The inside of a kettle turns black when boiled on gas stoves but not electric ones due to the difference in combustion processes and heat distribution.
When using a gas stove, the flame is directly in contact with the bottom of the kettle, and the combustion process is not as controlled as in electric stoves. Gas flames can produce soot or carbon particles as a byproduct of incomplete combustion. These tiny carbon particles can be released into the air and may come into contact with the bottom of the kettle, causing the blackening effect over time.
On the other hand, electric stoves use electric coils or heating elements to transfer heat to the kettle. The heating elements are enclosed and do not produce soot or carbon particles like gas flames do. As a result, there is no direct contact with a flame, and the inside of the kettle does not turn black when boiled on an electric stove.
It's worth noting that even on gas stoves, not all kettles will turn black. The extent of blackening can depend on various factors, such as the composition of the kettle's surface, the quality of the gas combustion, and the cleanliness of the gas burner. Regular cleaning and maintenance can help minimize the blackening effect on kettles used on gas stoves.