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How can a substance change continuously from a gas-like fluid to a liquid-like fluid without crossing a phase boundary?

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Diagram showing four physical states of matter: solid with tightly packed molecules, liquid with loosely packed molecules, gas with widely spaced molecules, and plasma with free electrons and nuclei, each labeled with descriptions and an arrow indicating increasing energy. — phase States of matter.

A substance can transition continuously from a gas-like fluid to a liquid-like fluid by bypassing the vapour-pressure curve—the line on a phase diagram where liquid and gas coexist in equilibrium.1

To achieve this, the substance must be moved along a path that avoids crossing any phase-change lines. This is possible by increasing the temperature and pressure beyond the critical point, the specific set of conditions where the densities of the liquid and gas phases become identical.12

Once a substance exceeds its critical temperature and pressure, it enters the supercritical fluid state. In this state, the distinction between liquid and gas disappears, and the substance exhibits properties of both—flowing like a gas while maintaining a density closer to that of a liquid. Because no phase boundary is crossed during this process, the change in physical properties, such as density and viscosity, occurs smoothly and continuously rather than abruptly.341