Define (a) critical point and (b) triple point in phase...

Define (a) critical point and (b) triple point in phase transformation.
Helium boils at 4.2 K under the atmospheric pressure $p=760 \mathrm{~mm}$ of mercury. What will be the boilding temperature of helium if $p$ is reduced to 1 mm of mercury?

Key Concepts

Boiling Point and Pressure Dependence

The boiling point is the temperature at which a liquid's vapor pressure equals the external pressure. This means that the boiling point is not a fixed property but varies with changes in external pressure. In lower pressure environments, the boiling point decreases, reflecting the critical role that external pressure plays in determining the conditions under which phase transitions occur.

Phase Transition and Equilibrium

Phase transitions refer to the transformation between different states of matter (solid, liquid, gas) that occur due to changes in temperature, pressure, or other thermodynamic variables. Phase equilibrium occurs when the different phases coexist without net change, meaning the rates of phase transition in both directions are equal. Phase diagrams graphically display these equilibria and the conditions under which each phase is stable.

Triple Point

The triple point is the specific set of conditions (temperature and pressure) at which all three phases of a substance—solid, liquid, and gas—can coexist in thermodynamic equilibrium. It is a unique and fixed point on the phase diagram and serves as an important reference in thermodynamics, allowing for precise calibration of temperature scales and the understanding of phase transitions.

Critical Point

The critical point is the end point of the phase equilibrium curve between the liquid and vapor phases. At this unique state, the properties of the liquid and vapor merge, meaning they become indistinguishable. In thermodynamic phase diagrams, this point marks the highest temperature and pressure at which the liquid and gas phases can coexist. Beyond this point, the phase boundary vanishes, and the substance exists in a supercritical fluid state.

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