In refrigerator, why are the freezer always locate at the top
Freezer is the source for the refrigerator's coldness. When the freezer is placed on top, the cold air produced from it is denser than the warmer air in the bottom. So cold air being dense sinks down and the warm air is forced to rise up so when the warm air rises up it and gets cold in the freezer. This is the principle for the uniform cooling of a single compartment refrigerator.