How can melting point equal freezing point?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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,