A freezing system starts cooling the room at the rate of 4°C per hour. If at the beginning, the temperature of the room is 30° C, find the number of hours it takes to lower down the room temperature to -2° C.
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Answer:
This phenomenon is true for not only glasses of frozen water but also frozen lakes. We know that water is the densest at 4 degrees Celsius. So when the process of freezing begins say … from the temperature T degrees Celsius which is greater that 4 degrees Celsius, the water keeps getting denser as it reaches 4 degrees. Now denser water sinks to the bottom, and that’s what happens till 4 degrees. Just as the top layer of water crosses 4 degrees, it’s density decreases and it cannot sink anymore, so it becomes an insulating layer for the water below as it freezes to 0 degrees. Once the top layer has frozen over, its the turn for the next layer to freeze, then the next, and so on, till the entire glass of water freezes. This is a brief picture of how and why the water starts freezing from the top.
You must be wondering about what happens to the layer of water touching the glass walls and bottom. Since the solid glass will conduct heat away from the water quicker than the air touching the surface ever can, it should also freeze…right? Actually, the water touching the sides do cool more rapidly, but as the temperature reaches 4 degrees, its density become lighter than the surrounding water and hence it rises to the 1st layer, where is contributes to the freezing.
Now coming to the thawing, as the temperature rises from X degree Celsius below 0 degrees to the melting point, the water starts gaining heat more rapidly from the sides and bottom of the glass because of the aforementioned property of conduction of solids.and they melt first. The water being naturally denser than the ice trickles to the bottom and starts accumulating there. Also that’s when an interesting thing happens. Since the melted down water is denser that the ice it had been, it also occupies less volume. So what happens is that a layer of ice is left behind at the surface while below the surface the level of water slowly goes down. This stays until the layer cannot maintain it’s own weight, and collapses. This phenomenon is not so evident in a small glass of water, but it is evident in big lakes, which make noise as the ice cracks from the strain of its own weight. have a look at the video here. It beautifully demonstrates this phenomenon.
Now as the glass is small, the entire ice layer at the top comes loose and starts floating on top until it too, melts away. And this is how the thawing of the glass of ice begins not only from the bottom, but the sides as well.