The discussion centers on identifying liquids with a lower density than petrol that are not classified as fuels. Liquid helium is mentioned, but it does not exist in liquid form at room temperature. Participants explore other candidates, emphasizing the need for substances that remain liquid at room temperature and have a density around 0.7 g/mL. Hydrogen cyanide is proposed as a potential option, with a boiling point of 25 degrees Celsius and a density of 0.68 g/cm^3. It is noted that while hydrogen cyanide is highly flammable, it is not typically used as a fuel, serving instead as a reactant and solvent in chemical synthesis. The conversation highlights the challenge of finding suitable liquids that meet the specified criteria.
