A dense white fume is formed when ammonia gas reacts with
Answer Details
When ammonia gas (NH3) reacts with hydrochloric acid (HCl), a dense white fume is formed, which is actually ammonium chloride (NH4Cl). This is because the ammonia reacts with the hydrogen chloride in the air to form ammonium chloride, which is a white solid that sublimes at room temperature, meaning it turns directly from a solid to a gas. Therefore, the correct answer would be "HCl".