The element that burns with a blue flame and gives off highly acidic gas is Sulphur (S). When sulphur burns in the presence of oxygen, it produces sulphur dioxide gas (SO2), which is highly acidic in nature. The blue flame is a characteristic of sulphur combustion due to the high energy released during the reaction. Sulphur dioxide gas has a pungent odour and can cause respiratory irritation and other health issues. It is also a major contributor to acid rain and environmental pollution.