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JUNIOR GRADE SCIENCE 



dips into the lime-water in the bottle A. When, however, you blow instead 

 of sucking, your breath passes out through the tube which dips into the 

 lime-water in the bottle B. Notice that the lime-water in A remains 

 clear, but that in />' is rendered milky by the air you breathe out. You 

 thus see that fresh air has little effect upon lime-water, but breathed air 

 quickly turns clear lime-water milky. 



vi. Oxygen from plants. Take a bunch of fresh watercress, or water 

 weeds, and put it into a beaker or glass jar very nearly filled with water 



saturated with carbon dioxide. 

 Cover the plants with a funnel 

 nearly as wide as the jar, as shown 

 in Fig. 113. Fill a test-tube with 

 water and invert over the funnel. 

 If properly managed there should 

 at first be no gas in the test-tube. 

 Place the jar in bright sunlight for 

 an hour or two and then examine 

 it. You will notice bubbles of a 

 gas have collected at the top of the 

 tube. Test the gas with a glowing 

 splinter of wood. It is found to be 

 oxygen. 



vii. Plants in sunlight and in 

 darkness. Repeat the whole ex- 

 periment, but instead of putting 

 the bottle in bright sunlight place 

 it in the dark. Observe that in 

 such circumstances no bubbles of 

 oxygen are formed. 



viii. Carbon in plants. Take 

 some green portions of a plant 

 (leaves will do) and heat them on a piece of tin plate over a laboratory 

 burner. Note that they become charred, showing the presence of carbon 

 in them. 



Production of carbon dioxide by burning. When things such as 

 candles, oil, gas and wocd are burnt, either in the air or in pure oxygen, 

 a gas is produced which has the power of turning lime-water milky. 

 All these substances contain, in one form or another, a constituent 

 called carbon. As has been seen in previous chapters, the gas produced 

 when these substances burn is carbon dioxide, that is, the gas obtained 

 by burning carbon in air or oxygen. In fact, whenever a substance 

 rich in carbon burns in a plentiful supply of air or oxygen, this carbon 

 dioxide is produced. Knowing how many fires there are in houses, 

 furnaces, engines and so on, it is not difficult to understand that at 

 every hour of the day very large quantities of carbon dioxide are 

 formed, which escape, sooner or later, into the air. 



Carbon dioxide is given off in breathing. If a person blows with 

 the mouth into clear lime-water, the lime-water is turned milkv. 



FIG. 113. Green plants in bright sunlight 

 can decompose carbon dioxide. They keep 

 the carbon for themselves and liberate the 

 oxygen. 



