Water has an unusually high critical point
The critical point of water is unusually high at 374 degrees Celsius and 218 atm.
Scientific Explanation
Every substance has a critical point — a specific combination of temperature and pressure above which the distinction between liquid and gas ceases to exist. For water, this point occurs at 374 degrees Celsius and 22.1 MPa (approximately 218 atmospheres). By comparison, hydrogen sulfide reaches its critical point at just 100 degrees Celsius and 9 MPa, while carbon dioxide becomes supercritical at a mere 31 degrees Celsius and 7.4 MPa.
Water’s unusually high critical point is a direct consequence of its strong hydrogen bonds. For the boundary between liquid and gas to vanish, the intermolecular attractive forces must be completely overcome. In water, these forces are so strong that extreme conditions are required to reach this state.
Above the critical point, water enters the supercritical state. Supercritical water behaves neither like a typical liquid nor like a gas: it has the density of a liquid but the diffusivity and viscosity closer to a gas. It can dissolve nonpolar substances that are insoluble in normal water, and its dielectric constant drops dramatically. These properties make supercritical water a remarkable solvent with unique chemical reactivities.
The high critical point also means that water can exist as a stable liquid over an extraordinarily wide range of temperatures and pressures — from 0 degrees Celsius at normal pressure up to 374 degrees Celsius under high pressure. This broad stability range is of critical importance for geological and hydrothermal processes.
Everyday Relevance
The high critical point has far-reaching consequences for both nature and technology. In the deep ocean, at hydrothermal vents (so-called “black smokers”), water reaches temperatures above 400 degrees Celsius but remains liquid due to the enormous pressure. Some of Earth’s oldest life forms evolved at these locations.
In industry, supercritical water is increasingly used as an environmentally friendly solvent — for example, in the destruction of toxic waste or the decaffeination of coffee. Power plants employ supercritical steam cycles to increase efficiency in electricity generation. Without water’s high critical point, none of these applications would be feasible.