You are working in your laboratory and decide to do some cleaning. you find a test tube with some brown substance congealed at the bottom. you take a piece of iron wool to scrub it out, but cannot remove it. you leave the iron wool in the test tube and set it aside to clean your beakers instead. you do not realize the test tube is next to the heater and after a few minutes, a brown gas starts smoking out of the test tube, followed by a reddish-brown gas. upon observation, the iron wool is coated in a yellow-brown substance. you realize that the mystery substance was

The mysterious substance is Nitrogen Dioxide

Explanation:

The mysterious substance is Nitrogen Dioxide in solid form. When heated it releases the reddish-brown gas that has a strong and pungent odor.

It is common to see this gas as a result of the combustion process carried out in thermal plants, especially when the material submitted to combustion is composed of oil or gases. This occurs a lot in many industrial processes and can be seen even in older motor vehicles.

It is important to protect yourself against this gas, because it can cause irritation of the eyes and nose, in addition to pulmonary emphysema, which can be very dangerous.

the answer would be a