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Why the temperature of any substance is remain constant during its melting point or boiling point?

Hello Nargis,

Consider a substance being cooled. Assume that heat is removed at a constant rate as it goes from liquid at some temperature greater than the freezing/melting point to some temperature colder than the freezing/melting point. While the temperature is greater than freezing/melting, the kinetic energy of the molecules decreases as heat is removed. That decrease in molecular kinetic energy translates to decreasing temperature. When the temperature reaches the freezing/melting point, the molecules of the substance initially have too much energy to willingly start to settle into the fixed matrix of the frozen state.

As energy is removed from various molecules, those molecules begin to organize into the fixed matrix of the frozen state. Before any of the molecules that have settled into the fixed matrix of the frozen state can cool any further, the remaining molecules in the liquid state have to have their excess energy removed and they will then settle into the fixed matrix of the frozen state. Otherwise they might disrupt the partially formed matrix of the frozen state that other molecules have settled into. Once all molecules are in the frozen state, reduction of the temperature can continue as further energy is removed.

Cooling a substance from higher than the boiling/condensing point works in a very similar way. When warming a substance through either the freezing/melting point or the boiling/condensing point, again a pause occurs as the molecular state changes.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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