How can melting point equal freezing point?

I don't understand how the freezing point of a substance is the same temperature as the melting point of the same substance.

For example, if liquid water freezes at 0 °C how can ice also melts at 0 °C?

• I'd like to see someone answering this considering phenomena in which the melting point is "not the same as" the freezing point, such as supercooling, hysterisis and superheated crystals.
– Alex
Jun 20 '13 at 15:23
• @Alex I agree, but that seems to be a different question that involves concepts that might muddy the waters for the OP. Jun 21 '13 at 20:09

Because melting point and freezing point describe the same transition of matter, in this case from liquid to solid (freezing) or equivalently, from solid to liquid (melting).

What you may not realize is that while water is freezing or melting, its temperature is not changing! It is stuck on $0\ \mathrm{^\circ C}$ during the entire melting or freezing process. It is easier to see this for boiling points. if you put a thermometer in water and heat it, the temperature will rise until it reaches $100\ \mathrm{^\circ C}$, and then it starts boiling. And while it boils, it will stay at $100\ \mathrm{^\circ C}$! All the way until the water has all boiled away. Now if you could somehow trap the steam (gaseous water) and keep heating it, the steam could have a temperature higher than $100\ \mathrm{^\circ C}$.

So to sum this all up, when matter is transitioning from solid to liquid (melting) or liquid to solid (freezing), its temperature is fixed at the melting/freezing point, which is the same temperature.

• The temperature doesn't change in an ideal situation but I believe that the ice would be under the effects of a temperature gradient (the entire 'ice object' wouldn't be exactly 100C). Oct 24 '13 at 18:48
• @LordStryker: Yes, of course you are correct.
– user467
Oct 25 '13 at 1:19

I think this is an interesting question where the confusion is mostly due to semantics.

Let's consider an ice cube in your freezer (a typical kitchen freezer has a temperature of -10 C). When that ice cube is taken out of the freezer and placed in a warm kitchen, heat from the surroundings (air, counter top, etc.) is transferred to the ice cube. We observe that the temperature of the ice cube increases. The ice cube stays a cube because the energy of the intermolecular forces that keep the water molecules together is greater than the heat energy added to the ice cube so far.

At the melting point, however, there is enough thermal energy to start breaking those intermolecular forces. What we observe is that the temperature does not rise, but bonds are breaking and the solid starts to melt. Once all the solid melts, the temperature of the (now liquid) water can increase when thermal energy is added.

A similar explanation can be used for the reverse process (freezing water) only in this case the thermal energy is being transferred from the water to the surroundings.

So we come to your question, how can melting point = freezing point. This "point" is the temperature at which the solid and liquid forms of the molecule are in equilibrium. When we use a term like melts oftentimes we mean melts completely. In this case, the temperature of the liquid would be just beyond above the melting point.

• Thanks, that makes sense now. Could you also help me understand why a stirred mixture of solid and liquid phases of a substance will always adjust to the temperature which is the melting point of the substance? For example, why does a mixture of water and ice always adjust to zero degrees.. Jun 21 '13 at 3:32
• @jaykirby The answer involves looking at the energy required to affect the phase change and the energy required to change the temperature of the phases. Careful with your "...always adjust to zero degrees" which is not entirely true. An ice cube thrown into Lake Michigan will not change the temperature of the water to 0 degrees. Relative amounts of ice and water need to be considered as well. Jun 21 '13 at 20:06

http://en.wikipedia.org/wiki/Triple_point

If a material's equilibrium melting and freezing points were non-identical, you would have a perpetual motion machine of the first kind. Go head, trace the energy flows for temperature versus specific heats and latent enthalpies of transition. Cycled divergent vapor pressures at the differing transition temperatures would power a perpetual wind generator. Ya gotta think about these things. e.g., water below, 