Question

Give reason why there is a metal grill at the back of the fridge

Answer 1

Cooling coils at the back of refridgerators are made of copper because copper is a very good conducter of heat and electricity.A gas called freon22 flows through the tubes. This cools the refridgerator.
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Answer 2

Metal grill is at the back of the fridge because it is involved in recirculation of refrigerant fluid.

Explanation:

While the fridge works it leads to elimination of small amount of water and it is called refrigerant fluid. This fluid must be circulated back into the refrigerator. For this purpose, three parts are involved namely compressor, Radiating fins and coolant tubes.

The compressor is located at the bottom of the fridge and it is involved in pressurizing the fluid and sends it back through the coolant which is made up of copper and is attached with the metal grill. After pressure, this refrigerant liquid ends up hot and it is required to diminish its temperature.  

Thus this hot fluid is sent through coolant a tube which reaches the radiating fin and thus the water is again sent inside the refrigerator.