[MatheusLinsPicollo]

Hello!

I am having some trouble answering a question that raised during my last lab class. It was a simple experiment: we have put a test tube with some naphthalene crystals and a thermometer on a bain-marie, waited until the whole sample melted, put the fire off and observed the change in temperature until the sample reached ~60ºC.

Here's an image of the structure assembled for the experiment. In addition to this, we used a plastic stopper to seal the test tube, with a little hole in the center for the thermometer:

The thing is, once the naphthalene started solidifying (at around 78,9ºC), the temperature dipped to 78ºC and then raised again to 78,7ºC, when it resumed the expected cooling.

Since then I started looking for why that happened. I took a look at some books (as Russell, Brady and Humiston, Kotz, Atkins...) and found nothing. Then I searched in the internet and I found some papers on recalescence, but it supposedly only happens on metals and the articles I've found about this are really sparse to draw any connection. The only interesting thing I've found on web was this lab report which covers a very similar experiment, and the same phenomenon happens, but the author simply ignores it 

So, long story short, why this temperature raise happens?
Any kind of help is welcome! You guys are my last resort!

Thanks in advance,
Matheus

PS: Sorry for my broken english.

[Borek]

Have you (separately) tracked water temperature?

[MatheusLinsPicollo]

Have you (separately) tracked water temperature?

Sadly, no.

I've also thought about the possibility of the water reheating the naphthalene (since it's temperature should be stable during freezing point and water was still hot), but the time it took from the formation of the first naphthalene crystal and the reheating simply doesn't match. Look:

-The experiment started at 0s, 98ºC;

-It took 540s (9 min) for the first crystal to appear, at 78,9ºC;

-For the next minute, the temperature continued falling (albeit slowy), down to 78ºC at 600s (10 minutes);

-At 620s the temperature bumped to 78,3ºC, then to 78,5ºC at 640s and 78,7ºC at 660s, and remained there until 720s, when it resumed the cooling.

To my mind, it's ok to assume that at the start of the experiment the water's temperature was greater than the naphthalene's. But it's also ok to assume that once the naphthalene started cooling down, the water's temperature and the naphthalene temperature's should've been close, since the tube was sealed and the only way for the sample to exchange heat was through the water. When the naphthalene started to solidify it should've stayed on it's freezing point range while the water continued it's natural cooling, having no business reheating the naphthalene.

I know I can be wrong, and if I am, please correct me!
I'm still looking foward to any suggestion!

[Enthalpy]

Uniform temperatures are difficult to obtain and require good stirring. Here less than 1K differences can well result from uneven distributions.

Just one possible direction: naphthalene stayed shortly liquid down to +78.9°C, or below its +80.3°C freezing point (something common)
https://en.wikipedia.org/wiki/Supercooling
and when it later froze, the released heat made it warmer.

Movements in the water with heterogeneous temperature could probably offer more explanations.