What is the significance of the critical point for water?

• A. It is the pressure and temperature at which liquid and vapor phases become indistinguishable.
• B. It is the pressure at the triple point.
• C. It is the pressure required to compress water to a supercritical state.
• D. It is the pressure at which water boils at 100°C.

Answer: (A)

The critical point is where the liquid–vapor boundary ends and the two phases become indistinguishable. At this point the densities of the liquid and the vapor converge and surface tension disappears, so you can’t tell apart a liquid from a gas. For water, this occurs at about 374°C and 22.064 MPa. Above these conditions, water becomes a single supercritical fluid whose properties change gradually with temperature and pressure, not through a distinct phase transition.

This is why the statement about being the pressure and temperature at which liquid and vapor phases merge is the best description. Other ideas mix in separate concepts: the triple point is a different reference point where solid, liquid, and gas coexist; and boiling at 100°C refers to the boiling point at standard pressure, not the critical point.