The least dense liquid in the world is often considered to be supercritical helium. At extremely low temperatures, close to absolute zero (-273.15°C or -459.67°F), helium can exist in a supercritical state where it exhibits both liquid-like and gas-like properties. In this state, helium becomes extremely low in density, making it the least dense liquid known.
Supercritical helium has a density of about 0.145 grams per cubic centimeter (g/cm³) at a temperature of 2.1768 Kelvin (-270.973°C or -455.751°F) and a pressure of 25 atmospheres. This makes it significantly less dense than other commonly encountered liquids, such as water, which has a density of approximately 1 g/cm³.
It's worth noting that the density of supercritical helium is highly dependent on temperature and pressure conditions. As the temperature or pressure changes, its density can vary accordingly. However, under the specific conditions mentioned above, supercritical helium remains the least dense liquid known.