Name: melting point and freezing equilibrium
Uploaded: 2020-09-24T15:06:49.000Z
Duration: 2 min 39 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.1

Phase Transitions: Melting and Freezing

The particles of a solid tightly pack together through attractive forces and vibrate at fixed positions without disrupting the lattice. 
The addition of heat causes the particles&#8217; thermal energy to rise, and they vibrate faster. The particles move about and rearrange by partially overcoming the intermolecular forces. Subsequently, the lattice collapses, and the solid melts.
This transition from solid to liquid is called melting or fusion, and the temperature at which it occurs is called the melting point or the fusion point. 
The change in enthalpy that is required to completely melt 1 mole of a solid at its melting point is called its molar heat of fusion or its molar enthalpy of fusion. Since melting nearly always requires energy, it is an endothermic process with a positive enthalpy value &#8212; with a few exceptions.
For example, when a mole of ice absorbs 6.02 kilojoules of heat energy from its surroundings, its temperature increases. When the temperature hits 0 &#176;C, it begins to melt.
For any substance, the heat of fusion is lower than the heat of vaporization. For instance, while melting a mole of ice requires merely 6.02 kilojoules of energy, vaporizing a mole of water requires 40.

phase transitions: melting and freezing

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy becomes large enough to partially overcome the forces holding the molecules or ions of the solid in their fixed positions, and the solid begins the process of transitioning to the liquid state or melting. At this point, the temperature of the solid stops rising, despite the continual input of heat, and it remains constant until all of the solid is melted. Only after all of the solid has melted will continued heating increase the temperature of the liquid.
If heating is stopped during melting and the solid-liquid mixture is placed in a perfectly insulated container so no heat can enter or escape, the solid and liquid phases will remain in equilibrium. This is almost the situation with a mixture of ice and water in a very good thermos bottle; almost no heat gets in or out, and the mixture of solid ice and liquid water remains for hours. In a mixture of solid and liquid at equilibrium, the reciprocal processes of melting and freezing occur at equal rates, and the quantities of solid and liquid, therefore, remain constant. The temperature at which the solid and liquid phases of a given substance are in equilibrium is called the melting point of the solid or the freezing point of the liquid.
Use of one term or the other is normally dictated by the direction of the phase transition being considered, for example, solid to liquid (melting) or liquid to solid (freezing). The enthalpy of fusion and the melting point of a crystalline solid depend on the strength of the attractive forces between the units present in the crystal. Molecules with weak attractive forces form crystals with low melting points. Crystals consisting of particles with stronger attractive forces melt at higher temperatures.
The amount of heat required to change one mole of a substance from the solid state to the liquid state is the enthalpy of fusion, &#916;Hfus of the substance. The enthalpy of fusion of ice is 6.0 kJ/mol at 0 &#176;C. Fusion (melting) is endothermic.
<img alt="Static equilibrium; &#931;F=0, &#931;M=0 formula; beam and forces diagram; equilibrium analysis tool." src="/files/ftp_upload/11349/11349_Equation%201.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq1" />
The reciprocal process, freezing, is an exothermic process whose enthalpy change is &#8722;6.0 kJ/mol at 0 &#176;C:
<img alt="Mass spectrometry setup; ion fragmentation process diagram for protein analysis." src="/files/ftp_upload/11349/11349_Equation_2.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq1" />
This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions">Openstax, Chemistry 2e, Section 10.3: Phase Transitions</a>.

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy ...

Education

JoVE Core

Liquids, Solids, and Intermolecular Forces

KEY TERMS AND CONCEPTS

melting point and freezing equilibrium

Melting point and freezing equilibrium describe how a crystalline solid changes between solid and liquid states. As a solid is heated, the average energy of its atoms, molecules, or ions increases, and the solid becomes hotter.

At a certain point, the added energy becomes large enough to partly overcome the forces that hold the particles in fixed positions. The solid then starts melting, or changing into a liquid. During melting, the temperature stops rising even though heat is still being added. It stays constant until all of the solid has melted. Only after that does continued heating raise the temperature of the liquid.

