Diffusion in liquids. Diffusion takes place in liquids, as you know. For when you put sugar in coffee or tea and do not stir it, although the upper part of the tea or coffee is not sweetened, the part nearer the sugar is very sweet. If you should let the coffee or tea, with the sugar in the bottom, stand for a few months, it would get sweet all through. Diffusion is slower in liquids than in gases, because the molecules are so very much closer together.
Osmosis. One of the most striking and important facts about diffusion is that it can take place right through a membrane. Try this experiment:
Experiment 86. With a rubber band fasten a piece of parchment paper, made into a little bag, to the end of a piece of glass tubing about 10 inches long. Or make a small hole in one end of a raw egg and empty the shell; then, to get the hard part off the shell, soak it overnight in strong vinegar or hydrochloric acid diluted about 1 to 4. This will leave a membranous bag that can be used in place of the parchment bag. Fill a tumbler half full of water colored with red ink, and add enough cornstarch to make the water milky. Pour into the tube enough of a strong sugar solution to fill the membranous bag at its base and to rise half an inch in the tube. Put the membranous bag down into the pink, milky water, supporting the tube by passing it through a square cardboard and clamping it with a spring clothespin as shown in Figure 151. Every few minutes look to see what is happening. Does any of the red ink pass through the membrane? Does any of the cornstarch pass through?
This is an example of diffusion through a membrane. The process is called osmosis, and the pressure that forces the liquid up the tube is called osmotic pressure. It is by this sort of diffusion that chicks which are being incubated get air, and that growing plants get food. It is in this way that the cells of our body secure food and oxygen and get rid of their wastes. There are no little holes in our blood vessels to let the air get into them from our lungs. The air simply diffuses through the thin walls of the blood vessels. There are no holes from the intestinal tract into the blood vessels. Yet the dissolved food diffuses right through the intestinal wall and through the walls of the blood vessels. And later on, when it reaches the body cells that need nourishment, the dissolved food diffuses out through the walls of the blood vessels again and through the cell walls into the cells. Waste is taken out of the cells into the blood and passes from the blood into the lungs and kidneys by this same process of diffusion. So you can readily see why everything would die if diffusion stopped.
Fig. 151. Pouring the syrup into the "osmosis tube."
Application 65. Explain how the roots of a plant can take in water and food when there are no holes from the outside of the root to the inside; how bees can smell flowers for a considerable distance.
Inference Exercise
Explain the following:
401. A shell in the bottom of a teakettle gathers most of the scale around it and so keeps the scale from caking at the bottom of the kettle.
402. There is oxygen dissolved in water. When the water comes in contact with the fine blood vessels in a fish's gills, some of this oxygen passes through the walls of the blood vessels into the blood. Explain how it does so.
403. Asphalt becomes soft in summer.
404. When the trolley comes off the wire the car soon stops.
405. You cannot see stars in the daytime on earth, yet you could see them in the daytime on the airless moon.
406. Although the carbon dioxid you breathe out is heavier than the rest of the air, part of it goes up and mixes with the air above.
407. On a cold day wood does not feel as cold as iron.
408. To make mayonnaise dressing, the oil, egg, and vinegar are thoroughly beaten together.
409. A solution of iodin becomes stronger if it is allowed to stand open to the air.
410. A drop of milk in a glass of water clouds all the water slightly.