Problem 58 - Chemistry The Central Science

Question

(a) What is the significance of the triple point in a phase dia gram? (b) Could you measure the triple point of water b measuring the temperature in a vessel in which water vapo liquid water, and ice are in equilibrium under one atmospher of air? Explain.

Step-by-Step Solution

Answer

The triple point in a phase diagram signifies a unique set of conditions (temperature and pressure) where solid, liquid, and gas phases coexist in equilibrium. However, measuring the triple point of water in a vessel under one atmosphere of air (101325 Pa) is not possible, as the triple point pressure for water is 611.657 Pa, and the necessary pressure and temperature conditions would not align.

Step 1: Triple Point Significance

The triple point in a phase diagram represents a unique set of conditions (temperature and pressure) at which all the three phases of a substance, i.e., solid, liquid, and gas, coexist in equilibrium. In other words, it is the point at which the substance can transform from one phase to another, and all three phases are stable at the same time.

Step 2: Measuring Triple Point of Water

We are now asked if we can measure the triple point of water by having water vapor, liquid water, and ice in equilibrium in a vessel under one atmosphere of air. The triple point of water occurs at a specific temperature (273.16 K) and pressure (611.657 Pa). However, one atmosphere of air is equivalent to a pressure of 101325 Pa, which is much higher than the triple point pressure for water. Thus, under one atmosphere of air, the conditions will not be the same as the triple point, and the three phases of water will not be in equilibrium. Therefore, it is not possible to measure the triple point of water by simply having the three phases under one atmosphere of air, as the pressure and temperature conditions would not align with the triple point values.

Key Concepts

Phase Diagram

A phase diagram is essential for understanding how a substance behaves under different conditions of temperature and pressure. It acts like a map, showing the states (solid, liquid, gas) in which a substance can exist. For every substance, the phase diagram looks different, marking distinct regions where a particular phase is stable.

The diagram includes lines known as phase boundaries, which separate different states. These lines indicate the conditions where two phases can coexist in equilibrium. One notable feature of the phase diagram is the triple point, a place where solid, liquid, and gas phases meet.

• Triple Point: where all three phases can coexist.
• Critical Point: the end of the liquid-gas boundary, beyond which distinct liquid and gas phases don't exist.

Understanding these elements in phase diagrams helps predict how substances will react when temperature and pressure change.

Equilibrium

Equilibrium in the context of a phase diagram refers to the state where multiple phases of a substance can exist together, without any net transformation from one phase to another. This means the amount of each phase does not change over time. When a substance is right on a phase boundary, say between solid and liquid, both phases can exist in equilibrium.

At the triple point, equilibrium is particularly special. It is the only condition under which all three major phases—solid, liquid, and gas—of a substance are in equilibrium at the same time. This is a precise condition relating to the specific temperature and pressure unique to the substance. For example, for water, this occurs at 273.16 K and 611.657 Pa.

• Dynamic Equilibrium: ongoing processes occur, but without changing overall phase proportions.
• Phase Boundaries: lines in phase diagrams showing all points where specific equilibrium exists.

Understanding equilibrium is crucial as it describes how phases adjust to maintain balance, providing insight into natural and industrial processes.

Water Phases

Water is a fascinating substance due to its ability to exist naturally in three phases: solid (ice), liquid (water), and gas (vapor). Each phase can transform into another when conditions like temperature or pressure change. These transformations are famously depicted in a phase diagram for water, demonstrating its phase behavior under different conditions.

The triple point of water is where ice, liquid water, and water vapor meet in equilibrium. This occurs at a unique temperature and pressure, specifically 273.16 K and 611.657 Pa, respectively. At this precise point, all three phases can coexist. However, it's essential to have the right conditions because merely having all phases present at one atmosphere of air won't recreate this state.

• Solid to Liquid: melting at 0°C under normal pressure.
• Liquid to Gas: boiling at 100°C under normal pressure.
• Solid to Gas: sublimation, primarily seen in conditions below freezing.

These properties of water phases make it a subject of extensive study, playing a vital role in natural phenomena and influencing environmental conditions globally.