Sciencemadness Discussion Board

Are there any substance that has thermal hysteresis?

RU_KLO - 8-7-2024 at 11:46

Ok, maybe the question is not worded correctly.
When talking of hysteresis I dont mean magnetic, but more like a thermostat or Schmitt triggers,

So I will try to explain what I mean.

Melting point is a characteristic of a substance and is a fixed value.

Is there a substance that, for example, melts at 80ºC, but solidifies at 60ºC.
So in the range of 60º to 80ºC is solid or liquid depending from where you start.

So if you heat it stays solid till 80ºC, but once liquid, you need to lower its temperature till 60ºC to solidify it.

Maybe a substance that at certain temperature changes its chemical properties/structure, but recovers them at a lower or higher than original temperature.

I asked this to google, found some, but they are more magnetic in nature:
* Thermal hysteresis evaluation of the MnAs compound near room temperature. https://www.sciencedirect.com/science/article/abs/pii/S02540...

*Iron(III) Spin-Crossover Compounds with a Wide Apparent Thermal
Hysteresis around Room Temperature
https://pubs.acs.org/doi/10.1021/ja0017920



[Edited on 8-7-2024 by RU_KLO]

j_sum1 - 8-7-2024 at 17:04

Change of state is a kinetic process. Anything that exhibits super-cooling or super-heating will display the hysteresis you describe. But it is never going to be terribly predictable -- unlike magnetic hysteresis. The change of state depends on nucleation sites which is a function of surface properties, agitation, and a whole lot of randomness.

As for chemical properties or other physical properties -- I suspect the answer to your question is no. As far as I know, curie temperature for example is a constant. I can't think of any other property that might change depending on the direction of the temperature change.

Rainwater - 8-7-2024 at 18:24

thermostat or Schmitt triggers use a feedback loop to generate hysteresis.
For a schmitty trigger, the input is connected to series resistor circuit, a small amount of current from the output is feed back into the center of this circuit.
To change state, the input has to sink this additional current.

I can not think of any chemical that could reproduce this without some other type of topology providing feedback.

BromicAcid - 8-7-2024 at 19:12

As j_sum1 said, super cooling/heating comes to mind but so does different polymorphs. Look at chocolate for example and the different types of crystalline mass that form:

https://www.hotelchocolat.com/uk/blog/chocolateknowledge/wha...

Metacelsus - 9-7-2024 at 17:14

Agarose gels do this. See for example: https://cybercolloids.net/information/technical-articles/int...
Quote:
Agar has a uniquely high hysteresis between its melting and setting temperature. Typically, agar needs to be heated above 90°C to form a good solution and depending on the seaweed source the setting temperate can be as low as 30°C but is typically between 30-45°C for a 1.5% solution.


[Edited on 2024-7-10 by Metacelsus]