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Why is ethanol used to make brown ethylene?
It is brown because it is mixed with concentrated H2SO4 as dehydrating agent.

The experiment is as follows:

The preparation of ethylene is an important experiment in senior high school chemistry (Volume II) and university organic chemistry experiments. In the current textbook [1-3], 95% ethanol is used as raw material and concentrated H2SO4 is used as dehydrating agent, and it is heated to 160℃ ~ 180℃. This method mainly has the following two disadvantages: (1) produces ethylene in a narrow temperature range, and ethylene can only be produced above 170℃, but when the temperature reaches 180℃, the solution turns black completely, which is difficult to control in the experiment. (2) There were side effects in the experiment from beginning to end. Ether is generated at 140℃ ~ 170℃, and carbonization begins above 170℃. With the increase of temperature, the generated carbon is oxidized by concentrated sulfuric acid into carbon dioxide, and the carbon dioxide itself is reduced to sulfur dioxide.