If heating stops partway through melting and the mixture is placed in a perfectly insulated container, no heat can enter or leave. In that case, the solid and liquid remain in equilibrium. A mixture of ice and water in a very good thermos is a close example, since very little heat moves in or out and the ice-water mixture can last for hours. In this equilibrium state, melting and freezing happen at equal rates, so the amounts of solid and liquid stay the same.

The temperature where the solid and liquid phases of a substance are in equilibrium is called the melting point of the solid or the freezing point of the liquid. Which term is used depends on the direction of the change, such as solid to liquid for melting or liquid to solid for freezing. The melting point and the enthalpy of fusion depend on the strength of the attractive forces in the crystal. Substances with weak attractive forces usually have low melting points, while crystals with stronger attractive forces melt at higher temperatures. The enthalpy of fusion is the heat needed to change one mole of a substance from solid to liquid. For ice, ΔHfus is 6.0 kJ/mol at 0 °C. Fusion, or melting, is endothermic. Freezing is the reverse process and is exothermic, with an enthalpy change of −6.0 kJ/mol at 0 °C.

Melting Point and Freezing Equilibrium

molecular comparison of gases, liquids, and solids

molecular-comparison-of-gases-liquids-and-solids

How Intermolecular Forces Shape States

Intermolecular forces and particle motion shape the states of matter: solids, liquids, and gases. In a solid, particles are tightly packed and often arranged in a regular pattern. A solid has a fixed shape, and its particles vibrate in place rather than flowing past one another. In a liquid, particles stay close together but do not have a regular arrangement. Liquids have no fixed shape, yet their particles move past each other while remaining in contact. That is why liquids can flow and take...

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Intermolecular forces are electrostatic attractions between molecules. Their strength depends on how far apart the molecules are. These forces matter most in solids and liquids, where particles are close together. In gases, they become important mainly under high pressure because the molecules are then closer together. Intermolecular forces help control key physical properties. They affect melting point, boiling point, density, and the enthalpies of fusion and vaporization. When a liquid is...

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How Molecule Shape Affects Intermolecular Forces

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phase transitions: vaporization and condensation

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Heat and Mass Transfer in Phase Changes

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clausius-clapeyron equation

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Melting point and freezing equilibrium describe how a crystalline solid changes between solid and liquid states. As a solid is heated, the average energy of its atoms, molecules, or ions increases, and the solid becomes hotter. At a certain point, the added energy becomes large enough to partly overcome the forces that hold the particles in fixed positions. The solid then starts melting, or changing into a liquid. During melting, the temperature stops rising even though heat is still being...

phase transitions: sublimation and deposition

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solids

Molecular and Ionic Crystal Properties

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x-ray crystallography

X-ray Diffraction in Crystal Structure

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lattice centering and coordination number

lattice-centering-and-coordination-number

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key terms and concepts

Explore how heating a crystalline solid leads to melting, why temperature levels off during the change, and how melting point, freezing point, and enthalpy of fusion are defined.

Melting Point and Freezing Equilibrium | JoVE K12

chemistry

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Physical Science

Matter and its Interactions

PS.PSc.1.2

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Ohio’s Learning Standards and Model Curriculum

Study Of Matter

Classification of matter

Structure And Properties Of Matter

Intermolecular forces of attraction

Virginia  Standards of Learning

HIgh School

CH.6

CH.6.a

PS.3

PS.3.b

Texas Essential Knowledge and Skills

High School

CHEM.11

CHEM.11.C

Alabama Course of Study Science

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9-12.C.1.5.d

9-12.PS.3.5.b

Florida Department of Education

SC.912.P.8

SC.912.P.8.2

SC.912.P.12

SC.912.P.12.11

Chemistry and Physics

IPC.7

IPC.7.C

Georgia Standards of Excellence

SPS5.

Energy and Phase Transitions

SPS7.

Phase Changes and Energy

SC5.

Heating Curves and Phase Changes

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PS.Chm.7.2

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PS.Chm.7.3

Understand the relationship among pressure, temperature, volume, and phase.

Melting Point and Freezing Equilibrium

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