When a bunsen burner is lit below a wire gauze the flame initially burns below the gauze after sometime the flame burns both below and above the gauze explain this observation?

Answer 1

At first, the air above the gauze is cold and heavy therefore, it cannot be ignite. After sometime, the hair above gets heated and becomes light hence can be ignited thus the flame burns above the gauze.

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Answer 2

This is difficult to demonstrate with modern steel gauzes, but works quite well with a copper gauze. A fuel and oxygen mixture will only ignite above a certain temperature, the flash point, which varies with the fuel. (This is why diesel engines may not start in cold weather and why some fires can be put out with water, which cools the fuel.) Copper is a very good conductor of heat, so it carries away the heat of the burning gas, and thus the gas mixture above the gauze is too cool to burn. However, the flame soon heats up the copper and the upper gases then reach their flash point.