 1 2 4 3 5 6 7 Rain falls onto the soil, flows into the rivers or seeps into the soil Rain falls directly into rivers, dams or the oceans Some rain seeps into the soil and becomes part of the ground water supply Condensation forms clouds Evaporation Surface water Ground water may feed into rivers or will eventually lead into the sea Figure 18.1: The water cycle contact with the colder air outside, its temperature drops very quickly and the water vapour in the air you breathe out condenses. The ’smoke’ that you see is actually formed in much the same way as clouds form in the upper atmosphere. 5. Precipitation Precipitation occurs when water falls back to the earth’s surface in the form of rain or snow. Rain will fall as soon as a cloud becomes too saturated with water droplets. Snow is similar to rain, except that it is frozen. Snow only falls if temperatures in the atmosphere are around freezing. The freeing point of water is 0 0 C). 6. Infiltration If precipitation occurs, some of this water will filter into the soil and collect underground. This is called infiltration. This water may evaporate again from the soil at a later stage, or the underground water may seep into another water body. 357
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18.3 CHAPTER 18. THE WATER CYCLE - GRADE 10 7. Surface runoff This refers to the many ways that water moves across the land. This includes surface runoff such as when water flows along a road and into a drain, or when water flows straight across the sand. It also includes channel runoff when water flows in rivers and streams. As it flows, the water may infiltrate into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.
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