4.1.8. Phase Changes

Table of Contents

States and Changes of State

Matter can exist in different physical states, mainly solid, liquid, and gas. A phase change happens when a substance moves from one state to another without changing its chemical identity. Water, for example, can be ice, liquid water, or water vapor, but in all three cases it is still $H_2O$.

Phase changes are caused by changes in energy, usually by heating or cooling, and sometimes by changing pressure. In this chapter, the main idea is that during a phase change, the substance is changing its internal arrangement rather than simply changing its temperature.

Common Types of Phase Changes

The most familiar phase changes are melting, freezing, vaporization, condensation, sublimation, and deposition.

Melting is the change from solid to liquid. Freezing is the reverse process, from liquid to solid. Vaporization is the change from liquid to gas. Condensation is the reverse, from gas to liquid. Sublimation is the change directly from solid to gas, and deposition is the reverse, from gas to solid.

The table below summarizes these changes.

Initial state	Final state	Name of phase change
Solid	Liquid	Melting
Liquid	Solid	Freezing
Liquid	Gas	Vaporization
Gas	Liquid	Condensation
Solid	Gas	Sublimation
Gas	Solid	Deposition

What Happens Microscopically

In a solid, particles are closely packed and mostly vibrate around fixed positions. In a liquid, particles remain close together but can move around one another. In a gas, particles are much farther apart and move freely.

When a substance melts, the added energy weakens the tight binding that keeps particles in fixed positions. When a liquid boils or evaporates, energy allows particles to escape into the gas phase. When a gas condenses or a liquid freezes, energy is removed, and particles become more ordered.

A phase change is therefore a change in the arrangement and motion of particles.

Temperature During a Phase Change

One of the most important facts about phase changes is that the temperature of a pure substance often remains constant while the phase change is happening, as long as the pressure stays constant.

For example, when ice at $0^\circ \mathrm{C}$ melts, heat can be added and the temperature still stays at $0^\circ \mathrm{C}$ until all the ice has melted. The energy goes into changing the phase, not into increasing the temperature.

During a phase change, added or removed heat usually changes the state of the substance, not its temperature.

This is why heating curves often show flat sections where temperature does not rise even though energy continues to enter the substance.

Melting and Freezing

Melting happens when a solid absorbs enough energy for its particles to break out of their fixed arrangement. The temperature at which this occurs is the melting point. For a pure substance at a given pressure, melting happens at a definite temperature.

Freezing is the reverse process. As energy is removed from a liquid, particle motion decreases and the particles settle into a more rigid arrangement.

For most substances, the freezing point and melting point are the same at the same pressure.

Vaporization and Condensation

Vaporization can happen in two ways, evaporation and boiling. Evaporation occurs at the surface of a liquid and can happen at many temperatures. Boiling occurs throughout the liquid and happens at a specific temperature called the boiling point, for a given pressure.

Condensation happens when gas particles lose enough energy to return to the liquid state.

Boiling is strongly affected by pressure. At lower pressure, the boiling point is lower. At higher pressure, the boiling point is higher. This is why water boils at a lower temperature on a mountain than at sea level.

Sublimation and Deposition

Some substances can change directly between solid and gas without becoming liquid. This process is sublimation if the change is from solid to gas, and deposition if the change is from gas to solid.

Dry ice is a common example. It is solid carbon dioxide, and at ordinary atmospheric conditions it changes directly into gas instead of melting into a liquid.

Heating Curve

A heating curve helps show how temperature changes as heat is added to a substance. The sloped parts of the graph represent temperature change within one phase. The flat parts represent phase changes.

Rendering scientific sketch…

Typical heating curve of a substance

The flat part during melting shows that energy is being used to change solid into liquid. The flat part during boiling shows that energy is being used to change liquid into gas.

Energy and Phase Change

A phase change requires energy input or energy release. Melting, vaporization, and sublimation require energy to be absorbed. Freezing, condensation, and deposition release energy.

The energy involved in changing phase is connected to intermolecular forces. More energy is needed when particles must be separated more strongly, especially in the transition from liquid to gas.

Phase changes involve energy transfer even when the temperature stays constant.

This idea is central in thermodynamics. A substance can absorb heat without becoming hotter if that heat is being used for a phase change.

Phase Equilibrium

At the temperature where a phase change occurs, two phases can coexist in equilibrium. For example, at the melting point, solid and liquid can exist together. At the boiling point, liquid and gas can exist together.

This means that melting and freezing, or vaporization and condensation, can occur at the same time at equal rates under the right conditions.

A glass of ice water is a simple example of solid and liquid existing together in phase equilibrium.

Effect of Pressure

Pressure affects phase changes, especially those involving gases. Increasing pressure generally makes it harder for a liquid to become a gas, so the boiling point rises. Lowering pressure makes boiling easier, so the boiling point falls.

For many substances, pressure also affects melting point, though usually less dramatically. Water is a special case because ice is less dense than liquid water, which leads to some unusual behavior.

Phase Diagram, Basic Idea

A phase diagram shows which state of matter is stable for different values of temperature and pressure. Each region corresponds to a phase, and the boundaries between regions represent conditions where phase changes occur.

A complete study of phase diagrams belongs in a more advanced treatment, but the main point here is that phase depends not only on temperature, but also on pressure.

Rendering scientific sketch…

Simple phase diagram

Everyday Examples

Many everyday processes involve phase changes. Ice melting in a drink is melting. Water droplets forming on a cold glass are condensation. Frost forming on a window is deposition. Clothes drying involve vaporization. Snow disappearing on a cold dry day without first turning to liquid is sublimation.

These examples show that phase changes are not rare or exotic. They are part of daily life.

Key Ideas to Remember

Phase changes are changes of physical state, not chemical composition. They happen because energy is transferred to or from a substance. During a phase change, the temperature often remains constant while the substance changes state. Pressure can affect when phase changes occur, especially boiling. The main phase changes are melting, freezing, vaporization, condensation, sublimation, and deposition.

A phase change is a change in state of matter caused by energy transfer and sometimes pressure change, while the substance itself remains chemically the same.

For a pure substance at fixed pressure, melting and boiling occur at specific temperatures, and temperature usually stays constant until the phase change is complete.

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