X-Message-Number: 33237
Date: Sat, 15 Jan 2011 13:32:25 -0500
From: "Perry E. Metzger" <>
Subject: re: latent heat of fusion
References: <>

> From: "Eisab" <>
> References: <>
> Subject:   latent heat of fusion
> Date: Fri, 14 Jan 2011 21:34:58 -0500
> "Despite the control applied to the cooling of cells, most of
> the water present will freeze at approximately -2 C to -5 C. The
> change in state from liquid to crystalline form results in the
> release of energy in the form of heat; this is known as the latent
> heat of fusion. Warming of the sample occurs until the equilibrium
> freezing point is reached, at which temperature ice
> continues to form. To minimize the detrimental effects of this
> phenomenon, undercooling must be minimized by artificially
> inducing the formation of ice. This can be accomplished by
> seeding the suspension with ice or some other nucleating
> agent, or by rapidly dropping the temperature of the external
> environment to encourage ice crystal formation."
> http://www.nalgenelabware.com/techdata/Technical/cryo.pdf
> I  wonder if the latent heat of fusion is a big problem in cryonics.

All substances release heat when they go from a more disordered to a
more ordered state, and require heat to move from a more ordered to a
more disordered state. This is a basic rule of thermodynamics. It is
the reason why energy is required to effect a phase change, such as
moving a system from crystal to liquid or from liquid to gas. (In
fact, there is a basic relationship between the heat the process
requires, the temperature, and the change in entropy of the system: in
an isothermal process, the change in entropy times the absolute
temperature is the energy change in the system. (The formula is a
nearly equivalent integral if the process is not isothermal, but most
phase transitions like this are in practice isothermal.))

When the text you quoted refers to "warming", what they're really
saying is that a system undergoing the phase change "tries" to remain
at the phase transition temperature through the whole change of
phase. For example, quite famously, if you add heat to a liquid that
is reasonably well stirred, you will note that the temperature rises
steadily until you hit the boiling point, at which time the
temperature plateaus at the boiling point. Similar effects are seen in
all phase transitions such as crystallization (aka freezing). This is
not a particularly complicated phenomenon to understand -- the phase
transition itself will consume the heat added to the system or will
produce all the heat removed from the system until the transition is
completed, at which point more heat added goes into raising the
temperature again (or in the case of cooling, the heat removed starts
resulting in lowered temperatures again). If a slightly supercooled
liquid begins to freeze, the temperature can rise, but only to the
freezing point of the substance because of the liberated heat (because
if it got above that, the substance would melt, self-limiting the
process), and that's the precise explanation of the phenomenon
described in your quotation.

In freezing tissues for preservation, cryoprotectants are typically
added to depress the temperature of the phase transition (or to
prevent it by forcing the formation of glasses). As the temperature at
which the phase change drops, so too does the heat involved in the
phase change (see the relationship of entropy, temperature and
enthalpy), so phase changes at depressed temperatures (because of the
addition of cryoprotectants) necessarily involve lower heats of
fusion, and thus less heat needs to be removed for the phase change to
complete itself. Further, for some processes, such as glass
transitions, the difference in entropy between the solid and liquid
forms is not apparent -- no true phase transition happens and no heat
of fusion is involved at all. (This is one reason glass transitions
are not abrupt.)

Perry E. Metzger		

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