Question

why is the pipe carrying the hot gases made of a copper coil

Answer 1

Because.....
Copper is only metal that doesn't react with any temperature of water. It even don't react with ice or gas.

Answer 2

Hello friend!!
Rajdeep here....

According to my knowledge,
Copper is a good conductor of heat with a low specific heat capacity of only 0.4J/gC. Hence, when hot gases (gases at high temperatures) pass through a copper pipe, the heat taken from them by the copper pipe to rise its temperature is minimum (i.e., less heat is taken by from them by copper to attain the same temperature as the gases are in). Thus, loss of heat is minimum. Again, copper can retain heat for the maximum time, which helps the gases to remain hot for a long time, inside the pipes.