Which chemical does not mix with oil and floats on top of it?

The correct answer is carbon dioxide, which is indeed a gas that does not mix with oil and will float on top of it. When carbon dioxide is present in oil, it will form bubbles or layers due to its lower density compared to the liquid oil. This characteristic is significant because, in various applications, including environmental concerns and engineering, understanding the behavior of gases like carbon dioxide when in contact with liquids (particularly oils) is crucial.

The other options involve substances that either dissolve in oil or have a different behavior when in contact with it. Ammonia, while it can mix with water and form ammonium ions, does not float on oil and instead has a different interaction with liquids. Methane, being a lighter hydrocarbon gas, also does not perform as carbon dioxide does and may not reliably float atop oil under standard conditions. Freon, a group of refrigerants, behaves differently in that it can have various interactions with oils, depending on their specific types, but traditionally does not exhibit the property of simply floating on top like carbon dioxide does. Understanding these distinctions is vital for proper substance management in situations involving flammable materials or chemical safety protocols.