Why does the thermometer present below the central position in Hope's Experiment remain constant?
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Why does the thermometer present below the central position in Hope's Experiment remain constant?
It is a common experience that when a liquid is heated, it expands, irrespective of the temperature range. However, water is an exception. When water at 0 ºC is heated, it contracts till 4 ºC, instead of expanding. On further heating from 4 ºC, water expands like other liquids do. The special behaviour of water between 0ºC and 4ºC is called the anomalous behaviour of water.
In the year 1805, the scientist T. C. Hope devised a simple arrangement, known as Hope’s apparatus, to demonstrate the anomalous behaviour of water.
Hope’s apparatus consists of a long cylindrical jar with two openings on the side, one near the top and the other near the bottom to fit thermometers in each of these openings. A metallic cylindrical air-tight trough with an outlet is also fitted onto the jar, on its central portion. Two thermometers are fitted air-tight in the two openings of the cylindrical jar. The thermometer near the bottom of the jar is T1, and the one near the top of the jar is T2. Now the cylindrical jar is filled with water. The cylindrical trough at the central portion of the jar is filled with a freezing mixture of ice and common salt.
Initially, both the thermometers show the same reading, say 12 ºC. As the trough is filled with the freezing mixture, the water in the central portion of the jar loses heat energy and gets cooled. Its volume decreases and density increases. As a result, the denser layers of water in the central portion move downwards, displacing the lighter layers of water there towards the central portion. This forms a convection of water. Due to this convection, thermometer T1 near the bottom shows a rapid fall in temperature till it reaches 4 ºC. At 4 ºC, the density of the water in this part of the jar is maximum.
Thus, the dense water remains in the lower portion. temperature recorded by thermometer T2 remains almost unchanged. Now, the water in the central portion of the jar cools to below 4 ºC. Due to the anomalous expansion of water, it begins to expand. As the volume of the water increases, its density decreases. As a result, it moves upwards. Thus, thermometer T2 shows a rapid fall in temperature. However, the temperature of water in the lower portion of the cylinder remains at 4 ºC.The temperature of water in the upper portion of the jar continues to decrease gradually below 4 ºC.
As the temperature of water decreases from 4 ºC to 0 ºC, it expands and its density decreases. Hence, the cold water rises to the surface and forms a thin layer of ice. The ice floating on the surface of water indicates that the density of ice is less than the density of water at 4 ºC. The temperature in thermometer T1 remains at 4 ºC, while that in thermometer T2 falls to 0 ºC. This is due to the anomalous behaviour of water. If you plot a graph for the variation of the temperature recorded by thermometer T1with time, it will be as shown here. Similarly, the graph for the temperature recorded by thermometer T2 with time will be as shown
It is a common experience that when a liquid is heated, it expands, irrespective of the temperature range. However, water is an exception. When water at 0 ºC is heated, it contracts till 4 ºC, instead of expanding. On further heating from 4 ºC, water expands like other liquids do. The special behaviour of water between 0ºC and 4ºC is called the anomalous behaviour of water.
In the year 1805, the scientist T. C. Hope devised a simple arrangement, known as Hope’s apparatus, to demonstrate the anomalous behaviour of water.
Hope’s apparatus consists of a long cylindrical jar with two openings on the side, one near the top and the other near the bottom to fit thermometers in each of these openings. A metallic cylindrical air-tight trough with an outlet is also fitted onto the jar, on its central portion. Two thermometers are fitted air-tight in the two openings of the cylindrical jar. The thermometer near the bottom of the jar is T1, and the one near the top of the jar is T2. Now the cylindrical jar is filled with water. The cylindrical trough at the central portion of the jar is filled with a freezing mixture of ice and common salt.
Initially, both the thermometers show the same reading, say 12 ºC. As the trough is filled with the freezing mixture, the water in the central portion of the jar loses heat energy and gets cooled. Its volume decreases and density increases. As a result, the denser layers of water in the central portion move downwards, displacing the lighter layers of water there towards the central portion. This forms a convection of water. Due to this convection, thermometer T1 near the bottom shows a rapid fall in temperature till it reaches 4 ºC. At 4 ºC, the density of the water in this part of the jar is maximum.
Thus, the dense water remains in the lower portion. temperature recorded by thermometer T2 remains almost unchanged. Now, the water in the central portion of the jar cools to below 4 ºC. Due to the anomalous expansion of water, it begins to expand. As the volume of the water increases, its density decreases. As a result, it moves upwards. Thus, thermometer T2 shows a rapid fall in temperature. However, the temperature of water in the lower portion of the cylinder remains at 4 ºC.The temperature of water in the upper portion of the jar continues to decrease gradually below 4 ºC.
As the temperature of water decreases from 4 ºC to 0 ºC, it expands and its density decreases. Hence, the cold water rises to the surface and forms a thin layer of ice. The ice floating on the surface of water indicates that the density of ice is less than the density of water at 4 ºC. The temperature in thermometer T1 remains at 4 ºC, while that in thermometer T2 falls to 0 ºC. This is due to the anomalous behaviour of water. If you plot a graph for the variation of the temperature recorded by thermometer T1with time, it will be as shown here. Similarly, the graph for the temperature recorded by thermometer T2 with time will be as shown
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