My physics text says this: 'The change of state from solid to liquid is called melting and from liquid to solid is called fusion'.

But according to my chemistry text in a different context, 'The enthalpy change that accompanies the melting of one mol of a solid substance in standard state is called standard enthalpy of fusion'.

I think it is a pretty good assumption to say that fusion and melting are equivalent from this statement.

According to Wikipedia,

Melting, or fusion is a physical process that results in the phase transition of a substance from a solid to a liquid.

Why is this difference? All the articles I have read so far define fusion as a transition from solid to liquid. So, why does my physics text say otherwise?

Also, in simple terms, fusion refers to the combination of things, right? So, won't it be more intuitive to say that fusion implies the transition to solid from liquid? (due to the formation of a more ordered structure).

I apologise if this was a silly question. But, I would still like to know the answers.

  • 1
    $\begingroup$ It seems there is a lot of conFUSION on the naming of the applicable latent heat. I wouldn't lose any sleep over it. The heat required to melt or solidify is the same except for the direction of heat flow. $\endgroup$
    – Bob D
    May 4, 2021 at 12:59

2 Answers 2


The English terminology on phase changes is imprecise from being mixed a lot with non-scientific daily usage.

  • Solid $\to$ liquid: Melting
  • Liquid $\to$ solid: Freezing/fusion/solidification

The ideal thermodynamic temperatures where these phase changes happen are the same, typically and interchangeably called melting point, freezing point or ice point (I've never heard fusion point, but why not). Also, the latent energies required for these phase changes are the same (with opposite signs), typically just called latent heat of fusion.

  • Liquid $\to$ gas: Evaporation/vaporization/boiling
  • Gas $\to$ liquid: Condensation

Here we typically talk about the boiling point or steam point as well as latent heat of evaporation for both. Sometimes evaporation refers specifically to a liquid-to-gas phase change below the boiling point, and boiling only above the boiling point.

  • Solid $\to$ gas: Sublimation (rarely evaporation, often within materials science and microfabrication circles)
  • Gas $\to$ solid: Deposition (rarely condensation, again sometimes in materials science and microfabrication contexts)

These phase changes are uncommon in daily life outside the lab and don't have colloquial terms for phase-change temperatures. We would simply say sublimation point and deposition point when needed.

  • 2
    $\begingroup$ Merriam-webster gives this definition for fusion : "the act or process of liquefying or rendering plastic by heat", i.e. solid $\to$ liquid $\endgroup$ May 4, 2021 at 14:13
  • 2
    $\begingroup$ @SolubleFish That is a different meaning - joining two solid materials by heating the interface between them so that they melt, and then allowing them to cool. This forms a strong bond between the materials if the liquids dissolve in each other. $\endgroup$
    – alephzero
    May 4, 2021 at 14:59
  • $\begingroup$ So, if somebody says that the magnitude of the latent heat for a particular reaction is X units, I won't be able to figure out what that reaction is, right? Dont these notations make the usage of these processes inconvenient and so how can their usage be justified?@Steeven. PS: I have heard of ice point and steam point as well $\endgroup$
    – Proxima
    May 9, 2021 at 17:05
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    $\begingroup$ @Proxima Correct, if you just have the latent heat value for, say, fusion, then you can't know from that value whether we are dealing with melting or solidification. It is just an amount of energy. Nevertheless, it still does represent the energy needed for either of these processes, so the use of it as a property is definitely justified. It just doesn't convey or contain as much information as it could have. You are right that it is inconvenient with ambiguous terminology in certain fields - sometimes traditional use overcomes exactness and sense (queue, the old SI system...) Context is key. $\endgroup$
    – Steeven
    May 9, 2021 at 17:23
  • $\begingroup$ @Proxima I have never heard the terms ice point and steam point before. Are they other terms for the freezing point and boiling point? I'm which context have you heard them? $\endgroup$
    – Steeven
    May 9, 2021 at 17:24

I have never heard of fusion as a word for melting. Other answers made it sound like there was a fusion (combination) of heat in matter, but that is odd. There is no "steam" point at 100c (373.15k) because water vapor can exist below 0C- Water condenses at DEW point that varies with relative humidity.


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