Melting and Freezing


Related Posts:

This figure shows four photos each labeled, “a,” “b,” “c,” and, “d.” Each photo shows a beaker with ice and a digital thermometer. The first photo shows ice cubes in the beaker, and the thermometer reads negative 12.0 degrees C. The second photo shows slightly melted ice, and the thermometer reads 0.0 degrees C. The third photo shows more water than ice in the beaker. The thermometer reads 0.0 degrees C. The fourth photo shows the ice completely melted, and the thermometer reads 22.2 degrees C.
Figure 1. (a) This beaker of ice has a temperature of −12.0 °C. (b) After 10 minutes the ice has absorbed enough heat from the air to warm to 0 °C. A small amount has melted. (c) Thirty minutes later, the ice has absorbed more heat, but its temperature is still 0 °C. The ice melts without changing its temperature. (d) Only after all the ice has melted does the heat absorbed cause the temperature to increase to 22.2 °C. (credit: modification of work by Mark Ott)

Melting and Freezing (OpenStax Chemistry 2e)

When we heat a crystalline solid, we increase 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 (Figure 1).

If we stop heating during melting and place the mixture of solid and liquid in a perfectly insulated container so no heat can enter or escape, the solid and liquid phases 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, ΔHfus of the substance. The enthalpy of fusion of ice is 6.0 kJ/mol at 0 °C. Fusion (melting) is an endothermic process:

The reciprocal process, freezing, is an exothermic process whose enthalpy change is −6.0 kJ/mol at 0 °C:


Flowers, P., Theopold, K., Langley, R., & Robinson, W. R. (2019, February 14). Chemistry 2e. Houston, Texas: OpenStax. Access for free at:


Related Research

Research Article: Kinetics of Decelerated Melting

Date Published: March 01, 2018 Publisher: John Wiley and Sons Inc. Author(s): Lothar Wondraczek, Zhiwen Pan, Theresia Palenta, Andreas Erlebach, Scott T. Misture, Marek Sierka, Matthieu Micoulaut, Uwe Hoppe, Joachim Deubener, G. Neville Greaves. Abstract: Melting presents one of the most prominent phenomena in condensed matter science. Its microscopic understanding, however, is still fragmented, … Continue reading

Research Article: The Human Genomic Melting Map

Date Published: May 18, 2007 Publisher: Public Library of Science Author(s): Fang Liu, Eivind Tøstesen, Jostein K Sundet, Tor-Kristian Jenssen, Christoph Bock, Geir Ivar Jerstad, William G Thilly, Eivind Hovig, Yves van de Peer Abstract: In a living cell, the antiparallel double-stranded helix of DNA is a dynamically changing structure. The structure relates to interactions … Continue reading

Research Article: A Comparison of Biocompatibility of a Titanium Alloy Fabricated by Electron Beam Melting and Selective Laser Melting

Date Published: July 8, 2016 Publisher: Public Library of Science Author(s): Hong Wang, Bingjing Zhao, Changkui Liu, Chao Wang, Xinying Tan, Min Hu, Jie Zheng. Abstract: Electron beam melting (EBM) and selective laser melting (SLM) are two advanced rapid prototyping manufacturing technologies capable of fabricating complex structures and geometric shapes from metallic materials using … Continue reading

Research Article: A stitch in time: Efficient computation of genomic DNA melting bubbles

Date Published: July 17, 2008 Publisher: BioMed Central Author(s): Eivind Tøstesen. Abstract: It is of biological interest to make genome-wide predictions of the locations of DNA melting bubbles using statistical mechanics models. Computationally, this poses the challenge that a generic search through all combinations of bubble starts and ends is quadratic. An efficient algorithm … Continue reading

Research Article: Counterintuitive DNA Sequence Dependence in Supercoiling-Induced DNA Melting

Date Published: October 29, 2015 Publisher: Public Library of Science Author(s): Rifka Vlijm, Jaco v.d. Torre, Cees Dekker, Fenfei Leng. Abstract: The metabolism of DNA in cells relies on the balance between hybridized double-stranded DNA (dsDNA) and local de-hybridized regions of ssDNA that provide access to binding proteins. Traditional melting experiments, in which short … Continue reading