Phase Diagram of Water

What is a Phase Diagram?

A phase diagram is a graphical representation of the various phases of a substance or mixture of substances that coexist in thermodynamic equilibrium, and undergo phase changes under different working conditions, such as temperature, pressure, or volume.

The water system is divided into three phases: ICE (S), WATER (L), and WATER VAPOUR (G)

Since H₂O is the only chemical compound present, as a result, it is a one-component system.

Table of Contents

• Depiction in Phase Diagram of Water
• Difference between General Phase Diagram and Phase Diagram of Water
• Phase Diagram for Water
• Frequently Asked Questions – FAQs

Depiction in Phase Diagram of Water

At different temperatures and pressures, a phase diagram depicts the preferred physical states of matter. Water is a liquid at typical room temperatures and pressures, but it becomes solid (i.e. ice) if the temperature is reduced below 273 K, and gaseous (i.e. steam) if the temperature is increased above 373 K at the same pressure.

Each line describes the conditions that exist when two phases coexist, but a change in temperature or pressure can cause the phases to abruptly change from one to the other.

When three lines come together, there is a ‘triple point,’ which is where three phases coexist but can abruptly and completely change into each other due to a change in temperature or pressure. Four lines cannot intersect at the same point.

A ‘critical point’ is defined as the point at which the properties of two phases become indistinguishable from one another.

Difference between General Phase Diagram and Phase Diagram for Water

There is one significant difference between the general phase diagram and the water phase diagram. The slope of the line connecting the solid and liquid states in the water diagram is negative rather than positive. Water is an unusual substance in that its solid state is less dense than its liquid state. In liquid water, ice floats. As a result, a change in pressure has the opposite effect on those two phases. When ice is near its melting point, it can be converted into liquid water by applying pressure. In the liquid phase, water molecules are actually closer together than in the solid phase.

Phase Diagram for Water

Curves:

1. OA- Water & Vapor

OA is the vaporisation curve. The curve OA comes to an end at A. It has a critical pressure of 218 atm and a temperature of 374℃. It represents the vapour pressure of a liquid at various temperatures.

Along the curve, two phases of water and water vapours coexist in equilibrium.

The vapour pressure is one atmosphere. The corresponding temperature in degrees Celsius is the boiling point of water, which is 100℃.

1. OB- Ice and Vapor

The OB curve is a sublimation curve. The curve OB comes to an end at B, which is the absolute zero temperature of -273℃. It depicts the vapour pressure of solid ice at various temperatures.

In equilibrium, the two phases of solid ice and water vapour coexist.

1. OC- Ice & Water

The OC curve is the fusion curve. The OC curve comes to an end at C, the critical pressure.

In equilibrium, the two phases of solid ice and liquid water coexist. The curve shows that the melting point of ice decreases as pressure increases. At one atm, the line intersects the curve at 0℃.

Areas:

The area AOC, AOB, and BOC the curves or areas between the curves represent the conditions at temperature and pressure under which a single phase, i.e., ice, water, and water vapour, can exist indefinitely.

1. BOC- Ice- denotes the solid phase.
2. COA- Water- denotes the liquid phase.
3. AOB- Vapour- denotes the gaseous phase

Triple Point:

All three curves, OA, OB, and OC, meet at point O, which is known as the triple point, where all three phases, solid, liquid, and vapour, are in simultaneous equilibrium.

At 0.0075℃ and 4.58 mm Hg pressure, the triple point occurs.

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