A supercritical fluid is formed whenever

Correct option is A

The correct answer is (a) A substance is heated above its critical temperature
Explanation: • A supercritical fluid is a state of matter that occurs when a substance is held at a temperature and pressure above its critical point. • The critical temperature is the highest temperature at which a substance can exist as a liquid, regardless of the pressure applied. • When a substance is heated above its critical temperature, the molecules possess enough kinetic energy to overcome intermolecular attractive forces, preventing the formation of a distinct liquid phase. • In this state, the distinction between the liquid and gas phases disappears, and the substance exhibits properties of both: it has the density of a liquid but the viscosity and diffusivity of a gas. • For a fluid to be truly supercritical, it must also be above its critical pressure, which is the minimum pressure required to liquefy a gas at its critical temperature.
Information Booster: • Supercritical Carbon Dioxide (CO2) is the most widely used supercritical fluid because its critical point is easily accessible (critical temperature about 31°C and critical pressure about 73 atm). • These fluids are highly valued as "green solvents" in industrial processes like decaffeinating coffee or dry cleaning, as they can be easily removed by reducing pressure.
Additional Knowledge:
(b) A substance is completely burnt (Incorrect) • Complete combustion is a chemical reaction that results in the formation of new products (like carbon dioxide and water) and heat; it is not a process for forming a supercritical state of the original substance.
(c) A substance is heated far below its critical temperature (Incorrect) • Below the critical temperature, a substance exists in distinct solid, liquid, or gas phases depending on the pressure. • If the temperature is below the critical temperature, increasing the pressure will eventually cause a gas to liquefy rather than become a supercritical fluid.