Why does the boundary between liquid phase and gaseous phase disappear on heating a liquid up to critical temperature in a closed vessel? In this situation what will be the state of the substance?

The boundary between the liquid phase and the gaseous phase disappears when a liquid is heated up to its critical temperature in a closed vessel due to the process of reaching the critical point. At the critical temperature, the distinction between the liquid and gas phases becomes indistinguishable, and the substance exists in a state known as the supercritical fluid state.

Here's why this occurs:

1. Critical Temperature: The critical temperature (Tc) is the temperature above which a substance cannot exist in the liquid phase, regardless of pressure. At this temperature, the distinction between the liquid phase and the gas phase disappears, and the substance transitions into a supercritical fluid state.

2. Critical Pressure: The critical pressure (Pc) is the pressure required to liquefy a gas at its critical temperature. Below the critical temperature, increasing pressure can cause a gas to liquefy. However, above the critical temperature, no amount of pressure can cause liquefaction.

3. Supercritical Fluid State: When a substance is heated above its critical temperature in a closed vessel, it enters the supercritical fluid state. In this state, the substance exhibits properties of both a liquid and a gas. It has the density of a liquid and the ability to diffuse through materials like a gas. The boundary between the liquid phase and the gas phase becomes blurred, and the substance behaves as a homogeneous fluid.

In this situation, the substance will exist in the supercritical fluid state. It will not have distinct liquid and gas phases, as they merge into one homogeneous phase. The substance will exhibit properties of both liquids and gases, such as high diffusivity and solubility, making it useful in various industrial processes such as extraction, chromatography, and chemical reactions.