Question

Why is the freezing chest (or cooling unit) in a refrigerator fitted near the top?

Answer 1

Hot air has lesser density than cold air. So it always rises up. Now the freezer cools it, thus increasing its density and this cold air descends to the lower part On the way down it absorbs the heat from the food items. As it reaches down its temperature increases and again it rises up. This is a very good example for a convection phenomenon. If the freezer was placed at the bottom of the fridge then this cycle would not occur and cold air would get accumulated in the lower portion of the fridge. This would cause unevenness in the temperature of air inside the fridge and there won't be any circulation.