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ENTERTAINING EXPERIENCES AT HOME
Why do berries release juice. Chemical experiments
Entertaining experiences at home / Chemistry experiments for children When a plant dries up, when its leaves turn yellow, this means that the plant cells do not have enough water. But each cell is enclosed in a shell-membrane. How does the moisture absorbed by the roots penetrate the membrane into the cell? And what makes water move against gravity, from bottom to top, from roots to leaves? Before we get answers to these questions, let's set up preliminary experiments with a membrane that is somewhat reminiscent of a cell membrane. If two solutions are separated by a dense partition, then, of course, they do not mix. If there is no partition at all, then the solutions, on the contrary, mix by themselves, even if they are not mixed. Well, what if the partition is semi-permeable? This will be the subject of the experiment, and a sheet of parchment or cellophane (but not polyethylene) will serve as a semi-permeable membrane. In order for it to acquire the properties of interest to us, it must be kept in water until it softens. Prepare sugar syrup - a saturated solution of sugar so thick that the sugar no longer dissolves. It is faster and more convenient to prepare such a solution in hot water. Pour the syrup to the top into a glass, cover with a soaked leaf and tie tightly. Make sure there are no air bubbles under the film. Place the glass in a jar or pot of water (the water should cover the glass) and leave for a few hours. When you look at the glass of syrup again, you will immediately notice that the film with which it is closed has swelled up: a bubble has formed over the glass.
To understand what happened, we must first understand what a semi-permeable membrane is. This is a film that traps some molecules and at the same time passes others. Both cellophane and parchment films are porous, but the pores in them are so small that they are impermeable to sugar molecules. There is water on both sides of our partition, but on the side where the sugar solution is located, there are fewer water molecules per surface area. Therefore, more molecules pass through the membrane from the water side, and this leads to the fact that the volume of liquid in the glass increases and, consequently, the semi-permeable film swells. In nature, everything tends to balance, in this case - to equalize the concentration of solutions. And soon equilibrium sets in: how many water molecules enter the glass with syrup, the same number leaves it and goes into the outer vessel. Therefore, the bubble is not too big. The physico-chemical phenomenon that we have just observed is called osmosis, and the pressure that causes the film to bend is called osmotic pressure. To observe osmosis, a partition and two liquids are required: a solution of some substance and a pure solvent (we have water) or at least a weaker solution. The membrane of living cells is always a semi-permeable membrane. It traps the molecules of many substances dissolved in water, but passes water through. Therefore, each animal and plant cell is a microscopic osmotic system, and osmotic pressure plays a very important role in the life of organisms. Osmosis can be observed in the simplest experiments. Cut a thin slice of lemon with a sharp knife and place it on a saucer. Note: there is almost no juice on the surface. Sprinkle the slice with granulated sugar or, even better, powdered sugar - and very soon the lemon will release the juice. A similar experience can be made with strawberries and other berries by putting them in dry jars. Berries sprinkled with sugar quickly release juice. In all these cases, osmosis works. A concentrated sugar solution forms on the surface of a lemon or berry, and the juice, much less concentrated, tends to dilute this solution, it penetrates through the cell membranes and comes out - just like in the previous experiment, water from a jar rushed into a glass of syrup. Our next object is cabbage. Naturally, we will sprinkle it not with sugar, but with salt. Chop the cabbage with a knife, sprinkle with salt and rub well - the cabbage will also give juice. This is what happens when cabbage is fermented; and cabbage salads are certainly advised to rub it properly so that the juice stands out and the cabbage becomes softer and more tender. The reason is the same: osmosis. Let's move on to potatoes. Cut three cubes from a potato, preferably the same size. Prepare three jars. Pour salted water into one, concentrated salt solution into another, and just tap water into the third. Drop a potato cube into each jar. After two or three hours, carefully examine the cubes. The one that was in salted water, you will not find any changes. But the other two have changed, and noticeably. The cube that lay in the concentrated salt solution became much smaller, and the one that you lowered into the water, on the contrary, became noticeably larger. First, about why the first cube has not changed. It was in a dilute solution, and the concentration of salt was about the same as in the potato juice itself. The cube, which was in a concentrated solution, began to give off water, reducing the concentration of this solution; the water drained from the potatoes and the cube shrank. And the last cube, the one that was in the water, began to absorb water and increased in size. Let's move on from potatoes to carrots and make them work like a pump. Cut off the tops of the carrots and stick a glass tube into the "crown". Put the carrot in a glass of water; you can probably figure out how to keep carrots upright. Pour half the salt solution into a glass tube and make observations. Soon the water level in the tube will begin to rise and, if the experiment is set up correctly, the water will even pour out of the tube. Carrots, as it were, pump water from a glass, makes it move up. When you water carrots growing in the garden, they pump water from the soil into the tops in much the same way. In its juice, the concentration of salts is higher than in irrigation water, and thanks to osmosis, not only the roots, but also all plant tissues receive life-giving moisture. Author: Olgin O.M.
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