# Thermodynamics cooling curve question

I'm given that a solid is heated, beyond melting point and then the temperature of the liquid is 90 Celsius. When it cools, the temp falls to around 80 Celsius and stays constant for a period of minutes before falling again.

What would 80 Celsius represent here in thermodynamics terms?

Am I right in assuming it's the temperature just above the solid - liquid phase change on a cooling curve?

• Have a look at this: bbc.co.uk/schools/gcsebitesize/science/ocr_gateway/home_energy/… – Gert Feb 29 '16 at 16:06
• Thanks, but in this question it mentions that the liquid remains at a constant temperature of 80 celcius for several minutes after falling from 90, and then falls again? – I come from a land down under Feb 29 '16 at 16:08
• I suspect it might be the given liquid re solidifying? – I come from a land down under Feb 29 '16 at 16:09
• So the "several minutes" must represent the phase change from liquid - solid right? – I come from a land down under Feb 29 '16 at 16:10
• Yes, of course. As the link shows. Pure substances have a definite melting/solidifying point. During melting/solidifying the temperature remains constant, at least for slow cooling/heating. – Gert Feb 29 '16 at 16:11

Considering water, it might start at $10 \;\mathrm{^\circ C}$ and is then cooled down to $-10 \;\mathrm{^\circ C}$. Along the way, there will be a while of constant-temperature cooling at exactly $0 \;\mathrm{^\circ C}$.