5A. Although we did not discuss this in detail in class, it turns out that caref

Question

5A. Although we did not discuss this in detail in class, it turns out that careful measurements show that water is slightly higher in density at 4 ?C than at any other temperature. This is a small, but real effect. We generally take the density of water to be 1000 kg/m3 and do not worry about any small change with temperature (just like we have done with other materials). But, in actual fact the density of water is a bit temperature dependent and the highest density is 1000 kg/m3 and this value is reached at 4 C and not

3 0 C. This is a small effect: the actual density at 0 C is 999.86 kg/m3 and this is also the

density at 8 C, with higher temperatures showing density values smaller than this; the higher the temperature, the smaller the density, but as mentioned these effects are quite small. So, for practical purposes we generally are happy to say that the density of water is 1000 kg/m3. But, for this question, think about the fact that the density of water does depend a bit on temperature. As a pond cools in the fall, the cooling takes place from the top of the pond (cold air cools the surface water). The cooler water sinks if it is denser than the water below it and it is replaced by warmer water from below. So, the pond cools. Think about all this and then respond:

What will happen to the pond as the fall season gets colder and heads into winter

a. The process described above will continue until all the water in the pond reaches 0 C.

b. The bottom of the pond will most likely remain at 4 ?C during the winter.
c. Any ice will naturally be found at the top of the pond because ice is less dense than water.

d. both a and c

e. both b and c

Explanation / Answer

Ans

Before reaching 4 oC, water sinks down from the top as cold water density is more than hot water density. When the pond reaches 4 oC , it has maximum density. so no further sinking, Hence top of the pond cools down further to get converted to Ice. Hence ice will naturally be found at the top of the pond and the bottom of the pond will be at 4 oC.

Hence option e is correct.

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